Sterrekunde

Hoe skat sterrekundiges die totale stofmassa in wolke en sterrestelsels?

Hoe skat sterrekundiges die totale stofmassa in wolke en sterrestelsels?


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Soos ek verstaan, skat sterrekundiges die massa sterre deur die wentelbane van hul metgeselle (ander sterre of planete) te bestudeer. Dit help hulle om verwantskappe uit te lei tussen die helderheid en massas sterre. Dit stel hulle in staat om die massa sterre met bekende helderheid af te lei.

Maar hoe skat sterrekundiges die massa kosmiese stof in bepaalde wolke of in hele sterrestelsels?


Stof absorbeer sterrelig (hoofsaaklik in die ultraviolet) en word verhit. Vervolgens word dit afkoel deur infrarooi, 'termiese' bestraling uit te gee. Uitgaande van 'n stofsamestelling en korrelgrootteverdeling, kan die hoeveelheid uitgestraalde IR-lig per eenheid stofmassa bereken word as 'n funksie van temperatuur. As u die voorwerp op verskillende IR-golflengtes waarneem, kan 'n Planck-kromme op die datapunte aangebring word, wat die stoftemperatuur lewer. Hoe meer UV-lig op die stof val, hoe hoër is die temperatuur.

Die resultaat is ietwat sensitief vir die aannames, en dus is die onsekerhede soms redelik groot. Hoe meer IR-datapunte verkry word, hoe beter. As slegs een IR-punt beskikbaar is, kan die temperatuur nie bereken word nie. Dan is daar 'n ontaarding tussen invallende UV-lig en die hoeveelheid stof, en die massa kan net binne 'n mate van grootte geskat word (dink ek).

As daar ook lyne van verskillende atoom- of molekulêre oorgange gesien word, kan die samestelling beter beperk word. Die grootteverspreiding kan bepaal word deur die teoretiese spektrum van 'n gegewe verspreiding op die waargenome stofspektrum te pas. Hierdie inligting is dikwels nie beskikbaar in 'n gegewe sterrestelsel nie, dus kan ons gedwing word om aan te neem dat die stof soortgelyk is aan 'plaaslike' stof, dit wil sê in die Melkweg en ons naaste bure.

As u belangstel in die relevante vergelykings, kan dit op baie plekke gevind word, bv. hier.

'N Ander manier om die stofmassa te skat, is om die metaalagtigheid van die gas waarmee die stof gemeng word, te meet, hetsy vanaf emissielyne of absorbsielyne as 'n agtergrondbron beskikbaar is. Die stofmassa word dan gevind uit 'n veronderstelde stof-tot-metaal-verhouding, wat redelik goed gevestig is in die plaaslike Heelal, en tot 'n mate ook by hoër rooiverskuiwings.


Al is die aanvaarde antwoord goed gestel en duidelik, sou ek 'n ander metode vir stofmetings noem. X-straalspektra kan ook help om die hoeveelheid stof af te lei en die absorpsie by lae energie van die waargenome spektra te ontleed.

Inderdaad word die neutrale waterstofkolomdigtheid $ N_H $ en die uitsterwing $ A (V) $, wat deur die stof veroorsaak word, in ons Melkweg aangetref as: (Guver & Ozel, 2009):

$ N_H = 2.2 keer 10 ^ {21} A (V) , $ mag

Uit die uitsterwing kan 'n mens die stofoptiese diepte kry (sien hier), en daaruit die volumetriese stofdigtheid.

'N Goeie voorbeeld van hoe u $ N_H $ kan verkry, word in die volgende plot gegee (S. Ikeda et al. 2009, ApJ 692 608):

Soos gesien kan word, word die spektra verskillend aangepas deur verskillende hoeveelhede neutrale waterstofkolomdigtheid $ N_H $. Dit kan natuurlik verband hou met verskillende oorvloed, om die spektrums die beste te pas en die hoeveelhede metale te vind.

Hierdie metode hou ook verband met die yster K $ alpha $ -lyn teen $ 6.4 , $ keV, wat nog 'n meting is van die metaalmetaal (Fabian et al. 2000):

Om tussen verskillende ionisasie-toestande te onderskei, is hoë-resolusie spektroskopie baie algemeen.


Hoe skat sterrekundiges die totale stofmassa in wolke en sterrestelsels? - Sterrekunde

Bronne: William Keel (205 / 348-1641) en Raymond White (205 / 348-1640). Beelde wat hierdie weergawe vergesel, is op die Wêreldwye web beskikbaar op http://www.astr.ua.edu/keel/research/dust.html

KOSMIESE SILHOUETTE GEE SELFDIENSTIGE GLIMPSE VAN GALAXIES SE STOF

Vir vrystelling: 09:20 EST, Vrydag 9 Januarie 1998

(Washington, DC) - Nuwe beelde wat vandag van 'n paar satellietteleskope aangebied word, bied 'n unieke uitsig op die verduisterende stof in sterrestelsels, en maak gebruik van seldsame kosmiese silhoeëtte om die debat oor die hoeveelheid stof in sterrestelsels op te los. is, en hoeveel dit saak maak.

Sterrekundiges William Keel en Raymond White III, van die Universiteit van Alabama in Tuscaloosa, het die bevindings tydens die Washington-byeenkoms van die American Astronomical Society aangebied met behulp van data van NASA's Hubble-ruimteteleskoop (HST) en die Europese Ruimteagentskap Observatorium vir infrarooi ruimtes (ISO) om die duidelikste kartering nog van die stof in verre sterrestelsels te bied.

Die hoeveelheid stof in tipiese sterrestelsels, en die effek daarvan op wat ons sien, was 'n onderwerp van kontroversie sedert 'n artikel van Edwin Valentijn uit 1990, wat die lang beskouing dat sterrestelsels basies deursigtig is, betwis het. Valentijn het statistiese bewyse uit sterrestelselkatalogusse gebruik om aan te dui dat die meeste sterrestelsels feitlik ondeursigtig is. Daaropvolgende statistiese ontledings het getoon dat sulke gevolgtrekkings bevooroordeeld kan wees deur die wyses waarop sterrestelsels vir die studie gekies is, of in die eerste plek gekies kan word om in die bestaande katalogusse in te skryf. Om sulke onduidelikhede te vermy, het Keel en White 'n meer direkte benadering gevolg deur die seldsame gevalle te ondersoek waar 'n sterrestelsel in die voorgrond gedeeltelik 'n verre agtergrondstelsel dek. Met behulp van teleskope op die grond het hulle duisende sterrestelselpare uitgeskakel om die paar simmetries genoeg te vind en op die regte manier gesien om hul stof teen die kollig van 'n agtergrondmaat te wys. Nuwe gegewens van twee sterrewagbane wentel 'n weergawe van 'n paar uitverkore voorwerpe in ongekende besonderhede.

Die ondersoekers het beelde, geneem met Hubble's Wide Field Planetary Camera 2 (WFPC2), aangebied van twee opvallende sterrestelsels, elk met 'n spiraalvormige sterrestelsel voor 'n gladde elliptiese metgesel. Albei is te flou om in die bekende NGC-katalogus te verskyn, en is die eerste keer in die Arp-Madore (AM) katalogus van die suidelike hemel gelys. AM1316-241 is ongeveer 400 miljoen ligjare weg (rooi verskuiwing z = 0.033) in Hydra, terwyl AM0500-620 in die diep suidelike konstellasie Dorado, ongeveer 350 miljoen ligjare weg (rooi verskuiwing z = 0,028). Die beelde los strukture in hierdie sterrestelsels op tot so klein as 175-200 ligjaar, en verbeter die meer as tien keer die diskriminasie van detail op die beste vroeëre beelde.

