Sterrekunde

Waarom glo ons dat die super massiewe swart gate in die middelpunte van twee samesmeltende sterrestelsels self sou saamsmelt?

Waarom glo ons dat die super massiewe swart gate in die middelpunte van twee samesmeltende sterrestelsels self sou saamsmelt?


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Wanneer ek na podcasts luister of op YouTube-video's kyk van sterrekundiges wat samesmeltings oor sterrestelsels bespreek, hoor ek gereeld hoe die super massiewe swart gate in hul sentrums self sal saamsmelt tydens of kort na die botsing. Waarom glo ons dat dit die geval is?

A priori sou ek verwag dat SMBH's dieselfde sal optree as alle ander galaktiese voorwerpe. Hulle kan buitengewoon massief wees, maar fisies is hulle steeds min in vergelyking met die groot leë ruimte tussen sterre in 'n sterrestelsel. Botsings tussen voorwerpe (sonder om reusagtige wolke van gas en stof te tel) is buitengewoon skaars, so waarom maak ons ​​'n uitsondering vir die SMBH's?

Ek kon sien dat hulle saamsmelt in die (seldsame?) Geval waar die gasheerstelsels mekaar op so 'n manier tref dat die wedersydse massamiddelpunt saamval met hul individuele massasentrums. In daardie geval kan die SMBH's naby genoeg wees om om mekaar te wentel, energie verloor aan swaartekraggolwe en uiteindelik saamsmelt. Ek sou dink dat hierdie scenario egter redelik skaars is. Ek vind dit aanneemliker dat 'n gemiddelde samesmelting van die sterrestelsels die SMBH's onafhanklik sal laat wentel om die middelpunt van die gekombineerde sterrestelsel, te ver van mekaar af om enige beduidende kinetiese energie aan swaartekraggolwe te verloor.

Die sterrekundiges wat oor galaktiese samesmeltings praat, weet baie meer as ek oor die onderwerp, en ek neem aan dat daar foute is in my aannames of my begrip van die fisika. Wat mis ek?


Die SMBH's lê in die bodem van die galaktiese potensiaal, wat oorheers word deur die sterrestelsels se donker materiehalo's. Maar hoewel donker materie die swaartekrag oorheers, veroorsaak botsings tussen gas- en stofdeeltjies in die interstellêre medium genoeg wrywing dat die baroniese komponent van die sterrestelsels vertraag word. Dit sal veroorsaak dat die ander komponente van die sterrestelsels ook vertraag deur aantrekkingskrag.

Ten spyte daarvan dat donker materie (en in die praktyk sterre en swart gate omdat hulle so klein is) botsingsloos is, is daar verskillende maniere om te "ontspan", dit wil sê om na 'n ewewig te ontwikkel. In die konteks van samesmelting van sterrestelsels, is die belangrikste meganisme (dink ek) 'gewelddadige ontspanning', waar die vinnige verandering van die swaartekragpotensiaal veroorsaak dat deeltjies ontspan, bv. massiewe deeltjies is geneig om meer energie na hul ligter bure oor te dra, en sodoende sterker gebind te word en sink dit na die middelpunt van die swaartekragpotensiaal.

Alhoewel SMBH's ... wel, supermassief, sal die potensiaal (gewoonlik) oorheers word deur donker materie, gas en sterre, sodat die nuwe gravitasiepotensiaal ook daartoe sal lei dat die SMBH's op dieselfde manier na die bodem sal soek en uiteindelik sal saamsmelt.


Die kort antwoord is 'dinamiese wrywing': massiewe voorwerpe wat deur 'n veld van minder massiewe voorwerpe beweeg, skep 'n 'wakker' wat daarop terugtrek, wat lei tot verlies aan energie. Omdat die SMBH's baie massiewer is as die sterre, die molekules en atome van die gas en die deeltjies van die donker materie (wat dit ook al mag wees), is hulle veral geneig hiertoe. Die netto effek is dat die SMBH's energie verloor en in die middel van die (gekombineerde) stelsel gaan sit.

Sodra hulle 'n binêre vorm, kan hulle ook energie verloor deur drie-liggaam ontmoetings met sterre naby die (gekombineerde) sterrestelsel: 'n ster is in wisselwerking met die SMBH-binêre en verkry energie (word gewoonlik uit die sterrestelselkern uitgegooi), terwyl die binêre energie verloor deur te krimp. Massiewe elliptiese sterrestelsels het dikwels sterre "kerne" met lae digtheid, wat gewoonlik die oorblyfsels van een of meer rondes SMBH-binêre samesmeltings is. As daar baie gas in die melkwegsentrum is, kan hulle ook krimp deur gravitasie-interaksies met die gas.


Vra 'n astrofisikus

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Biblioteek van vorige vrae en antwoorde

Kwasars

Wie het die kwasar ontdek en wanneer?

Die ontdekking van kwasars het regtig oor tyd versprei. Quasar is 'n verkorting van 'kwasi-sterre radiobron', en hulle is ook kwasi-sterre voorwerpe of QSO's genoem. In die laat 50's is verskeie radiobronne gekoppel aan baie dowwe optiese voorwerpe wat soos sterre gelyk het, maar met vreemde spektra met baie ultraviolet oortolligheid. Een daarvan, 3C273, se posisie is baie akkuraat gemeet deur C. Hazard en medewerkers met behulp van maan okkultasies. In 1962 verkry M. Schmidt 'n spektrum van hierdie 'ster', wat 'n rooi verskuiwing van 0.158 toon. Dit is toe QSO geskep is, want dit was 'n baie verre voorwerp wat voorgekom het as 'n ster, 'n kwasi-sterre voorwerp.

Hierdie beskrywing is geparafraseer uit 'n boek, "High Energy Astrophysics", deur M.S. Longair.

Eric Christian en Maggie Masetti
vir Vra 'n astrofisikus

Word die aarde enigsins beïnvloed deur kwasars? En wat is kwasars presies?

Die woord 'kwasar' is 'n inkrimping van 'kwasi-sterre radiobron'. Die eerste kwasars is in die vroeë sestigerjare ontdek toe sterrekundiges hul baie sterk radiovrystellings gemeet het. Wetenskaplikes was daarna verbaas om flou blou steragtige ligpunte te sien, eerder as sterrestelsels, toe hulle na dieselfde dele van die ruimte met optiese teleskope gekyk het. Toe die kwasar se lig ontleed is, is gesien dat patrone wat bekend is uit laboratoriumstudies van atoomprosesse teenwoordig is met baie groot rooi verskuiwings (wat beteken dat die voorwerpe teen hoë snelheid van ons af wegbeweeg!).

Ons weet nou dat die meeste kwasars 'radiostil' is, dit wil sê dat hulle baie min radiogolfemissie het, maar die naam kwasar is in elk geval behoue ​​gebly. Ons weet ook nou dat baie (miskien alle) kwasars klein streke van intense aktiwiteit in andersins normale sterrestelsels is.

Wat is verantwoordelik vir al die energie wat kwasars glo produseer - soms honderde keer die energie uit normale sterrestelsels? Die beste verklaring blyk te wees dat kwasars super-massiewe swart gate in die sentrums van sterrestelsels is. Soos materiaal in die swart gate inloop, word 'n groot deel van die massa in energie omgeskakel. Dit is hierdie energie wat ons sien.

As gevolg van hul groot afstande van ons af, het kwasars geen werklike uitwerking op die aarde nie.

Was die melkweg 'n kwasar in die verre verlede?

Die antwoord is dat ons nie regtig seker is nie. Ons sterrestelsel het 'n supermassiewe swart gat in die middel, wat volgens sterrekundiges die enorme uitstoot in kwasars bevorder. Of dit ooit 'n kwasar was al dan nie, kan nog bespreek word. Dit is heeltemal moontlik, maar ons het geen bewys op die een of ander manier nie.

Barbara en Stefan
Vir die span "Vra 'n astrofisikus"

Ek is 'n student in fisika uit Kanada, en ek het my afgevra hoe ek op 'n baie gedetailleerde vlak oor kwasars kon uitvind.

Aangesien u van swart gate weet, neem ek aan dat u weet dat kwasars 'n deelversameling is van 'n klas sterrestelsels genaamd aktiewe galaktiese kerne (AGN) wat waarskynlik aangedryf word deur 'n supermassiewe swart gat. As u dit nog nie gedoen het nie, gaan kyk na 'Aktiewe sterrestelsels' onder 'Advance High-Energy Astrophysics' by ons leersentrum. As u meestal belangstel in die fisika van aanwas op 'n swart gat, is die standaardteks "Accretion Processes in Astrophysics" van Frank, King en Raine.

Aan die ander kant, as u meer belangstel in AGN in die algemeen, is die basiese handboek "Die astrofisika van gasagtige newels en aktiewe galaktiese kern" van Osterbrock (die klem val hier op optiese spektra, maar dit bevat baie van die fisika van foto-ionisering wat belangrik is in AGN).

Kwasars is AGN wat baie helder en radiohelder is, en ons dink dat dit oor die algemeen radiohelder is omdat ons sinchrotron-emissie sien vanaf 'n straal relativistiese deeltjies wat van die AGN af kom. In radio-stil AGN is die straal nie teenwoordig nie, of word dit van ons af weggerig (aangesien die deeltjies relatiwisties is, word die emissie in die rigting van die straal gestraal). Daar is 'n paar nabygeleë radiostelsels wat afstammelinge van kwasars kan wees met 'n lae helderheid, wat radiostrale en bewyse van baie hoë-energie deeltjies toon (sien boeke hieronder). Blazars is 'n uiterste geval van kwasars waar ons dink ons ​​kyk direk in die straler.

'N Paar boeke en artikels oor Radio Galaxies and Jets is:

  • Hoofstuk 13 van "Galactic and extragalactic Radio Astronomy", onder redaksie van G.L. Vershuur en K.I. Kellermann, 1988, Springer-Verlag.
  • "Beams and Jets in Astrophysics", deur P.A. Hughes, c. 1991 Cambridge University Press
  • "Extragalactic Radio Jets" in Ann. Resensies van astrofisika, 1984, 22: 319-58 deur A.H. Bridle en R.A. Perley

Andy Ptak en Jonathan Keohane
- vir Imagine the Universe!

Hoeveel kwasars dink jy is daar in die heelal?

Daar is vandag ongeveer 12 000 kwasars bekend. Ek is seker dat die getal sal styg namate ons teleskope beter word. As 'n raaiskoot sou ek baie skat.

Hier is 'n paar ander inligting oor kwasars:

Hoop dit help,
Mike Arida
vir Vra 'n astrofisikus

Het iemand die hoë en lae limiete van die massa in die waarskynlike aantal kwasars bereken?

Vir u eerste vraag blyk dit dat die massa kwasars by die massa sterrestelsels ingesluit is, dus dit is nie die ontbrekende massa nie.

Vir nabygeleë sterrestelsels word die massa dikwels geskat deur die rotasiekurwe (vir spiraalstelsels) of die snelheidsverspreiding (vir elliptiese sterrestelsels) te meet. Hoe hoër die snelhede, hoe groter moet die massa wees. Dit is eintlik 'n reguit toepassing van die Newtonse swaartekrag (wat nog steeds 'n redelike benadering ver van die swart gat af is). Hierdie metode bevat die massa van enige sentrale swart gat (of sterre swart gate) wat binne-in skuil.

As ons hierdie massas met die helderheid van die sterrestelsel in verband bring, ontwikkel ons 'n massa-helderheidsverhouding vir sterrestelsels wat toegepas word om massas sterrestelsels te ver te skat om 'n rotasiekurwe of snelheidsverspreiding te verkry.

Aangesien onlangse Hubble-resultate daarop dui dat swart gate in die sentra van baie, indien nie alle sterrestelsels, kan lê nie, is die massa van hierdie swart gate alreeds opgeneem in ons massa-beramings van die sterrestelsels en dus van die heelal. Ons huidige modelle van kwasars dui daarop dat dit bloot gewone sterrestelsels is wat in 'n straal gesien word wat naby die sentrale swart gat uitgegooi word. Die strale word waarskynlik gevorm deur ingewikkelde magneto-hidrodinamiese interaksies in die aanwasskyf met die draai van die swart gat.

In elk geval, as gevolg hiervan word die kwasars en die massa daarvan opgeneem in ons massa-beramings van die Heelal. Daar is 'n paar studies gedoen wat ondersoek instel na moontlike populasies van klein swart gaatjies wat tussen sterrestelsels rondloop, maar die waarnemingsbeperkings dui nie daarop dat dit die bron van die 'donker materie' of 'ontbrekende lig' kan wees nie, wat u ook al noem.

Tom Bridgman en David Palmer
vir Vra 'n astrofisikus

Mees bekende "naakte" kwasars het geen newelagtigheid nie. Dit hou 'n duidelike bedreiging in vir sommige prominente teorieë oor kwasars. Het u enige idees oor die gebrek aan gasheersterrestelsels?

Baie dankie vir die vraag rakende kwasars. Dit is beslis 'n raaisel: waarom sien ons in sommige gevalle nie die newels wat ons sou verwag om omliggende kwasars te sien nie, in die konteks van die huidige hipotese dat kwasars kernkerne is. Die meeste sterrekundiges glo dat daardie 'naakte kwasars' bloot kern van relatief flou sterrestelsels is, en ons het dit eenvoudig nie opgespoor met die sensitiefste instrumente wat tans beskikbaar is nie, soos die kameras aan boord van die Hubble-ruimteteleskoop. Dit is 'n gebied van intense studie en daar is nog geen konsensus bereik nie. Dit is onnodig om te sê dat die ontdekking van 'n alternatiewe antwoord 'n enorme uitbetaling sou hê: die afskaffing van die huidige teorie oor die aard van kwasars sou tot roem lei (maar miskien geen fortuin nie, aangesien sterrekundiges oor die algemeen nie baie hoë salarisse betaal word nie.) Dit is natuurlik moontlik dat die sentrale streek van die gasheerstelsel eerste sterre gevorm het, en dat die afgeleë streke die sterre tempo baie laer het. (die newels wat ons sien is te danke aan sterre).

Greg Madejski en Damian Audley
vir Vra 'n astrofisikus.

Glo u regtig dat rooi skofte as 'n geldige manier van afstandsaanduiding gebruik kan word, en indien wel, op grond waarvan? Ek kom teë met 'n toenemende aantal individue wat beweer dat groot rooi skofte nie in die spektra van kwasars voorkom nie (alhoewel 'n paar beweer dat klein rooi skuiwe (z Hoe lank is die lewe van 'n Quasar en wat gebeur as dit sterf?)

Hoe lank is die lewe van 'n Quasar en wat gebeur as hy sterf?

Die vraag wat u oor kwasarleeftye gestel het, is uitstekend, maar ons weet dit net aan die hand van teoretiese argumente wat ooreenstem met die waarnemingsgegewens. Alle kwasars wat ooit ontdek is (die eerste ontdekking was ongeveer 35 jaar gelede) is nog steeds "daar". dit is die enigste werklik onbetwiste waarnemingsmeting van hul leeftyd. Laat my beskryf wat ons van kwasars weet, en hieruit gee ek u argumente vir hul leeftyd.

Oor die algemeen is kwasars relatief helderpuntbronne. Ons glo dat dit middelpunte of 'kern' van sterrestelsels is. Hulle toon groot rooi verskuiwings, wat beteken dat hulle teen groot snelhede van ons af wegbeweeg. In die huidige aanvaarde scenario dat die heelal sodanig uitbrei dat die uitbreidingsnelheid ongeveer eweredig is aan die afstand van ons tot 'n voorwerp, beteken dit dat kwasars baie ver voorwerpe is, wat dikwels halfpad tot aan die rand van die sigbare heelal geleë is. . Twee punte is hier belangrik:

Eerstens, aangesien kwasars relatief helder is, maar tog baie ver, so intrinsiek, moet dit uiters helder wees - miskien duisend keer meer helder as al die sterre in 'n sterrestelsel. Tog moet hierdie geweldige krag ontstaan ​​in 'n gebied wat vergelykbaar is met die sonnestelsel, en ons weet dit uit die feit dat hul helderheid op 'n relatiewe kort tydskaal wissel. Aangesien geen voorwerp groter kan wees as die afstand wat die lig kan beweeg gedurende 'n tyd waartydens die voorwerp sy helderheid verander deur byvoorbeeld 'n faktor van twee nie, beteken dit dat hul geweldige liguitset in 'n relatiewe klein volume ontstaan. Ons glo dat die beste scenario is dat kwasars deur 'n stort van materie op 'n baie massiewe swart gat aangedryf word, met 'n massa van so groot as 'n miljoen tot 100 miljoen Sonne.