"'N Kykie na hierdie skouspelagtige beelde maak dieselfde punte as wat ons jare spandeer het deur gedetailleerde ontleding en modellering op grondgebaseerde data", sê Keel. "Dit is net so interessant vir wat ons nie verwag het nie, as vir wat ons gedoen het."

Trouens, die sterrekykers het wel gesien wat hulle verwag om te vind, dat die stof pap en klonterend is, grootliks in lyn met die spiraalarms. "Die feit dat die meeste absorberende stof in die spiraalarms is, waar die meeste lig oorsprong het, is dat die statistiese studies verkeerdelik tot die slotsom gekom het dat spirale ondeursigtig is," sê White. Hierdie feit was implisiet in vergelyking met hul vroeëre resultate deur middel van verskillende filters in sigbare en naby-infrarooi lig, maar van die grond af kon net die heel grootste van hierdie stofvlekke, duisende ligjare groot, duidelik gesien word. Alhoewel dit nie juis verbasend is nie - hierdie soort verspreiding blyk duidelik uit die mees gedetailleerde prente van spiraalstelsels - maar hierdie tegniek met behulp van oorvleuelende sterrestelsels vermy baie onduidelikhede wat opduik om die stofinhoud van 'n individuele sterrestelsel te meet.

Die nuwe data openbaar ook verrassings. Die stowwerigste kolle wat in die HST-beelde verskyn, is nie baie donker nie, want minstens 20% van die blou lig kom deur en nog meer van die naby-infrarooi lig. Dit lyk asof dit die verwagtinge oortree gebaseer op sywaartse blik deur die arms van ons eie melkweg. Boonop vertoon die stowwerige spiraalarms nie soveel fyn struktuur as wat die navorsers verwag het nie. Die stofvlekke glad uit in groottes van 500 ligjare of meer, eerder as op die volledige reeks groottes, wat so klein afneem as wat gemeet kan word, voorgestel deur fraktale modelle van interstellêre materiaal. Die twee sterrestelsels wat deur HST waargeneem word, verskil ook in die breedte en die tekstuur van die stof in spiraalarms, alhoewel hulle spiraalvormige sterrestelsels van soortgelyke algehele tipe is.

Die ISO-infrarooi-waarnemings vul die voortreflike Hubble-beeldvorming aan in hul vermoë om in die stowwerige streke te kyk vir spore van stervorming en om die totale hoeveelheid stof te meet, gebaseer op sy eie emissie van ver-infrarooi straling. Hulle toon aan dat daar geen noemenswaardige stervorming is in die stowwerige streke wat met HST gekarteer is nie, wat verdere beperkings gee aan hoeveel aksie in hierdie sterrestelsels plaasvind waar dit moeilik is om te sien.

Die kennis van die hoeveelheid stof wat in sterrestelsels voorkom, is belangrik in 'n wye verskeidenheid vrae in astrofisika. Hierdie resultate dui byvoorbeeld daarop dat die absorpsie deur stof in tussenliggende sterrestelsels waarskynlik nie die rede sal gee dat ons waarskynlik nie veel kwasars met 'n baie hoë rooiverskuiwing sien nie, maar ons kyk verder as die tyd waarin dit gevorm is. Nader aan die huis is pogings om die boeke oor energievloei in sterrestelsels te balanseer, sowel wat sterre uitgee as wat deur stof opgeneem word en daarna diep in die infrarooi uitgestraal word, gestimuleer deur ons gebrek aan kennis van net waar die stof lê en hoe dit geleë is ten opsigte van die helderste en warmste sterre.

Keel en White wys vinnig gebiede vir verdere werk in hierdie rigting aan. Hierdie sterrestelselpare vertel ons slegs van hul buitenste streke, waar die agterste beligting die sterkste is, die binneste dele van sterrestelsels, die rykste aan swaar chemiese elemente en miskien stof, baie moeiliker is om te ondersoek. HST-waarnemings wat vir die volgende jaar beplan word, sowel as 'n direkte sterrestelsel-superposisie wat hul eie program uitbrei, en deur verskeie ander navorsingsgroepe wat die nuwe infrarooi-instrument NICMOS aan boord van HST gebruik, sal waarskynlik hierdie situasie verbeter. Namate die analise van die ISO-data in die ver-infrarooi gaan, waar al die stofkomponent van sterrestelsels gesien word, verbeter die algemene begrip van die mees tipiese stofkorrels ook. Dit is hierdie soort meting wat ons die totale hoeveelheid stof kan vertel, wat belangrik is om te bepaal hoe dit op skale saamgevoeg word, selfs buite die vermoë van selfs die Hubble-teleskoop om direk te onderskei.

Hierdie navorsing is deur NASA befonds, in 'n HST-navorsingstoelaag deur die Space Telescope Science Institute en deur ondersteuning van die Amerikaanse deelname aan die ISO-missie.

AGTERGROND: STOF IN GALAXIES

Sterrekundiges het geleer, gewoonlik op die harde manier, dat 'wat jy sien nie altyd is wat jy kry nie' op 'n kosmiese skaal. In onlangse jare het verskeie sterrekundiges tot die besef gekom in die konteks van iets wat baie lank reeds as 'n vaste saak beskou het - kan ons dit sien deur sterrestelsels? Uit die vroegste foto's van spiraalvormige sterrestelsels blyk dit dat daar absorberende materiaal voor sommige van die sterlig is. In werklikheid het James Keeler van Lick Observatory 'n gedetailleerde vergelyking getoon van die voorkoms van stofstrukture in spiraalstelsels jare voordat Edwin Hubble hul aard as onafhanklike sterrestelsels soos ons eie Melkweg vasgestel het. Nietemin was die meeste sterrekundiges meer as 40 jaar gelede deur Erik Holmberg se resultate oortuig dat die algehele verlies aan stof deur stof klein is, en dat stof dus nie 'n belangrike faktor is in ons metings van gewone sterrestelsels nie. Holmberg vergelyk sterrestelsels se gemiddelde helderheid van die oppervlak met hul hellings met verwysing na ons siglyn, en redeneer dat as stof nie belangrik is nie, randstelsels 'n hoër helderheid van die oppervlak sal hê met dieselfde hoeveelheid lig in 'n skraler gebied verpak.

Hierdie vertroostende gevolgtrekking het rondom 1990 ter sprake gekom, met 'n kombinasie van teoretiese studies deur sterrekundiges aan die Universiteit van Wallis in Cardiff en 'n statistiese ontleding deur Edwin Valentijn van die European Southern Observatory, wat saam getoon het dat die beskikbare data ewe ooreenstem met die opvatting dat spiraalvormige sterrestelsels was baie stowwerig en dat ons miskien minder as die helfte van hul sterlig sien, aangesien dit deur stofkorrels opgeneem word. Hierdie verslae het 'n vlaag van verdere navorsing aangeraak, aangesien daar 'n groot astronomiese belegging is om te weet hoeveel sterre daar in sterrestelsels is, in vergelyking met hoeveel sterlig ons eintlik sien. Of die spiraalvormige sterrestelsels grotendeels deursigtig of ondeursigtig is, het gevolge vir die aard van donker materie, die sterrevorming in sterrestelsels en die waarneembaarheid van kwasars, onder die verste voorwerpe in die heelal.