Daar is egter net soveel materie per tydseenheid dat 'n swart gat kan "sluk" - dit is gewoonlik ongeveer 1 sonmassa per jaar vir 'n 1 - 10 miljoen sonmassa swart gat. Dus, tot die eerste orde, moet die lewensduur van 'n kwasar minstens een tot tien miljoen jaar wees.

Die tweede punt het te make met die getaldigtheid van kwasars as 'n funksie van hul afstand. Op groot afstande - sê ons halfpad tot aan die rand van die heelal - is daar baie meer kwasars as wat ons in ons plaaslike omgewing sien. Aangesien die lig egter met 'n eindige spoed beweeg, neem ons hierdie voorwerpe waar wanneer hulle ongeveer 1/2 so oud was soos nou. Wat het van hierdie kwasars gebeur? Laat ons op die oomblik die ouderdom van die heelal (wat die tyd van die oerknal verstreke is) neem tot 12 miljard jaar. Op die afstand wat ooreenstem met die tyd van ongeveer 9 miljard jaar gelede, sien ons al baie sterrestelsels en kwasars, en ons lei af dat die massa van hul swart gate ongeveer 1-10 miljoen sonmassas is. Wat het met hulle gebeur? Om die kwasars van die verlede te verreken, moet ons baie taamlike anonieme plaaslike sterrestelsels hê wat baie massiewe, maar onaktiewe swart gate in hul middel bevat.

Dit blyk dat onlangse teoretiese werk impliseer dat vir 'n gegewe aantal of gram materie wat op 'n swart gat per sekonde val, die doeltreffendheid van die omskakeling van die gravitasie-energie na straling (lig) daal namate die massa van 'n swart gat toeneem, en uiteindelik , word die kwasaar al hoe flouer, selfs met dieselfde tempo van massa-aanwas. Dit kan dus verklaar dat plaaslike kwasare dof is. In werklikheid dui baie onlangse werk aan die hand van die Hubble-ruimteteleskoop duidelik aan dat daar baie plaaslike sterrestelsels is (sommige navorsers dink dat soveel as 50% van alle sterrestelsels!) 'Stil' swart gate in hul sentrums het. Dit impliseer dat die kwasarfase waarskynlik korter is as 'n paar miljard jaar.

Dit gee u 'n algemene idee: ons weet dit nie, maar die beste ramings van vandag is dat die kwasar - wat beteken 'n ligfase van 'n aanvaarde supermassiewe swart gat - waarskynlik 10 tot 'n paar miljard jaar duur.

Greg Madejski vir Ask a Astrophysicist

Waarom is aktiewe sterrestelsels vandag aansienlik minder helder as kwasars met 'n hoë rooiverskuiwing (hul vermeende stamvader)?

Dit is 'n goeie vraag wat aan die grens van huidige navorsing staan. Daar is dus geen definitiewe antwoord nie. Dit is moontlik dat die skynbare tekort aan hoë ligsterkte nabygeleë brandwonde 'n artefak is van onvolledige waarnemings, en dat sensitiewer soektogte meer lae helderheidsvoorwerpe met 'n hoë rooiverskuiwing kan openbaar. Die waarskynlikheid is groter dat die fase met 'n hoë helderheid relatief kortstondig is en dat baie sterrestelsels met 'n lae rooiverskuiwing sluimerende AGN het. Die AGN-verskynsels kan gereguleer word deur die toevoer van gas vanaf die sterrestelsel na 'n massiewe swart gat in die middel, en 'n relatief ongewone en gewelddadige gebeurtenis (soos 'n botsing tussen sterrestelsels of 'n uitbarsting van stervorming) is nodig om voorsiening te maak voldoende brandstof.

In teenstelling met die meeste ander voorwerpe kan massiewe swart gate nie vernietig word nie, en moet dit op 'n sekere vlak waarneembaar wees deur hul swaartekrag-invloed op die sterre in hul gasheerstelsels. Namate teleskooptegnologie verbeter, word hierdie soektogte sensitiewer, en meer bewyse vir massiewe kompakte voorwerpe in andersins onaktiewe sterrestelsels kom na vore.

Tim Kallman
vir Vra 'n astrofisikus

Ek is ietwat vertroud met sterrekunde en relatiwiteit. My vraag is dit: quasar 3C273 het 'n straal wat 9 keer ligspoed beweeg vanaf die aarde. Dit is deur die wetenskap weggelê deur te sê dat die hoek van die straal na die aarde net klein genoeg is om eintlik net 'n illusie van vinniger as ligsnelhede in vergelyking met die aarde te gee.

Daar moet egter 'n teenvliegtuig wees en deur 'n sterrekundige van Flagstaff, AZ, is dit vir my gesê dat dit waar is. Die bestaan ​​van 'n teenstraal wat dieselfde relatiewe snelhede toon, sal die 'kleinhoek'-uitleg sekerlik verhoed en sal dan bewys dat daar vinniger as ligsnelhede in die heelal in verhouding tot die aarde bestaan. Dit dui dan aan dat relatiwiteit verkeerd is in die bewering dat FTL relatief tot die aarde nie kan wees nie. Kan u my asseblief sê waar ek verkeerd is? Dit is 'n belangrike struikelblok in my aanvaarding van relatiwiteit. Baie dankie en kan u, indien moontlik, aandui wie antwoord as ek 'n opvolgvraag sou hê. Weereens, dankie.

Ek weet nie dadelik of die toonbank vir 3C273 gesien is nie.

impliseer dat dit nie het nie. Daar sou verwag word dat dit dowwer sou wees (aangesien bestraling geneig is om in die rigting van beweging te straal) en dat dit nie 'n skynbare superluminale beweging toon nie.

U het gelyk dat AS 'n superluminale teenstraal gesien is, dit sou beteken dat iets baie verkeerd is met ons begrip van die stelsel of relatiwiteit.

Ek weet wel dat die 'microquasar' in ons sterrestelsel 1915 + 105 albei stralers vertoon, en die een wat na ons gerig is blykbaar superluminaal is, terwyl die een wat weggewys is, nie is nie.

David Palmer en Samar Safi-Harb
vir Vra 'n astrofisikus

Ander aktiewe sterrestelsels

Ek het absoluut geen ervaring in astrofisika nie, maar 'n vriend van my het 'n tesis oor agn gedoen. Sy kon dit nie aan my verduidelik nie, want sy is 'n Pools en ek is 'n Amerikaner. so my vraag kom in twee dele voor. Wat is AGN presies? Watter implikasies het dit vir die begrip van hoe die heelal werk?

'N AGN is 'n afkorting vir' Active Galactic Nucleus '. Sommige sterrestelsels het kerne (sentrums) wat 'aktief' is, wat beteken dat dit groot hoeveelhede straling uitstraal (radio, optiese, X-strale, gammastrale, deeltjiestrale, ens.) En / of baie veranderlik is. (Byvoorbeeld, 'n galaktiese kern wat 30 miljard keer so helder as die son begin en dan binne net 'n halfuur tot 45 miljard keer so helder soos die son word).

Aangesien hulle so vinnig wissel, moet die belangrike streek klein wees, nie groter as die binneste sonnestelsel nie (aangesien die tyd waaroor iets kan wissel, beperk is tot die tyd wat dit neem om van die een kant na die ander te kom). Aangesien hulle so helder is, moet daardie klein streek ongelooflike energie bevat.

Gigantiese swart gate, miljarde kere so massief soos die son, wat sterre en gaswolke insluk, is die enigste redelike teorieë wat lyk asof dit by die gegewens pas.

Soek op ons webwerf na 'aktief' om meer inligting te vind

David Palmer
vir Vra 'n astrofisikus

Ek is 'n hoërskoolleerling en stel belang in hierdie onderwerp. Waarom het sommige sterrestelsels 'n aktiewe kern en ander nie?

Volgens huidige teorieë is 'n galaktiese kern aktief as dit 'n groot swart gat het wat groot hoeveelhede materie verbruik.