Sterrekundiges skat die sigbare massas van spiraalvormige sterrestelsels deur die lig van hul sterre op te tel en deur die tipiese massas van sulke sterre te ken uit metings in ons eie Melkwegstelsel. Maar spiraalvormige sterrestelsels bevat veel meer massa as wat in sigbare sterre en gas waargeneem word. Hierdie bykomende massa, wat slegs deur die swaartekrag-effekte op sterrestelsels opgespoor word, word toegeskryf aan die sogenaamde 'donker materie', waarvan die aard een van die belangrikste onopgeloste probleme in die sterrekunde is. As spiraalvormige sterrestelsels ondeursigtig is vir hul eie sigbare lig, kan die sterrekundiges die hoeveelheid normale sterrestelsel wat hulle bevat, onderskat en die "donker materie" te veel toeskryf.

Infrarooi teleskope, soos die Infrarooi astronomiese satelliet, het baie sterrestelsels gevind wat sterk uitstralers van ver-infrarooi bestraling is, wat beskou word as 'n teken van intense uitbarstings van stervorming. Aangesien die meeste sterre diep in stowwerige wolke van interstellêre gas gevorm word, is hul sterlig nie direk in die optiese sigbaar nie, maar die lig van die jong sterre verhit die omringende stof, wat weer uitstraal op infrarooi golflengtes wat die stofwolke kan ontsnap. As stof egter baie wydverspreid en dik in sterrestelsels is, is die sterre wat die omliggende stof verhit dalk nie net die jong sterre nie, dus is die waargenome infrarooi-bestraling nie die goeie maatstaf vir die vorming van sterre wat gewoonlik aanvaar word nie. Om die stofinhoud van tipiese sterrestelsels te verduidelik, is belangrik om te verstaan ​​hoe dit mettertyd ontwikkel, aangesien interstellêre gas in sterre omskep word.

Die deursigtigheid of dekking van spiraalvormige sterrestelsels kan ook beperk hoe diep ons in die ruimte kan sien. Die bekendste voorwerpe is kwasars, baie energieke galaktiese kerne, wat in groter en groter getalle gesien word hoe verder sterrekundiges ondersoek. Maar buite 'n sekere afstand, ongeveer 85 persent van die weg tot aan die rand van die waarneembare heelal, word kwasars nie meer bespeur nie. Sommige sterrekundiges stel voor dat hierdie afsnyding te wyte is aan deurkykende sterrestelsels op pad na verre kwasars, en hierdie sterrestelsels skyn hul lig. As spiraalvormige sterrestelsels egter deursigtig is, kan dit nie die kwasarafsny veroorsaak nie.

Die korrels in die interstellêre ruimte, wat in die buitenste atmosfeer van sterre naby die jongste stadiums van hul leeftyd en in sterre ontploffings geproduseer word, is klein, duisendste van 'n millimeter of minder. Soos die stof in die aarde se atmosfeer, laat dit meer rooi as blou lig deur, wat agtergrondvoorwerpe rooi maak. In voldoende hoeveelheid kan hierdie korrels byna al die sigbare lig van ver voorwerpe blokkeer.


Sterrekundiges vind massiewe stofomhulde sterrestelsel uit die vroeë heelal

Een van die belangrikste onbeantwoorde vrae in die sterrekunde is hoe ons moderne stelsel van sterrestelsels in die eerste plek ontwikkel het tot sy huidige konfigurasie. Nou het navorsers bewyse gevind van 'n massiewe sterrestelsel wat gevorm het toe die heelal baie jonger was as vandag, met 'n heel ander opset as die sterrestelsels wat ons in die moderne era sien.

Sterrekundige Christina Williams, wat die studie geskryf het, het saam met die Atacama Large Millimeter Array (ALMA) gewerk toe sy 'n uiters flou sterrestelsel waargeneem het in 'n gebied waar voorheen geen sterrestelsel bekend was nie.

& # 8220 Dit was baie geheimsinnig, want dit lyk asof die lig glad nie aan 'n bekende sterrestelsel gekoppel is nie, & # 8221 het Williams, 'n postdoktorale genoot van die National Science Foundation aan die Steward Observatory, gesê. & # 8220Toe ek sien dat hierdie sterrestelsel op enige ander golflengte onsigbaar is, het ek regtig opgewonde geraak omdat dit beteken dat dit waarskynlik baie ver weg was en deur stofwolke weggesteek is. & # 8221

So was dit ook. En die ontdekking daarvan kan astronome help om 'n langdurige probleem met bestaande teorieë oor die vorming van sterrestelsels op te los. Omdat dit natuurlik vir astronome onmoontlik is om 'n botteluniversum te skep en dan te sien hoe sterrestelsels vorm, moet ons op rekenaarmodelle staatmaak wat resultate genereer op grond van die aanvanklike voorwaardes. As die model nie 'n heelal produseer wat lyk soos die een waarin ons woon nie, weet u dat die model op een of ander manier verkeerd is.

Antenna sterrestelsels NGC 4038 & amp 4039 middel samesmelting. Blou gebiede is gebiede met stervorming. Beeld van Wikipedia

Op die oomblik suggereer teorieë dat stervorming 'n hoogtepunt van ongeveer 3,5 miljard jaar na die oerknal bereik het, teen 'n rooiverskuiwingswaarde (uitgedruk in Z) van 1.9. Rooiverskuiwings word nie lineêr geskaal nie, maar dit neem vinnig toe as ons die begin van die heelal nader. Die kosmiese mikrogolf-agtergrondstraling, wat dateer uit

389,000 jaar na die oerknal het 'n Z waarde van 1089. Die hoogste rooiverskuiwingsstelsel wat nog opgespoor is, is GN-z11, wat waargeneem word soos dit ongeveer 13.4B jaar gelede, 400 M jaar na die oerknal, bestaan ​​het en 'n rooiverskuiwingswaarde van 11,09 het. Lig van hierdie nuut bespeurde sterrestelsel (nog nie naam nie) het ongeveer 12,5B jaar gereis om ons te bereik en het 'n waargenome rooi verskuiwingswaarde van Z = 5.5 met 'n reeks van +/- 1.1.

Een van die uitdagings vir bestaande teorieë oor vroeë sterrestelselvorming is dat dit lyk asof vroeë sterrestelsels baie vinnig, baie vinnig geword het. Daar is 'n hoeveelheid bewyse wat daarop dui dat hierdie seldsame, maar massiewe sterrestelsels by die rooiverskuiwingswaardes van 3 of minder die helfte van die kosmiese stervormingstempo (CSFRD) kan uitmaak. Optiese en naby-infrarooi sterrestelsels beslaan die ander helfte van waargenome sterre. Daarbuite Z & gt 3, die situasie is egter onduidelik. Alhoewel 'n kaal handvol van hierdie groot, stofversteekte sterrestelsels op groter afstande van rooiverskuiwing waargeneem is, skryf die outeurs dat & # 8220hulle slegs die punt van die sterreformasie (SFR) verspreiding in vroeë tye opspoor & # 8230 Die totale bydrae van stof verduisterde stervorming, en daarom is die sensus van sterrevorming in die vroeë heelal onbekend. & # 8221

Die lig & # 8212 die bietjie wat ons bereik & # 8212 word waarskynlik veroorsaak deur sterre wat die gaswolke verhit wat tussen onsself en die verre sterrestelsel sit. Die sterrestelsel self word heeltemal verduister deur hierdie mis, alhoewel sterrekundiges skat dat dit die benaderde grootte van die Melkweg is. Dit is egter baie meer aktief as ons huis. Sterrevormingskoerse kan tot 100 keer hoër wees as wat die Melkweg tans ervaar.

Sterrekonstruksiesnelhede van hierdie hoogtepunt kan verklaar hoe die vroeë heelal so vinnig, so vinnig geword het, maar ons moet nog baie sterrestelsels soos hierdie vind om die geïmpliseerde tempo van stervorming in die vroeë heelal volledig te verklaar.

& # 8220 Ons verborge monsterstelsel het presies die regte bestanddele om daardie ontbrekende skakel te wees, & # 8221 Williams verduidelik, & # 8220 omdat dit waarskynlik baie meer algemeen is. & # 8221 Die bekendstelling van die James Webb-ruimteteleskoop in 2021 behoort te help skyn. meer lig op hoe algemeen hierdie groot sterrestelsels voorkom.


20.1: Die interstellêre medium

  • Bydrae deur Andrew Fraknoi, David Morrison, & amp Wolff et al.
  • Afkomstig van OpenStax

Aan die einde van hierdie afdeling is u in staat om:

  • Verduidelik hoeveel interstellêre materie daar in die Melkweg is, en wat die tipiese digtheid daarvan is
  • Beskryf hoe die interstellêre medium verdeel word in gasvormige en vaste komponente

Sterrekundiges verwys na al die materiaal tussen sterre as interstellêr materie die hele versameling interstellêre materie word die genoem interstellêre medium (ISM). Sommige interstellêre materiaal word in reuse wolke gekonsentreer, wat elkeen bekend staan ​​as 'n newel (meervoud & ldquonebulae, & rdquo Latyn vir & ldquoclouds & rdquo). Die bekendste newels is die wat ons kan sien gloei of sigbare lig weerkaats. Daar is baie foto's hiervan in hierdie hoofstuk.

Interstellêre wolke duur nie die hele leeftyd van die heelal nie. In plaas daarvan is dit soos wolke op die aarde, wat voortdurend verskuif, saamsmelt, groei of versprei. Sommige word dig en massief genoeg om onder hul eie swaartekrag inmekaar te stort en nuwe sterre te vorm. As sterre sterf, werp hulle weer hul materiaal in die interstellêre ruimte uit. Hierdie materiaal kan dan nuwe wolke vorm en die siklus weer begin.

Ongeveer 99% van die materiaal tussen die sterre is in die vorm van a gasDit beteken, dit bestaan ​​uit individuele atome of molekules. Die meeste elemente in hierdie gas is waterstof en helium (wat ons gesien het, is ook die meeste elemente in die sterre), maar die gas bevat ook ander elemente. Sommige van die gas is in die vorm van molekules en atome met kombinasies. Die oorblywende 1% van die interstellêre materiaal is vaste en mdash bevrore deeltjies wat bestaan ​​uit baie atome en molekules wat genoem word interstellêre korrels of interstellêre stof (Figuur ( PageIndex <1> )). 'N Tipiese stofkorrel bestaan ​​uit 'n kern van rotsagtige materiaal (silikate) of grafiet wat omring word deur 'n mantel yswater, metaan en ammoniak is waarskynlik die meeste ysies.

Figuur ( PageIndex <1> ) Verskeie tipes interstellêre materie. Die rooierige newels in hierdie skouspelagtige foto gloei met lig wat deur waterstofatome uitgestraal word. Die donkerste gebiede is stofwolke wat die lig van sterre daaragter blokkeer. Die boonste deel van die prentjie is gevul met die blou liggloed wat weerkaats word van warm sterre wat in die buitewyke van 'n groot, koel wolk van stof en gas ingebed is. Die koel superreus-ster Antares kan gesien word as 'n groot, rooierige kol in die onderste linker-deel van die prentjie. Die ster vergiet 'n deel van sy buitenste atmosfeer en word omring deur 'n wolk van sy eie vorm wat die rooi lig van die ster weerkaats. Die rooi newel in die middel regs omring die ster Sigma Scorpii gedeeltelik. (Aan die regterkant van Antares sien u M4, 'n baie verre tros uiters ou sterre.)

As al die interstellêre gas in die Melkweg glad versprei, sou daar slegs ongeveer een atoom gas per cm 3 in die interstellêre ruimte wees. (Daarenteen het die lug in die kamer waar u hierdie boek lees ongeveer 1019 atome per cm3.) Die stofkorrels is nog skaarser. 'N km3 ruimte sou slegs 'n paar honderd tot 'n paar duisend klein korreltjies bevat, wat gewoonlik minder as een tienduisendste millimeter in deursnee was. Hierdie getalle is egter net gemiddeldes, omdat die gas en stof op 'n onbeduidende en onreëlmatige manier versprei word, net soos waterdamp in die Aarde en die atmosfeer dikwels in wolke gekonsentreer word.

In sommige interstellêre wolke kan die digtheid van gas en stof die gemiddelde soveel as duisend keer of meer oorskry, maar selfs hierdie digtheid is byna 'n vakuum as wat ons op Aarde kan maak. Laat & rsquos 'n vertikale buis lug voorstel wat van die grond af tot bo-op die Aarde en die rsquos-atmosfeer strek met 'n dwarsdeursnee van 1 vierkante meter. Laat ons nou dieselfde buis van die top van die atmosfeer uitsteek tot by die rand van die waarneembare heelal en ongeveer 10 miljard ligjare weg. Alhoewel dit wel is, sal die tweede buis steeds minder atome bevat as die een in ons planeet en rsquos-atmosfeer.

Terwyl die digtheid van interstellêre materie baie laag is, is die volume ruimte waarin sulke materie voorkom, groot, en so ook totaal massawesenlik is. Om te sien waarom, moet ons in gedagte hou dat sterre slegs 'n klein fraksie van die volume van die Melkwegstelsel beslaan. Dit neem byvoorbeeld net ongeveer vier sekondes lig om 'n afstand gelyk aan die deursnee van die son te beweeg, maar meer as vier jare om van die son na die naaste ster te reis. Al is die ruimtes tussen die sterre yl bevolk, is daar baie ruimte daar buite!

Sterrekundiges skat dat die totale massa gas en stof in die Melkwegstelsel gelyk is aan ongeveer 15% van die massa in sterre. Dit beteken dat die massa van die interstellêre materie in ons Melkweg ongeveer 10 miljard keer die massa van die Son is. Daar is baie grondstowwe in die Melkweg om generasies nuwe sterre en planete (en miskien selfs sterrekundestudente) te maak.

Voorbeeld ( PageIndex <1> ): Skatting van interstellêre massa

U kan 'n ruwe skatting maak van hoeveel interstellêre massa ons sterrestelsel bevat en ook hoeveel nuwe sterre daaruit gemaak kan word. Al wat u moet weet, is hoe groot die Galaxy is en die gemiddelde digtheid met behulp van hierdie formule:

U moet onthou om konsekwente eenhede en mdashsuch soos meter en kilogram te gebruik. Ons sal aanneem dat ons melkweg die vorm van 'n silinder het, en die volume van 'n silinder is gelyk aan die oppervlakte van sy basis maal sy hoogte

waar (R ) die radius van die silinder is en (h ) die hoogte daarvan is.

Gestel die gemiddelde digtheid van waterstofgas in ons Melkweg is een atoom per cm3. Elke waterstofatoom het 'n massa van 1,7 & keer 10 & minus27 kg. Wat is die massa van hierdie gas as die Galaxy 'n silinder is met 'n deursnee van 100.000 ligjaar en 'n hoogte van 300 ligjaar? Hoeveel sterre in die sonmassa (2,0 en keer 10 30 kg) kan uit hierdie gasmassa geproduseer word as dit in sterre verander word?