As 'n kern nie 'n groot swart gat het nie, is dit nie 'n agn nie. As dit 'n swart gat het, maar geen sterre ens. Val nie, is dit ook nie 'n AGN nie. As al die sterre wat in 'n baan was wat baie naby die swart gat kom, al geëet is, dan is die oorblywende sterre veilig, sodat die kern nie meer aktief is nie.

David Palmer
vir Vra 'n astrofisikus

Ek weet nie baie oor sterrekunde nie, maar ek moet dit vir die skool doen. Ek was baie verward oor u inligting oor Blazars. Ek wil graag 'n eenvoudige antwoord hê op: Wat is baadjies? Dankie!

Dit is 'n goeie vraag, en dit kan goed wees om dit in twee dele op te deel: ten eerste, hoe lyk 'n baadjie vir sterrekundiges, en tweedens, wat gaan eintlik in 'n baadjie aan? Dit blyk dat sterrekundiges self nie eers die eerste vraag met sekerheid kan beantwoord nie. Ek sou sê dat 'n voorwerp die volgende kenmerke moet hê om 'n baadjie te kan noem:

1) Dit moet puntagtig aan die lug lyk, dit wil sê nie vaag soos 'n sterrestelsel of 'n newel nie. Sommige blazars het newels rondom hulle, maar die meeste lig kom van 'n puntbron af.

2) Hulle spektra lyk glad (dit wil sê geen sterk absorpsie lyne wat 'n ster mag hê nie) en platter as 'n ster. Hierdie twee eiendomme op sigself sou dit 'n kwasar maak.

3) Hul sigbare lig is dikwels gedeeltelik gepolariseer.

4) Die produksie daarvan in alle golflengtes wissel vinniger en groter as 'n kwasar.

Nou, wat gaan aan? Soos op die webwerf gesê word, kan 'n materiaalstraal naby 'n swart gat kom, met die gas in die straal wat amper die ligspoed beweeg en amper reguit na ons toe kom.

Tim Kallman
vir die Ask a Astrophysicist-span

U bladsy oor aktiewe sterrestelsels sê dat kwasars agns is wat in die straal gesien word. Hoe kan dit ooreenstem met beelde van 3C273 wat die straalvliegtuig duidelik aan die kant wys?

Ek is nie seker na watter bladsy u spesifiek verwys nie. Oor die algemeen is blazars die kwasars waarop jy kan kyk, terwyl die quasars baie helder en kompak AGN is. Dus is 'n kwasar soos 3C273 met 'n sigbare straal nie 'n baadjie nie.

Dit is belangrik om te onthou dat in die geval van AGN (en tot 'n sekere mate sterrekunde in die algemeen), benaming dikwels gedoen is op grond van waargenome kwaliteite eerder as 'n begrip van die fisiese aard van dinge (wat eers later verstaan ​​is), dus sommige benoemings skemas is nie perfek of besonder duidelik nie.


PHYS 1303, Sterre en sterrestelsels, Hfst. 25, Huiswerk- en amptoetsoorsig, prof. Kaim, DMC

Deur byvoorbeeld die bewegings van sterrestelsels in 'n bepaalde groep te meet, word die massa van die groep bepaal tot 800 MMW (waar MMW 'n massa-eenheid is gelykstaande aan die massa van die Melkwegstelsel). As die massa van die sigbare materie bymekaargetel word, het die individuele sterrestelsels 'n totale massa van 40 MMW, en die warm intrakluster gas tussen sterrestelsels het 'n totale massa van 80 MMW. Watter gevolgtrekkings kan gemaak word oor die aard van die massa van die sterrestelselgroep? (Merk alles wat van toepassing is.)

Deel A - Samesmeltings en die effekte daarvan op sterrestelsel-eienskappe

Sterrekundiges glo dat vroeë sterrestelsels gegroei het uit die herhaalde samesmelting van kleiner gaswolke. As hierdie idee waar is, moet die eienskappe van sterrestelsels mettertyd verander het. Bepaal of elke eiendom hieronder mettertyd vermeerder of afneem, en sorteer dan elke eiendom in die toepaslike asblik.

Verminder:
c) onreëlmatige struktuur van sterrestelsels
a) aantal sterrestelsels

Waarom glo sterrekundiges dat sterrestelsels vroeg in die geskiedenis van die heelal ontstaan ​​het deur samesmeltings en verkrygings? Hierdie idee word ondersteun deur verskeie stukke teoretiese en waarnemingsbewyse. Sorteer elke bewys hieronder, of dit teoretiese ondersteuning, waarnemingsondersteuning of geen hiërargiese samevoeging bied nie.

Waarnemingsondersteuning:
c) Sterrestelsels is nader aan mekaar in die ruimte geleë
d) Klein onreëlmatige sterrestelsels kom baie ver van die Aarde voor
e) Blou sterrestelsels kom baie ver van die Aarde voor

Geen ondersteuning:
f) Groot, gereelde sterrestelsels word baie ver van die aarde af gevind
b) Rooi sterrestelsels kom baie ver van die Aarde voor

Die meeste volledig gevormde sterrestelsels is in klein groepies en trosse, waar hulle met ander sterrestelsels kan kommunikeer. Hierdie interaksies kan groot effekte hê op die eienskappe van 'n sterrestelsel. Gebruik die byskrifte om die beskrywings te voltooi van hoe interaksies kan veroorsaak dat sterrestelsels oor tyd ontwikkel.

Pas die woorde in die linkerkolom by die regte spasies in die sinne aan die regterkant. Maak seker dat elke sin volledig is voordat u u antwoord indien.

Deel A - Bewyse vir supermassiewe swart gate

In die wetenskap moet sterrekundiges baie waarnemings ontleed en diegene wat bewyse lewer ter onderskeiding van 'n spesifieke teorie, onderskei. Om die geloofwaardigheid van 'n teorie te versterk, moet sterrekundiges na verskeie onafhanklike bewyse soek. Beskou die volgende voorbeeld: Sterrekundiges glo dat elke helder sterrestelsel 'n supermassiewe swart gat in sy middel bevat. Sorteer die volgende waarnemings volgens of dit hierdie teorie ondersteun of nie.

Moenie ondersteun nie:
b) Spiraalarms in die buitenste skywe van sterrestelsels
e) Sterre swart gate in interaksie met binêre stelsels
f) Globulêre trosse in die buitenste galaktiese hale

Sterrekundiges het 'n belangrike korrelasie gevind tussen die massas van die sentrale swart gate en die eienskappe van die sterrestelsels waarin hulle woon. Die grootste swart gate kom meestal voor in die sterrestelsels met die massiefste uitbultings (sien figuur).

Die rede vir hierdie korrelasie word nie ten volle verstaan ​​nie, maar die meeste sterrekundiges meen dat die evolusie van normale en aktiewe sterrestelsels baie nou moet verband hou. Deur sterrestelsels op verskillende afstande en ooreenstemmende terugskouingstye waar te neem, het sterrekundiges tot die gevolgtrekking gekom dat supermassiewe swart gate geskep is toe sterrestelsels saamgesmelt het gedurende (of net voor) die kwasartydperk 10-12 miljard jaar gelede. Hoe het galaktiese samesmeltings bygedra tot die ontwikkeling van supermassiewe swart gate? (Merk alles wat van toepassing is.)

Sterrekundiges glo vandag dat normale helder sterrestelsels moontlik van aktiewe sterrestelsels kon ontwikkel het, wat die groter voorkoms van intense kwasaraktiwiteit gedurende die kwasartydperk 10 tot 12 miljard jaar gelede sou verklaar. Voltooi elke stelling oor die evolusie van sterrestelsels aan die hand van die onderstaande figuur met die toepaslike frase.
Die figuur toon 'n aantal beelde wat van links na regs met mekaar verbind is. Die linkerkantste is vier klein geel ligte vaaghede. Elke paar van hulle loop na regs in 'n geel ligsirkel, met 'n swart punt in die middel. Een van die sirkels word as 'n 'gemerk'. Hierdie twee sirkels loop in 'n groter sirkel, gemerk as b, met 'n swart punt in die middel en bewolkte sterte om dit. Twee b-sirkels loop in 'n wit, uiters helder sirkel, gemerk as c, met twee teenoorgestelde rookstrale wat daaruit skiet en 'n swart punt. Uiteindelik loop c stadium in 'n wit ligsirkel, gemerk as d, met 'n swart punt in die middel. Een pyl wys vanaf die b-sirkel êrens buite die figuur.