Onthou dat 1 ligjaar = 9,5 en keer 10 12 km = 9,5 en keer 10 17 cm, dus die volume van die Melkweg is

[V = pi R ^ 2 h = pi links (50.000 keer 9.5 keer 10 ^ <17> teks regs) ^ 2 links (300 keer 9.5 keer 10 ^ <17 > text right) = 2.0 & times10 ^ <66> text ^ 3 nonumber ]

Die totale massa is dus

[2.0 keer 10 ^ <66> teks ^ 3 keer links (1 teks ^ 3 regs) keer 1.7 keer 10 ^ <& ndash27> teks = 3.5 keer 10 ^ <39> teks nonumber ]

Dit is voldoende om te maak

sterre gelyk aan massa soos die son. Dit & rsquos ongeveer 2 miljard sterre.

U kan dieselfde metode gebruik om die massa interstellêre gas rondom die son te skat. Die afstand van die son na die naaste ander ster, Proxima Centauri, is 4,2 ligjaar. Ons sal in Interstellar Matter around the Sun sien dat die gas in die onmiddellike omgewing van die Son minder dig is as die gemiddelde, ongeveer 0,1 atome per cm 3. Wat is die totale massa interstellêre waterstof in 'n sfeer wat op die son sentreer en tot by Proxima Centauri strek? Hoe kan dit vergelyk word met die massa van die son? Dit is handig om te onthou dat die volume van 'n sfeer verband hou met die radius daarvan:

Die volume van 'n bol wat van die son tot by Proxima Centauri strek, is:

[V = (4/3) pi R ^ 3 = (4/3) pi links (4.2 keer 9.5 keer 10 ^ <17> teks regs) ^ 3 = 2.7 & keer10 ^ <56> teks ^ 3 nonumber ]

Daarom is die massa waterstof in hierdie sfeer:

[M = V keer links (0.1 teks ^ 3 regs) keer 1.7 keer 10 ^ <& ndash27> teks = 4.5 keer 10 ^ <28> teks < kg> nonumber ]

Dit is slegs ( links (4.5 keer 10 ^ <28> teks regs) / links (2.0 keer 10 ^ <30> teks regs) = 2.2 /% ) die massa van die son.

As u kyk na die onderskrifte vir sommige van die skouspelagtige foto's in hierdie hoofstuk en Die geboorte van sterre en die ontdekking van planete buite die sonnestelsel, sien u die verskeidenheid name wat aan die newels gegee word. Sommige, wat in klein teleskope lyk soos iets wat herkenbaar is, word soms na die wesens of voorwerpe genoem wat hulle lyk. Voorbeelde hiervan is die krap-, tarantula- en sleutelgatnebulae. Maar die meeste het slegs getalle wat in 'n katalogus van astronomiese voorwerpe verskyn.

Miskien is die bekendste katalogus van newels (sowel as sterretrosse en sterrestelsels) saamgestel deur die Franse sterrekundige Charles Messier (1730 & ndash1817). Die passie van Messier & rsquos was om komete te ontdek, en sy toewyding aan hierdie saak besorg hom die bynaam & ldquoThe Comet Ferret & rdquo van koning Lodewyk XV. Wanneer komete vir die eerste keer na die son sien kom, lyk dit soos klein vaag ligkolle in klein teleskope; dit is maklik om te verwar met newels of met groeperings van baie sterre so ver dat hul lig saamgevoeg word. Keer op keer het Messier & rsquos se hart gespring toe hy gedink het hy het een van sy kosbare komete ontdek, net om te ontdek dat hy 'n newel of tros waargeneem het.

In frustrasie het Messier die posisie en voorkoms van meer as 100 voorwerpe wat met komete verwar kan word, gekatalogiseer. Vir hom was hierdie lys bloot 'n instrument in die baie belangriker werk van komeetjag. Hy sal baie verbaas wees as hy vandag terugkom om te ontdek dat niemand meer aan sy komete dink nie, maar sy katalogus van & ldquofuzzy dinge wat nie komete & rdquo is nie, word steeds wyd gebruik. Wanneer Figuur ( PageIndex <1> ) na M4 verwys, dui dit op die vierde inskrywing in die Messier & rsquos-lys.

'N Veel meer uitgebreide notering is saamgestel onder die titel van die Nuwe Algemene Katalogus (NGC) van Nebulae en Star Clusters in 1888 deur John Dreyer, werksaam by die sterrewag in Armagh, Ierland. Hy het sy samestelling gebaseer op die werk van William Herschel en sy seun John, plus baie ander waarnemers wat hulle gevolg het. Met die toevoeging van twee verdere aanbiedings (genaamd die Indeks katalogusse), Het Dreyer & rsquos-samestelling uiteindelik 13 000 voorwerpe ingesluit. Sterrekundiges gebruik vandag nog sy NGC-nommers as hulle na die meeste newel- en stergroepe verwys.


Jammer, sterrekundiges: die hele saak van die heelal ontbreek nog steeds

As ons opkyk in die groot afgrond van die heelal, word ons begroet deur 'n enorme reeks sterre, sterrestelsels en newels wat beide lig uitstraal en absorbeer. Op grond van alles wat ons waarneem en bespeur, kan ons alles wat ons ontdek deur die wetenskap van sterrekunde bymekaar tel, en uitvind hoeveel dit alles weeg. Dit gee ons 'n getal: hoeveel materie daar tans in die heelal is.

Maar ons het 'n ander metode wat ons kan gebruik, wat heeltemal onafhanklik is. Deur te let op hoe materie en lig beweeg of verander deur die invloed van swaartekrag, kan ons die totale hoeveelheid massa in die heelal meet. As ons die getalle bymekaar kan kry, sal ons uiteindelik verstaan ​​waar die saak in die heelal vandaan kom. Ons kan nie net nie, maar 85% daarvan word steeds nie verantwoord nie. Ondanks onlangse berigte dat ons die heelal se ontbrekende saak gevind het, was dit slegs 'n klein fraksie van wat ons nodig het. Hier is die volledige verhaal.

Die idee van ontbrekende materie strek tot in die dertigerjare. Op daardie stadium het ons begryp hoe sterre (soos ons son) goed genoeg gewerk het, dat as ons die lig wat daaruit kom, kon aflei hoe massief hulle was. Dit het nie net vir individuele sterre gewerk nie, maar ook vir groot versamelings sterre. Deur dit wat ons van sterre weet, toe te pas op die lig uit sterrestelsels in die verte, kan ons 'n skatting kry van hoeveel materie daar in een goed verstaanbare tipe voorwerp is: sterre.

Ons kan ook meet hoe hierdie sterrestelsels beweeg binne die groter struktuur waarvan almal deel uitmaak: 'n sterrestelselgroep. Omdat ons weet hoe gravitasie werk, leer ons die meet van die bewegings van hierdie sterrestelsels wat die totale massa van die groep moet wees om hulle stabiele wentelbane te gee.

Die groot probleem? Die tweede nommer was nie net groter as die eerste nie, maar dit was 160 keer so groot!

Sterrekundiges het lankal geweier om dit as 'n betekenisvolle ontdekking te aanvaar. There were many objections raised, some valid and some not so valid.