1. Op posisie A smelt klein onreëlmatige sterrestelsels saam om ______________ te skep.
2. Op posisie B lewer die brandstof van die supermassiewe swart gat in 'n massiewe uitputting ______________.
3. Op posisie C produseer samesmeltings van groot sterrestelsels ______________.
4. Op posisie D word _____________ waargeneem as kernbrandstof en aktiwiteit afneem.

e) die heelal
d) die plaaslike superkluster
c) die plaaslike groep
h) die Melkwegstelsel
f) ons sonnestelsel
b) die son
a) Jupiter
g) Aarde

g) die afstand vanaf die Melkwegstelsel na die Andromedastelsel
a) die afstand vanaf die son na die middel van die melkwegstelsel
f) die afstand van die aarde na Alpha Centauri
c) een ligjaar
b) die afstand oor die sonnestelsel (tot Neptunus)
d & amp e) die gemiddelde afstand vanaf die aarde na die son EN een astronomiese eenheid (AU)

Die plot toon die intensiteit van die lig as 'n funksie van die golflengte. Die golflengte word gemeet van ongeveer 610 tot ongeveer 70 nanometer en afneem in die rigting van die positiewe x-as. Die ligintensiteit word op die y-as van 0 tot 100 persent gemeet. Die intensiteit neem toe van 610 nanometer en 30 persent tot ongeveer 560 nanometer en ongeveer 98 persent wat 'n konvekse kurwe vorm. In hierdie streek wissel dit effens. Dan, van 560 tot 70 nanometer, het dit baie gereelde en skerp dalings en pieke. Die omhulsel van intensiteit neem af van 560 nanometer en 98 persent tot 470 nanometer en 20 persent wat 'n konvekse kurwe vorm. Dan styg dit amper lineêr tot 160 nanometer en 42 persent met 'n piek van 52 persent op 170 nanometer. Die golflengte van 122 nanometer word aangedui as die Lyman-Alpha-lyn, en die gebied van hierdie golflengte tot hoër waardes word as rooiverskuiwing gemerk.

As lig van 'n verre kwasar nie deur enige tussenliggende atoomwaterstofwolke gaan nie, dan moet die figuur (& quotAbsorptielyn & quot) geteken word om aan te toon


10 vrae wat u dalk het oor swart gate

'N Swart gat is 'n uiters digte voorwerp in die ruimte waaruit geen lig kan ontsnap nie. Terwyl swart gate geheimsinnig en eksoties is, is dit ook 'n belangrike gevolg van hoe swaartekrag werk: As baie massa in 'n klein genoeg ruimte saamgepers word, skeur die voorwerp die weefsel van ruimte en tyd, en word dit 'n singulariteit genoem. . 'N Swaartekrag met 'n swart gat is so kragtig dat dit in staat is om materiaal in die omgewing in te trek en dit aan te haal & quot.

Wil u 'n swart gat besoek? Ons beveel dit nie aan nie. Vind uit waarom hierdie gravitasie-raaisels van ver af beter bestudeer word. & rsaquo Meer

Hier is tien dinge wat u dalk oor swart gate wil weet:

1. Hoe kan ons van swart gate leer as dit lig vang en kan dit nie gesien word nie?

Geen lig van enige aard, insluitend röntgenstrale, kan binne die gebeurtenishorison van 'n swart gat ontsnap nie, die gebied waarvandaan daar geen terugkeer is nie. NASA's telescopes that study black holes are looking at the surrounding environments of the black holes, where there is material very close to the event horizon. Matter is heated to millions of degrees as it is pulled toward the black hole, so it glows in X-rays. The immense gravity of black holes also distorts space itself, so it is possible to see the influence of an invisible gravitational pull on stars and other objects.

2. How long does it take to make a black hole?

A stellar-mass black hole, with a mass of tens of times the mass of the Sun, can likely form in seconds, after the collapse of a massive star. These relatively small black holes can also be made through the merger of two dense stellar remnants called neutron stars. A neutron star can also merge with a black hole to make a bigger black hole, or two black holes can collide. Mergers like these also make black holes quickly, and produce ripples in space-time called gravitational waves.

More mysterious are the giant black holes found at the centers of galaxies &mdash the "supermassive" black holes, which can weigh millions or billions of times the mass of the Sun. It can take less than a billion years for one to reach a very large size, but it is unknown how long it takes them to form, generally.

3. How do scientists calculate the mass of a supermassive black hole?

The research involves looking at the motions of stars in the centers of galaxies. These motions imply a dark, massive body whose mass can be computed from the speeds of the stars. The matter that falls into a black hole adds to the mass of the black hole. Its gravity doesn't disappear from the universe.

4. Is it possible for a black hole to "eat" an entire galaxy?

No. There is no way a black hole would eat an entire galaxy. The gravitational reach of supermassive black holes contained in the middle of galaxies is large, but not nearly large enough for eating the whole galaxy.

5. What would happen if you fell into a black hole?

It certainly wouldn't be good! But what we know about the interior of black holes comes from Albert Einstein's General Theory of Relativity.

For black holes, distant observers will only see regions outside the event horizon, but individual observers falling into the black hole would experience quite another "reality." If you got into the event horizon, your perception of space and time would entirely change. At the same time, the immense gravity of the black hole would compress you horizontally and stretch you vertically like a noodle, which is why scientists call this phenomenon (no joke) "spaghettification."

Fortunately, this has never happened to anyone &mdash black holes are too far away to pull in any matter from our solar system. But scientists have observed black holes ripping stars apart, a process that releases a tremendous amount of energy.

6. What if the Sun turned into a black hole?

The Sun will never turn into a black hole because it is not massive enough to explode. Instead, the Sun will become a dense stellar remnant called a white dwarf.

But if, hypothetically, the Sun suddenly became a black hole with the same mass as it has today, this would not affect the orbits of the planets, because its gravitational influence on the solar system would be the same. So, Earth would continue to revolve around the Sun without getting pulled in &mdash although the lack of sunlight would be disastrous for life on Earth.

7. Have black holes had any influence on our planet?

Stellar-mass black holes are left behind when a massive star explodes. These explosions distribute elements such as carbon, nitrogen and oxygen that are necessary for life into space. Mergers between two neutron stars, two black holes, or a neutron star and black hole, similarly spread heavy elements around that may someday become part of new planets. The shock waves from stellar explosions may also trigger the formation of new stars and new solar systems. So, in some sense, we owe our existence on Earth to long-ago explosions and collision events that formed black holes.

On a larger scale, most galaxies seem to have supermassive black holes at their centers. The connection between the formation of these supermassive black holes and the formation of galaxies is still not understood. It is possible that a black hole could have played a role in the formation of our Milky Way galaxy. But this chicken-and-egg problem &mdash that is, which came first, the galaxy or the black hole? &mdash is one of the great puzzles of our universe.

8. What is the most distant black hole ever seen?

The most distant black hole ever detected is located in a galaxy about 13.1 billion light-years from Earth. (The age of the universe is currently estimated to be about 13.8 billion years, so this means this black hole existed about 690 million years after the Big Bang.)

This supermassive black hole is what astronomers call a &ldquoquasar,&rdquo where large quantities of gas are pouring into the black hole so rapidly that the energy output is a thousand times greater than that of the galaxy itself. Its extreme brightness is how astronomers can detect it at such great distances.

9. If nothing can escape from a black hole, then won't the whole universe eventually be swallowed up?

The universe is a big place. In particular, the size of a region where a particular black hole has significant gravitational influence is quite limited compared to the size of a galaxy. This applies even to supermassive black holes like the one found in the middle of the Milky Way. This black hole has probably already "eaten" most or all of the stars that formed nearby, and stars further out are mostly safe from being pulled in. Since this black hole already weighs a few million times the mass of the Sun, there will only be small increases in its mass if it swallows a few more Sun-like stars. There is no danger of the Earth (located 26,000 light years away from the Milky Way's black hole) being pulled in.

Future galaxy collisions will cause black holes to grow in size, for example by merging of two black holes. But collisions won't happen indefinitely because the universe is big and because it's expanding, and so it's very unlikely that any sort of black hole runaway effect will occur.

10. Can black holes get smaller?

Ja. The late physicist Stephen Hawking proposed that while black holes get bigger by eating material, they also slowly shrink because they are losing tiny amounts of energy called "Hawking radiation."