  • Maybe you’re only seeing the brightest stars, but the fainter ones have most of the mass.
  • Maybe most of the matter isn’t in stars, but consists of smaller, non-luminous clumps: planets, gas, dust, and perhaps even black holes.
  • Or maybe we don’t understand stars and solar systems as well as we think we do, and we’ve simply calculated the “mass in stars” incorrectly.

As the years and decades went by, we learned a lot about what we both were and weren’t seeing. The stars we see in other galaxies aren’t dominated by stars like our Sun, but rather by more massive, luminous, and (generally) bluer stars: the mismatch was more like 50-to-1 than 160-to-1. In addition, there really was a lot of dust and gas in these galaxies, which X-ray emitting galaxies and clusters truly helped reveal.

In addition to that, there’s also evidence for matter — normal matter, made of protons, neutrons and electrons — existing in the space between galaxies and galaxy clusters: the warm-hot intergalactic medium. This ionized plasma has been very difficult to detect, but has long been thought to exist in large quantities, making up significantly more mass than all the stars in the Universe combined.

Recently, to the highest precision ever, this sought-after matter has been detected as pulses of light known as fast radio bursts travel through them on their way to Earth. This is the “missing matter” that’s finally been discovered, as reported in numerous outlets over the past week or two. It’s an extremely important discovery for astrophysics, but it doesn’t come close to solving the problem of what or where the actual “missing mass” in the Universe actually is.

When you add up all the sources of matter that we have, know, and can identify, we find that:

  • black holes, planets, and dust make up significantly less that 1% of the total mass,
  • stars contribute about 1–2% of the total mass,
  • neutral gas, including gas found within galaxies, makes up about 5–6% of the total mass,
  • and the ionized plasma in the warm-hot intergalactic medium makes up about another 7–8% of the total mass.

Add up everything we understand, and we finally come out to about 15% of the total. That’s great, but it’s nowhere close to 100%.

And we knew it couldn’t be. All of this “missing matter” is normal, regular, proton/neutron/electron-based matter: the same building blocks that we’re made out of. But even before we discovered it, we already knew, without a doubt, how much normal matter needed to be out there.

That’s because one of the things we’ve been able to do is measure, from very pristine clouds of gas that have never (or only rarely) formed stars, which elements were present (and in what ratios) in the aftermath of the Big Bang. These “primordial abundances” teach us how protons and neutrons fused together to make the lightest elements in the Universe at extremely early times: before any stars had ever formed.

Because nuclear physics is now very well-understood, and we know about the presence of both radiation and neutrinos in the early Universe, measuring the abundances of these light elements teaches us how many baryons — i.e., how much total normal matter — there is in the Universe. We’ve measured our Universe’s hydrogen, helium-4, helium-3, deuterium, and lithium-7, all to incredible precision. And when we look at what they teach us, it’s the answer we fully expect: about 15% of all the matter in the Universe is normal matter.

4–5% of the critical density is in the form of normal matter. With another

25–28% in the form of dark matter, only about 15% of the total matter in the Universe can be normal, with 85% in the form of dark matter. (NASA / WMAP SCIENCE TEAM)

So it’s great that we found the missing baryons, or the missing normal matter, but that doesn’t teach us where the remaining 85% of the Universe’s mass is. That’s the heart of the real dark matter problem. It’s not, “where are the dark baryons, or the normal matter that we don’t directly see?”

Instead, the real question is, “what is responsible for the majority of mass in the Universe?” That’s the key to unlocking our big cosmic mystery: working to understand what dark matter is, and why it has the effects on the Universe that it does.

And we see the evidence for dark matter everywhere, which is to say, wherever we’re capable of making measurements of gravitational mass.

We see it when we look at the patterns of temperature fluctuations in the cosmic microwave background. If we didn’t have dark matter of any type, the heights, ratios, and number of “bumps” in the cosmic microwave background would be all wrong they don’t align with what we observe. (And definitively haven’t, by the way, since the first WMAP results came back in 2003. Once the third peak was discovered, scenarios without dark matter were altogether ruled out.)

When we look at systems of gravitational lenses, we can not only measure the total mass of the lens, but the distribution of various mass clumps in between ourselves and the objects we’re looking at. They help teach us that dark matter is not only real, but that it must have been moving quite slowly at relatively early times: a necessary condition to form the tiny clumps of mass that agree with our observations.

We have other means of measuring dark matter’s presence, too. The cosmic web wouldn’t have the shape or structure that it has with normal matter alone adding in 85% dark matter and just 15% normal matter leads to an agreement between theoretical predictions and our observed Universe. The absorption features of gas clouds along the line-of-sight from quasars — known as the Lyman-alpha forest — agrees with cold dark matter scenarios only.

And, perhaps most spectacularly, we’ve observed more than a dozen galaxy groups and clusters in various stages of mergers. Wherever we do, we can identify where the normal matter is from the presence of light, X-ray, and radio emissions. But we can also reconstruct where the mass is from weak gravitational lensing. The fact that the majority of the mass doesn’t line up with where the normal matter is may be the most important clue we have that dark matter, and not just normal matter alone, is required to explain our Universe.

It’s an amazing detective story to finally acquire the observational evidence needed to identify where the normal matter in the Universe has been hiding, and a very clever result to get it from an unexpected and poorly-understood phenomenon: fast radio bursts. While some of the normal matter is in the form of stars, a little less than half of it is in the form of gas, while the remaining half is an ionized plasma residing in the space between the Universe’s galaxies. Everything else — dust, planets, stars, asteroids, etc. — is completely negligible.

But the overwhelming majority of the total matter in the Universe, the remaining 85%, is still missing. We call it dark matter we know it can’t be made out of the stuff normal matter is made of about 1% (or slightly less) of it is neutrinos the remaining 99%+ is still unknown. That’s the great mystery of our time, and this new research doesn’t put a dent in it. Practically all of the Universe’s matter is still missing, and that’s a mystery still waiting to be solved.


Inhoud

Large surveys with sensitive, but low resolution radio telescopes like Arecibo or the Parkes Telescope look for 21 cm emission from atomic hydrogen in galaxies. These surveys are then matched to optical surveys to identify any objects with no optical counterpart, i.e. sources with no stars.

Another way astronomers search for dark galaxies is to look for hydrogen absorption lines in the spectra of background quasars. This technique has revealed many intergalactic clouds of hydrogen, but following up candidate dark galaxies is difficult, since these sources tend to be too far away, and are often optically drowned out by the bright light from the quasar.

Origin Edit

In 2005, astronomers found gas cloud VIRGOHI21 and attempted to determine what it was and why it caused such a gravitational pull on galaxy NGC 4254. After years of running out of other explanations, some have concluded that VIRGOHI21 is a dark galaxy, due to the massive effect it had on NGC 4254. [9]

Size Edit

The actual size of dark galaxies is unknown because they cannot be observed with normal telescopes. There have been various estimations, ranging from double the size of the Milky Way [10] to the size of a small quasar.

Structure Edit

Dark galaxies are composed of dark matter. Furthermore, dark galaxies are theoretically composed of hydrogen and dust. [9] Some scientists support the idea that dark galaxies may contain stars. [11] Yet the exact composition of dark galaxies is unknown because there is no conclusive way to spot them so far. However, astronomers estimate that the mass of the gas in these galaxies is approximately 1 billion times that of the Sun. [12]

Methodology to observe dark bodies Edit

Dark galaxies contain no visible stars, and are not visible using optical telescopes. The Arecibo Galaxy Environment Survey (AGES) is a current study using the Arecibo radio telescope to search for dark galaxies, which are predicted to contain detectable amounts of neutral hydrogen. The Arecibo radio telescope is useful where others are not because of its ability to detect the emission from this neutral hydrogen, specifically the 21 cm line. [13] Unfortunately, the decommissioning and subsequent catastrophic collapse of the Arecibo Radio Telescope on December 1st, 2020 has somewhat limited its efficacy in facilitating further data collection.