Hawking radiation occurs because empty space, or the vacuum, is not really empty. It is actually a sea of particles continually popping into and out of existence. Hawking showed that if a pair of such particles is created near a black hole, there is a chance that one of them will be pulled into the black hole before it is destroyed. In this event, its partner will escape into space. The energy for this comes from the black hole, so the black hole slowly loses energy, and mass, by this process.

Eventually, in theory, black holes will evaporate through Hawking radiation. But it would take much longer than the entire age of the universe for most black holes we know about to significantly evaporate. Black holes, even the ones around a few times the mass of the Sun, will be around for a really, really long time!


Thread: Why is it so bright at the center of some galaxies?

Hi, and welcome to BAUT!

The light is from stars! The stars in a galaxy tend to be concentrated towards its centre - they are closer together there, than they are out near the edges.

In general, speedfreak's answer is appropriate.

However, there are some galaxies which have particularly bright centres (nuclei), nearly always appearing as a point-like source. These galaxies are called Seyferts, after Carl Seyfert, who first identified them as a class of galaxy, back in 1943. Seyferts also generally have very strong emission lines - of hydrogen, helium, oxygen, etc - originating from their nuclei.

The nuclei of these galaxies are called active galactic nuclei, AGN for short, and the source of the point-like brilliance is an accretion disk around a super-massive black hole, plus the glowing gas surrounding that (and, in the infrared, a dusty torus surrounding the accretion disk), as well as the nuclear star cluster that is also often found. Quasars are AGNs of exceptional brilliance, so much so that the galaxies they are the nuclei of is almost always invisibly faint (or hidden in the glare).

Ahh yes, good point there Nereid, thanks!

I was just describing how galaxies in general seem much brighter in the centre than they do at the edges, rather than why certain galaxies have regtig bright centres.

Concerning the brightest galactic centers: Some galaxies are known to have very bright polar jets coming from their galactic cores, thought to be black holes, and at right angles to their galactic plane. By chance if one of these jets is faced in our direction the galactic core sometimes is so relatively bright that the galaxy itself cannot be seen in some cases. Some believe this is the explanation of quasars.

That's not correct, not by a long shot.

What we see with quasars, in the UV/optical/NIR, are the accretion disks, not the polar jets.

If we look down a polar jet (i.e. it's pointing directly at us), then it's a BL Lac object (or similar).

What I said was "some believe," concerning quasars.

quotes from the link below:

". high-energy quasars are being viewed with the jet pointed towards us.."

"Many astronomers believe that Seyfert galaxies and high-energy quasars are basically the same type of objects, but we are simply viewing them differently."

What I said was "some believe," concerning quasars.

quotes from the link below:

". high-energy quasars are being viewed with the jet pointed towards us.."

"Many astronomers believe that Seyfert galaxies and high-energy quasars are basically the same type of objects, but we are simply viewing them differently."

Well, someone is going to have to write to Phil Newman and his colleagues to update that website! It's astonishing that they have some of the basics quite wrong.

For the record, AGNs is the general class, and quasars, blazars, BL Lac objects, DRAGNs, Seyferts, (and more no doubt) are specific types of AGNs. The viewing angle makes a great deal of difference those seen from the side, through the dusty torus, are detectable only in the x-ray and gamma regions (though their twin jets, if they have them, are easily detected in the radio), or, sometimes, as a Type II (or Type 2) quasar. Not all AGNs have jets, in fact only giant cluster ellipticals (or late-type galaxies) seem to have them. The other big difference is in the intrinsic luminosity the brightest quasars are many orders of magnitude more luminous, in an absolute sense, than Seyferts, for example.

What the "high-energy quasars are being viewed with the jet pointed towards us" seems to be saying is that we can see the accretion disk more or less unobscured, and that the most luminous quasars are those found in giant ellipticals, which also (almost always) have twin jets. The jets themselves are not particularly luminous, except when viewed 'down the barrel'.


As the name suggests, supermassive black holes contain between a million and a billion times more mass than a typical stellar black hole. Although there are only a handful of confirmed supermassive black holes (most are too far away to be observed), they are thought to exist at the centre of most large galaxies, including the centre of our own galaxy, the Milky Way.

For many years, astronomers had only indirect evidence for supermassive black holes, the most compelling of which was the existence of quasars in remote active galaxies. Observations of the energy output and variability timescales of quasars revealed that they radiate over a trillion times as much energy as our Sun from a region about the size of the Solar System. The only mechanism capable of producing such enormous amounts of energy is the conversion of gravitational energy into light by a massive black hole.

More recently, direct evidence for the existence of supermassive black holes has come from observations of material orbiting the centres of galaxies. The high orbital velocities of these stars and gas are easily explained if they are being accelerated by a massive object with a strong gravitational field that is contained within a small region of space – i.e. a supermassive black hole.

Astronomers are still not sure how these supermassive black holes form. Stellar black holes result from the collapse of massive stars, and some have suggested that supermassive black holes form out of the collapse of massive clouds of gas during the early stages of the formation of the galaxy. Another idea is that a stellar black hole consumes enormous amounts of material over millions of years, growing to supermassive black hole proportions. Yet another, is that a cluster of stellar black holes form and eventually merge into a supermassive black hole.

Whatever their formation mechanism, most astronomers agree that accretion of material onto the supermassive black hole drives both active galactic nuclei and galactic jets.

Study Astronomy Online at Swinburne University
All material is © Swinburne University of Technology except where indicated.


Five Reasons We Think Dark Matter Exists

Any recent article about the remaining mysteries of the Universe will include dark matter close to the very top of the list of unsolved problems. What is it? Where is it? And if it’s there, how do we measure it? These are important questions still at the forefront of research in Cosmology. But this elusive substance that affects the motion of our galaxy and is the reason that galaxies exist with the properties they have, has only been detected indirectly, and has yet to be measured via direct detection. Earlier this year, the most sensitive dark matter experiment to date, LUX, released its results showing no direct evidence for dark matter and failing to confirm potential detections by two groups of experiments, DAMA/Libra and CoGeNT and Super-CDMS.

Despite this, fellow scientists are pushing forward, determined to measure direct evidence of dark matter. The U.S. Department Of Energy and National Science Foundation are on board with this plan, as they recently announced a new round of funding for 3 upcoming dark matter experiments: LZ (the successor to LUX), SuperCDMS-SNOLAB, and ADMX-Gen2. So if we haven’t measured dark matter directly yet, what is keeping researchers on the scent and funding agencies interested?

The idea of dark matter is very well motivated by other observations. Completely independent cosmological and astrophysical phenomena that aren’t explained within other theoretical frameworks can be solved by the existence of dark matter alone. Here are five of the most compelling reasons we think* dark matter exists:

1.) Galaxy Clusters

Throughout space, astrophysical objects of all sizes swirl and orbit: planets revolve around our sun, stars orbit around our galactic center, and individual galaxies in groups whiz around themselves. To keep these objects tightly bound together, the gravitational pull felt by an object must be strong enough to balance the energy it has due to its motion. A fast-moving object with more kinetic energy is harder to keep gravitationally bound.

In 1933, Fritz Zwicky (below) was studying the nearest very large cluster of galaxies to us in space: the Coma cluster (above).

He used the virial theorem, an equation which relates the average kinetic energy of a system to its total potential energy, to infer the gravitational mass of the cluster. He then compared that to the mass inferred from the bright, luminous matter (stars and gas) in the galaxies. You’d expect those two numbers — gravitational mass and mass due to luminous matter — to match, wouldn’t you? But instead, he found that the mass from the luminous matter was not enough to keep the cluster bound, and was several times smaller than the inferred gravitational mass. Assuming that the luminous matter constituted all of the mass in each galaxy, they should have been flying apart! He thus coined the term “dark matter” for the material that must therefore be present, quietly holding the galaxy cluster tightly together.

2.) Galactic Rotation Curves

Similar evidence was observed within galaxies themselves. From standard Newtonian dynamics, we expect the velocity of stars to fall as you move from the near the center of mass of a galaxy to its outer edges. But when studying the Andromeda galaxy in the 1960s, Vera Rubin and Kent Ford found something very different: the velocity of stars remained approximately constant, regardless of how far they were from the galactic center.

This and many future observations of the velocities of stars in spiral galaxies hinted that the mass of the galaxy must not be entirely defined by the objects we could see with our telescopes, which Rubin and Ford presented at an American Astronomical Society meeting in 1975. If instead a large fraction of the galaxy’s mass resided in a diffuse dark matter ‘halo’ that extended well beyond the edges of the luminous matter, the observed galactic rotation curves could be explained.

3.) The Cosmic Microwave Background

The Cosmic Microwave Background (CMB) is the earliest photograph of our Universe. The patterns that we see in observations of the CMB were set up by competition between two forces acting on matter the force of gravity causing matter to fall inward and an outward pressure exerted by photons (or particles of light). This competition caused the photons and matter to oscillate into-and-out-of dense regions. But if the Universe consisted partially of dark matter in addition to normal matter, that pattern would be affected dramatically. The existence of dark matter leaves a characteristic imprint on CMB observations, as it clumps into dense regions and contributes to the gravitational collapse of matter, but is unaffected by the pressure from photons.

We can predict these oscillations in the CMB with and without dark matter, which we often present in the form of a power spectrum. The power spectrum of the CMB shows us the strength of oscillations at different sizes of the photons and matter. The Wilkinson Microwave Anisotropy Probe (WMAP) was the first instrument to measure the CMB power spectrum through the first peak of oscillations, and showed that the existence of dark matter is favored.

4.) The Bullet Cluster

In 2006, astronomers working on the Hubble Space Telescope and the Chandra X-ray Observatory released exciting information about an object known as the bullet cluster. This cluster is actually two galaxy clusters which have recently undergone a high-speed collision, forcing the contents of each cluster to merge together. Observations from the two telescopes allowed us to measure the location of the cluster mass after the collision using two methods: optical observations of X-ray emission and gravitational lensing.

One way we can tell two clusters have just collided is through X-ray astronomy. An extremely hot gas of particles pervades the space between each galaxy in a cluster, which accounts for for about 90% of the mass from ordinary matter (rather than stars). When two galaxy clusters collide, the gas particles become even hotter from crashing into each other, causing an increase in brightness of the X-ray emission. From this we can tell how energetic the gas is and where it is located.

Gravitational lensing occurs because matter isn’t the only thing that feels the effects of gravity: light does as well. This means that a massive object can act as a lens a background source that emits light in all directions will have some of that light focused if it passes by a massive object. By measuring these focused images, we can infer the location and mass of the lens between us and the source.

If the clusters were entirely comprised of ordinary matter, the location of mass from the optical observations and the location calculated from gravitational lensing in the bullet cluster should overlap. Instead, the observations showed a glaring inconsistency. The optically visible matter told us the mass should be concentrated near the center of the image shown, highlighted in red. The mass distribution from gravitational lensing, highlighted in blue, shows that the concentration of mass is actually in two pieces, just outside of the luminous matter in the galaxy! Invoking dark matter, this behavior is easy to explain as follows:

a.) Dark matter interacts with its surroundings significantly less frequently than ordinary matter.

b.) During the cluster collision, the dark matter of one cluster would have slipped through all of the objects in the other cluster with relative ease.

c.) The luminous matter, on the other hand, would have bounced off of other particles around it, causing it to slow and separate from the dark matter.

The net result? High-velocity collisions between galaxy clusters should have the majority of their mass — in the form of dark matter — pass through one another unimpeded, while the normal matter collides, slows down, and heats up, emitting X-rays.

5.) Large-Scale Structure Formation

When telescopes like the Sloan Digital Sky Survey map the locations the galaxies in the Universe, with the biggest features being referred to as large-scale structure, it sees a set of patterns that couldn’t happen with only the gravity due to ordinary matter at work. We know that before the CMB, ordinary matter wasn’t able to efficiently clump into dense objects due to the oscillations from the competing forces of gravity and pressure from radiation. The structure we observe is much more advanced in its evolution given the amount of time available for objects to gravitationally collapse after the time of the CMB.

Instead, dark matter provides a reasonable explanation. Because dark matter didn’t undergo the same oscillations with matter and light, it was free to collapse on its own to form dense regions that helped structure formation get a head start, and allowed the distribution of galaxies and clusters to be what we observe today.

These five independent pieces of evidence, when taken all together, provide a compelling reason that dark matter must exist. Reading through each explanation again, there is a common theme: gravity. Each piece of the puzzle relies on the way dark matter affects things around it via the gravitational force.

An Alternative

If I had to place bets, my money would fully be on the “dark matter” square. At conferences and seminars, astronomers, astrophysicists, and cosmologists speak about dark matter as though it’s a certainty (and most think it is). So why do I say “five reasons we think dark matter exists”? Since we haven’t measured it directly yet, and the evidence for dark matter’s existence centers on its gravitational interactions, a responsible scientific community would ask “what if we just don’t understand gravity as well as we think we do?” Some research groups have been tackling that question, investigating theories like MOND (MOdified Newtonian Dynamics), which are often grouped together under the umbrella “modified gravity.” So far, these theories have had successes in describing one of these peculiarities: galactic rotation curves, but have not yet provided an explanation for the complete set of observations like dark matter does.

Modifying the theory of gravity is no easy game. We have fantastically precise measurements of gravity’s influence on objects throughout our solar system which fit precisely within the current understanding of gravity from General Relativity (a fact that underpins the precision of modern GPS). If you want to change the theory of gravity, you have to preserve its behavior as we’ve already measured it in the solar system. Further, the idea of modified gravity extends beyond trying to explain away dark matter. Modified gravity is an incredibly active field of research, with many ideas trying to explain the even more elusive phenomenon of dark energy. Often, these theories still require dark matter of some sort to exist.

But wait, there’s more!

These five reasons don’t constitute the total observational evidence we have for dark matter. Big Bang Nucleosynthesis (BBN), which explains the way light elements such as Helium were formed fractions of a second after the Big Bang, tells us abundance of baryonic matter doesn’t account for the total matter content of the Universe inferred from other observations, and that dark matter can’t be just be things like protons and neutrons. Observations of molecular clouds — neutral hydrogen gas — absorbing light from background galaxies and quasars, known as the Lyman-alpha forest, gives us information about the location of dark matter clumps as well as how much energy dark matter particles are allowed to have.

In almost every place we look, the Universe seems to be hinting that dark matter exists. The indirect evidence, from the early Universe to the present day, and from galactic scales up to the largest ones observable in the Universe, all point to the same conclusion. Direct detection is the next logical step. But that may be the biggest challenge of all: we still have to find it.

* “Think” here is used in a very scientific sense. We say “think” to mean “evidence strongly shows.” It is not meant in the same sense as something like “I think I turned the oven off…” or “I think that movie starred Nicolas Cage, but it could have been John Travolta.” “We think” means “we’re very sure, but we haven’t detected it yet so we can’t say ‘we know.’”

This article was written by Amanda Yoho, a graduate student in theoretical and computational cosmology at Case Western Reserve University. You can reach her on Twitter at @mandaYoho.


Why Do Supermassive Black Holes Erupt?

Astronomers are dragging the inner workings of black holes out into the light.

The powerful X-ray flares seen erupting from supermassive black holes are tied to the motion of these behemoths' surrounding "coronas," mysterious features that are sources of high-energy light, a new study suggests.

Specifically, supermassive black holes likely flare when their coronas launch away from them, researchers said. [Images: Black Holes of the Universe]

"This is the first time we have been able to link the launching of the corona to a flare," study lead author Dan Wilkins, of Saint Mary's University in Halifax, Canada, said in a statement. "This will help us understand how supermassive black holes power some of the brightest objects in the universe."

No light escapes from black holes themselves, but many of these objects are surrounded by an "accretion disk" of fast-moving, superheated material that emits light in various wavelengths.

Supermassive black holes lurk at the heart of most (if not all) galaxies, including Earth's own Milky Way. These monsters can contain as much mass as hundreds of millions, or even billions, of suns.

Wilkins and his team studied a supermassive black hole called Markarian 335 (Mrk 335), which is found 324 million light-years away from Earth. In September 2014, NASA's Swift satellite detected a bright flare coming from Mrk 335 the astronomers asked NASA to focus its NuSTAR (Nuclear Spectroscopic Telescope Array) spacecraft on the object to study it further in X-ray light.

Using these various observations, the study team determined that Mrk 335's corona launched away from the black hole at about 20 percent the speed of light, and then eventually collapsed.