Alternative theories Edit

Scientists say that the galaxies we see today only began to create stars after dark galaxies. Based on numerous scientific assertions, dark galaxies played a big role in many of the galaxies astronomers and scientists see today. Martin Haehnel, from Kavli Institute for Cosmology at the University of Cambridge, claims that the precursor to the Milky Way galaxy was actually a much smaller bright galaxy that had merged with dark galaxies nearby to form the Milky Way we currently see. Multiple scientists agree that dark galaxies are building blocks of modern galaxies. Sebastian Cantalupo of the University of California, Santa Cruz, agrees with this theory. He goes on to say, "In our current theory of galaxy formation, we believe that big galaxies form from the merger of smaller galaxies. Dark galaxies bring to big galaxies a lot of gas, which then accelerates star formation in the bigger galaxies." Scientists have specific techniques they use to locate these dark galaxies. These techniques have the capability of teaching us more about other special events that occur in the universe for instance, the “cosmic web”. This “web” is made of invisible filaments of gas and dark matter believed to permeate the universe, as well as “feeding and building galaxies and galaxy clusters where the filaments intersect.” [12]

HE0450-2958 Edit

HE0450-2958 is a quasar at redshift z=0.285. Hubble Space Telescope images showed that the quasar is located at the edge of a large cloud of gas, but no host galaxy was detected for the quasar. The authors of the Hubble study suggested that one possible scenario was that the quasar is located in a dark galaxy. [14] However, subsequent analysis by other groups found no evidence that the host galaxy is anomalously dark, and demonstrated that a normal host galaxy is probably present, [15] [16] so the observations do not support the dark galaxy interpretation.

HVC 127-41-330 Edit

HVC 127-41-330 is a cloud rotating at high speed between Andromeda and the Triangulum Galaxy. Astronomer Josh Simon considers this cloud to be a dark galaxy because of the speed of its rotation and its predicted mass. [17] [18]

Smith's Cloud Edit

Smith's Cloud is a candidate to be a dark galaxy, due to its projected mass and survival of encounters with the Milky Way. [19]

VIRGOHI21 Edit

Initially discovered in 2000, VIRGOHI21 was announced in February 2005 as a good candidate to be a true dark galaxy. [11] [20] [21] [22] It was detected in 21-cm surveys, and was suspected to be a possible cosmic partner to the galaxy NGC 4254. This unusual-looking galaxy appears to be one partner in a cosmic collision, and appeared to show dynamics consistent with a dark galaxy (and apparently inconsistent with the predictions of the Modified Newtonian Dynamics (MOND) theory). [23] However, further observations revealed that VIRGOHI21 was an intergalactic gas cloud, stripped from NGC4254 by a high speed collision. [24] [25] [26] The high speed interaction was caused by infall into the Virgo cluster.


How do you weigh a galaxy? Especially the one you're in?

Pinning down the mass of a galaxy may seem like an esoteric undertaking, but scientists think it holds the key to unraveling the nature of the elusive, yet-to-be-seen dark matter, and the fabric of our cosmos.

A new technique for estimating the mass of galaxies promises more reliable results, especially when applied to large datasets generated by current and future surveys, according to a research team led by Ekta Patel at the University of Arizona. Published in the Astrofisiese joernaal, the study is the first to combine the observed full three-dimensional motions of several of the Milky Way's satellite galaxies with extensive computer simulations to obtain a high-accuracy estimate for the mass of our home galaxy.

Determining the mass of galaxies plays a crucial part in unraveling fundamental mysteries about the architecture of the universe. According to current cosmological models, a galaxy's visible matter, such as stars, gas and dust, accounts for a mere 15 percent of its mass. The remaining 85 percent is believed to reside in dark matter, a mysterious component that never has been observed and whose physical properties remain largely unknown. The vast majority of a galaxy's mass (mostly dark matter) is located in its halo, a vast, surrounding region containing few, if any, stars and whose shape is largely unknown.

In a widely accepted cosmological model, dark-matter filaments span the entire universe, drawing luminous ("regular") matter with them. Where they intersect, gas and dust accumulate and coalesce into galaxies. Over billions of years, small galaxies merge to form into larger ones, and as those grow in size and their gravitational pull reaches farther and farther into space, they attract a zoo of other small galaxies, which then become satellite galaxies. Their orbits are determined by their host galaxy, much like the sun's gravitational pull directs the movement of planets and bodies in the solar system.

"We now know that the universe is expanding," says Patel, a fourth-year graduate student in the UA's Department of Astronomy and Steward Observatory. "But when two galaxies come close enough, their mutual attraction is greater than the influence of the expanding universe, so they begin to orbit each other around a common center, like our Milky Way and our closest neighbor, the Andromeda Galaxy."

Although Andromeda is approaching the Milky Way at 110 kilometers per second, the two won't merge until about 4.5 billion years from now. According to Patel, tracking Andromeda's motion is "equivalent to watching a human hair grow at the distance of the moon."

Because it's impossible to "weigh" a galaxy simply by looking at it -- much less when the observer happens to be inside of it, as is the case with our Milky Way -- researchers deduce a galaxy's mass by studying the motions of celestial objects as they dance around the host galaxy, led by its gravitational pull. Such objects -- also called tracers, because they trace the mass of their host galaxy -- can be satellite galaxies or streams of stars created from the scattering of former galaxies that came too close to remain intact.

Unlike previous methods commonly used to estimate a galaxy's mass, such as measuring its tracers' velocities and positions, the approach developed by Patel and her co-authors uses their angular momentum, which yields more reliable results because it doesn't change over time. The angular momentum of a body in space depends on both its distance and speed. Since satellite galaxies tend to move around the Milky Way in elliptical orbits, their speeds increase as they get closer to our galaxy and decrease as they get farther away. Because the angular momentum is the product of both position and speed, there is no net change regardless of whether the tracer is at its closest or farthest position in its orbit.

"Think of a figure skater doing a pirouette," Patel says. "As she draws in her arms, she spins faster. In other words, her velocity changes, but her angular momentum stays the same over the whole duration of her act."

The study, which Patel presents on Thursday, June 7, at the 232nd meeting of the of the American Astronomical Society in Denver, is the first to look at the full three-dimensional motions of nine of the Milky Way's 50 known satellite galaxies at once and compare their angular momentum measurements to a simulated universe containing a total of 20,000 host galaxies that resemble our own galaxy. Together those simulated galaxies host about 90,000 satellite galaxies.

Patel's team pinned down the Milky Way's mass at 0.96 trillion solar masses. Previous estimates had placed our galaxy's mass between 700 billion and 2 trillion solar masses. The results also reinforce estimates suggesting that the Andromeda Galaxy (M31) is more massive than our Milky Way.

The authors hope to apply their method to the ever-growing data as they become available by current and future galactic surveys such as the Gaia space observatory and LSST, the Large Synoptic Survey Telescope. According to co-author Gurtina Besla, an assistant professor of astronomy at the UA, constraints on the mass of the Milky Way will improve as new observations are obtained that clock the speed of more satellite galaxies, and as next-generation simulations will provide higher resolution, allowing scientists to get better statistics for the smallest mass tracers, the so-called ultra-faint galaxies.