"The corona gathered inward at first and then launched upwards like a jet," Wilkins said. "We still don't know how jets in black holes form, but it's an exciting possibility that this black hole's corona was beginning to form the base of a jet before it collapsed."

The new results also suggest that coronas are relatively compact rather than diffuse, as some researchers have posited, study team members said.

"The nature of the energetic source of X-rays we call the corona is mysterious, but now with the ability to see dramatic changes like this, we are getting clues about its size and structure," NuSTAR principal investigator Fiona Harrison, who's based at the California Institute of Technology in Pasadena, said in the same statement.

Harrison is not affiliated with the new study, which was published in the journal Monthly Notices of the Royal Astronomical Society.


Why do we believe that the super massive black holes at the centers of two merging galaxies would themselves merge? - Sterrekunde

Seeing Galaxies of the Past

  • Very distant galaxies are very faint, and we cannot see them very well even with the largest of today's telescopes. New, super-sized telescopes are planned, to allow us to see even farther into the past.
  • Very distant galaxies have very large recession velocities -- they are moving away at a good fraction of the speed of light. This causes extreme doppler redshifts, which means that distant galaxies are brightest in the infrared part of the spectrum. To observe these, we need to put large infrared telescopes into space.
  • Galaxies formed so quickly that it is not easy to see differences in galaxies until we reach close to the limit of our current capabilities.
We can learn a lot about galaxy formation by studying the parts of our own galaxy. Recall the schematic view of a spiral galaxy like our own: Here we see that there are objects (stars and globular clusters) in a more-or-less spherically symmetric "halo," which becomes thicker toward the center to make up the central bulge, but there are other objects (young stars, dust and gas, etc.) that are concentrated in a thin disk. From this we envision that our galaxy formed from a more-or-less spherical cloud of hydrogen and helium that collapsed due to gravity, just as our solar system formed from a far smaller cloud. In the early history of the collapsing cloud, the cloud would have remained cool due to radiating its heat away, so particularly dense regions would have formed the globular clusters and halo stars.

It is important to realize that once a star forms, it will not change its orbit to form a disk of stars. It will forever orbit at whatever inclination angle it was when it formed. By studying the orbits of globular clusters and halo stars, we can see that the gas of the original protogalaxy cloud was once more spherically distributed. However, the gas that remained did continue to collapse toward a thin disk, because the gas molecules could interact and lose their random motions. The law of conservation of momentum was at work to ensure that the remaining gas clouds orbited close to the plane of the disk. Stars that formed later, when the gas was more and more nearly a disk, kept their inclinations just as the halo stars did, so stars in the thick disk should be older than stars in the thin disk. In fact, stars in the halo and thick disk should be among the oldest stars in the galaxy, and because the original cloud was only hydrogen and helium (no heavier elements) the oldest stars should have no heavy elements (metals). In fact, we do see that the halo and thick disk stars are older and have fewer metals, while disk stars are younger and have more metals.

  • Lack of Rotation: Perhpas the initial cloud simply had very little angular momentum (very little rotation). If this were the case, no disk would form and the stars would remain rather evenly distributed in the cloud. However, this would suggest that ellipticals would have dust and gas from generations of stars, but the gas would be distributed throughout the elliptical. In fact, ellipticals seem not to have any dust at all.
  • Rate of Cooling: If the protocloud were cool enough, so many stars would form early on that there would have been little dust left to form a disk. One way to make the cloud extra cool would be to make it have a high starting density. Evidence for this scenario comes from very distant ellipticals, which seem to be redder than their recessional redshift would give. These galaxies must have no young blue or white stars, indicating that there is no new star formation going on, even though they are only a few billion years old.
  • Shaped by Collisions: We know that collisions among galaxies are quite common. Recall that galaxies are very different from stars in this respect. Stars are so far apart relative to their size that they essentially never collide. Recall that if we scale things down so that the Sun were the size of an orange, Alpha Centauri (the next nearest star) would be another orange separated by 3000 miles. If we then think about shrinking the galaxy to the size of an orange, then the Andromeda galaxy would be another orange only a few meters away! So relative to their sizes, galaxies are much much closer together. And recall that the universe is expanding. In the distant past, when the universe was smaller, the distance between galaxies was far smaller than it is today.
Some galaxies are called "active galaxies" because they have some sort of engine in their nucleus that is producing huge amounts of energy, sometimes 100s of times higher than a normal galaxy. The very brightest and most energetic are called quasars, which is a shortening of the term quasi-stellar object . As we mentioned earlier, quasars and other active galactic nuclei are a product of the early universe, and nearby (and hence present-day) galaxies do not seem to show such activity. It may be that the engine is present in nearby galaxies (even our own), but is currently not active.

For many years the cause of the phenomena seen in active galactic nuclei was completely unknown, but now we believe that all can be explained by a central, super-massive black hole in the nucleus of some galaxies. These giant black holes, of sometimes billions of solar masses, are active when new matter is being fed to them in accretion disks. The accretion disks produce lots of X-rays and ultraviolet light. The magnetic fields threading the disks create jets of particles that zoom far out into space to form radio galaxies.

Using quasars to probe the universe.


Researcher shows that black holes do not exist

This artist's concept depicts a supermassive black hole at the center of a galaxy. The blue color here represents radiation pouring out from material very close to the black hole. The grayish structure surrounding the black hole, called a torus, is made up of gas and dust. Credit: NASA/JPL-Caltech

Black holes have long captured the public imagination and been the subject of popular culture, from Star Trek to Hollywood. They are the ultimate unknown – the blackest and most dense objects in the universe that do not even let light escape. And as if they weren't bizarre enough to begin with, now add this to the mix: they don't exist.

By merging two seemingly conflicting theories, Laura Mersini-Houghton, a physics professor at UNC-Chapel Hill in the College of Arts and Sciences, has proven, mathematically, that black holes can never come into being in the first place. The work not only forces scientists to reimagine the fabric of space-time, but also rethink the origins of the universe.

"I'm still not over the shock," said Mersini-Houghton. "We've been studying this problem for a more than 50 years and this solution gives us a lot to think about."

For decades, black holes were thought to form when a massive star collapses under its own gravity to a single point in space – imagine the Earth being squished into a ball the size of a peanut – called a singularity. So the story went, an invisible membrane known as the event horizon surrounds the singularity and crossing this horizon means that you could never cross back. It's the point where a black hole's gravitational pull is so strong that nothing can escape it.

The reason black holes are so bizarre is that it pits two fundamental theories of the universe against each other. Einstein's theory of gravity predicts the formation of black holes but a fundamental law of quantum theory states that no information from the universe can ever disappear. Efforts to combine these two theories lead to mathematical nonsense, and became known as the information loss paradox.

In 1974, Stephen Hawking used quantum mechanics to show that black holes emit radiation. Since then, scientists have detected fingerprints in the cosmos that are consistent with this radiation, identifying an ever-increasing list of the universe's black holes.

But now Mersini-Houghton describes an entirely new scenario. She and Hawking both agree that as a star collapses under its own gravity, it produces Hawking radiation. However, in her new work, Mersini-Houghton shows that by giving off this radiation, the star also sheds mass. So much so that as it shrinks it no longer has the density to become a black hole.

Before a black hole can form, the dying star swells one last time and then explodes. A singularity never forms and neither does an event horizon. The take home message of her work is clear: there is no such thing as a black hole.

The paper, which was recently submitted to ArXiv, an online repository of physics papers that is not peer-reviewed, offers exact numerical solutions to this problem and was done in collaboration with Harald Peiffer, an expert on numerical relativity at the University of Toronto. An earlier paper, by Mersini-Houghton, originally submitted to ArXiv in June, was published in the journal Physics Letters B, and offers approximate solutions to the problem.

Experimental evidence may one day provide physical proof as to whether or not black holes exist in the universe. But for now, Mersini-Houghton says the mathematics are conclusive.

Many physicists and astronomers believe that our universe originated from a singularity that began expanding with the Big Bang. However, if singularities do not exist, then physicists have to rethink their ideas of the Big Bang and whether it ever happened.

"Physicists have been trying to merge these two theories – Einstein's theory of gravity and quantum mechanics – for decades, but this scenario brings these two theories together, into harmony," said Mersini-Houghton. "And that's a big deal."