"Our method allows us to take advantage of measurements of the speed of multiple satellite galaxies simultaneously to get an answer for what cold dark matter theory would predict for the mass of the Milky Way's halo in a robust way," Besla says. "It is perfectly suited to take advantage of the current rapid growth in both observational datasets and numerical capabilities."


How do astronomers estimate the total mass of dust in clouds and galaxies? - Astronomy

Based on tons of scientific data and decades of research, here is an artist s impression of the Milky Way Galaxy, as seen from above the galactic North pole . Credit: NASA. All of the basic elements have been established including its spiral arm pattern and the shape of its central bulge of stars. To directly answer this question, however, is a difficult, if not impossible, task. The problem is that we cannot directly see every star in the Milky Way because most are located behind interstellar clouds from our vantage point in the Milky Way. The best we can do is to figure out the total mass of the Milky Way, subtract the portion that is contributed by interstellar gas and dust clouds ( about 1 - 5 percent or so), and then divide the remaining mass by the average mass of a single star.

From a number of studies, the mass of the Milky Way inside the orbit of our sun can be estimated to an accuracy of perhaps 20 percent as 140 billion times the mass of the Sun, if you use the Sun's speed around the core of the galaxy. Radio astronomers have detected much more material outside the orbit of the Sun, so the above number is probably an underestimate by a factor of 2 to 5 times in mass alone.

Now, to find out how many stars this represents, you have to divide by the average mass of a star. If you like the sun, then use 'one solar mass' and you then get about 140 billion sun-like stars for what's inside the sun's orbit. But astronomers have known for a long time that stars like the sun in mass are not that common. Far more plentiful are stars with half the mass of the sun, and even one tenth the mass of the sun. The problem is that we don't know exactly how much of the Milky Way is in the form of these low-mass stars. In text books, you will therefore get answers that range anywhere between a few hundred billion and as high as a trillion stars depending on what the author used as a typical mass for the most abundant type of star. This is a pretty embarrasing uncertainty, but then again, why would you need to know this number exactly?

The best estimates come from looking at the motions of nearby galaxies such as a recent study by G. R. Bell (Harvey Mudd/USNO Flagstaff), S. E. Levine (USNO Flagstaff):

Using radial velocities and the recently determined proper motions for the Magellanic Clouds and the dwarf spheroidal galaxies in Sculptor and Ursa Minor, we have modeled the satellite galaxies' orbits around the Milky Way. Assuming the orbits of the dwarf spheroidals are bound, have apogalacticon less than 300 kpc, and are of low eccentricity, then the minimum mass of our galaxy contained within a radius of 100 kpc is 590 billion solar masses, and the most likely mass is 700 billion. These mass estimates and the orbit models were used to place limits on the possible maximum tangential velocities and proper motions of the other known dwarf spheroidal galaxies and to assess the likelihood of membership of the dwarf galaxies in various streams.

Again, you have to divide this by the average mass of a star. say 0.3 solar masses, to get an estimate for the number of stars which is well into the trillions!

Another factor that confuses the prpoblem is that our Milky Way contains a lot of dark matter that also produces its own gravity and upsets the estimates for actual stellar masses. Our galaxy is embedded in a roughly spherical cloud of dark matter (shown in the artists rendering above courtesy ESO) that is strongly concentrated towards the galactic center regions. By using the motions of distant galaxies astronomers have 'weighed' the entire Milky Way and deduce that the dark matter halo is likely to include around 3 trillion solar masses of dark matter.


Hunting in Total Darkness: The Search for Dust-Obscured Galaxies at Cosmic Dawn

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Titel: Illuminating the dark side of cosmic star formation two billion years after the Big Bang
Skrywers: M. Talia et al.
Instelling van die eerste outeur: University of Bologna & INAF, Italy
Status: Aanvaar tot ApJ

The modern terminology of galaxies is extraordinarily anthropomorphic blue, star-forming galaxies are “alive”, and red galaxies that have ceased star formation are “dead”. So then how do galaxies “live”? In other words, why do some galaxies form lots of stars while others do not? Are the dead galaxies older, or do they simply mature faster? What role do external forces such as galaxy mergers play in the lives of galaxies? How can their internal structures (bars, arms, and bulges) or internal forces (supernovae and active supermassive black holes) work to enhance or inhibit star formation? These details have been the focus of the past two decades of galaxy studies, trying to answer the question: How and when did galaxies assemble their mass of stars?

The highest-level diagnostic we can construct to help us understand the big picture of star formation in galaxies is the cosmic star formation rate density (SFRD) diagram. It maps the average rate at which stars are formed in the universe at a given time, per unit volume. The physics, then, is a matter of both supply and efficiency: how much gas is available to be formed into stars (supply), and how well did galaxies turn that gas into stars (efficiency)? Constructing the SFRD diagram can then help us to understand the interplay between gas and the processes that can act to enhance or inhibit star formation.

Figure 1: The star formation rate density diagram, including many literature measurements focusing on the early universe (Z > 3). The results from this paper indicate that a missing population of galaxies might account for a large portion of the SFRD at Z > 4. [Talia et al. 2021]

75% of the history of our universe by carefully constructing unbiased, representative samples of galaxies such that specific inferences of that sample also hold for the general, wider population of galaxies. As shown in Figure 1, the SFRD rises for the first 3 billion years before peaking at a redshift of Z

2, after which it declines for the remaining 10 billion years until today.

Observing star formation rates during the first 2 billion years of the universe (Z > 3) is incredibly difficult. Not only were the first galaxies intrinsically smaller and fainter than galaxies we see today, but starting at Z

6, the universe is pervaded by a dense fog of neutral hydrogen (from which galaxies formed!) that obscures their light. Given these difficulties, these incredibly early galaxies are only now being observed in large numbers.

The authors of today’s paper point out that the existing samples of Z > 3 galaxies are not at all representative. For the most part, and almost exclusively at Z > 6, these galaxies are discovered via their bright ultraviolet (UV) emission, which has been redshifted so that it is observed in the optical and infrared. Not only must these galaxies be incredibly bright to be found at such large distances, but their intensive UV emission translates directly to an enormous star formation rate. That is, the feature that makes them easy to find also makes their star formation rates high. This is a huge bias in our samples! To overcome this bias, the authors turn to radio wavelengths. They used a large radio survey VLA-COSMOS to find 197 radio sources that have no counterpart in near-infrared wavelengths. These, they argue, are heavily dust-obscured galaxies without any UV emission — the missing link.

Figure 2: Median galaxy template (top) fitted to stacked observations in many broadband filters (bottom). The derived average physical parameters, as well as the redshift distribution, are also shown. [Talia et al. 2021]

The authors’ first test was to stack the broadband brightness measurements of each galaxy together so that they can predict what the average total spectrum would look like for these galaxies, and hence their average properties. The lack of blue light on the left-hand side of the spectrum indicates that there is no luminous UV component as seen in the UV-bright galaxies of previous samples. Moreso, the authors estimate an incredibly high dust extinction of a whopping 4.2 magnitudes (nearly a factor of 50)! These galaxies are super dusty indeed.

Using a similar approach to the stacked analysis, the authors then estimate the redshift and star formation rate for each of the 98 galaxies for which they could reliably measure an infrared brightness. Due to their unique radio selection approach, the authors are able to compile a large sample of very high redshift galaxies at Z > 4.5. They estimate the redshifts and star formation rates for the remaining 99 sources as well, but with much greater uncertainty.

Lastly, the authors compute the SFRD using their sample, taking care to correct for any dusty galaxies they may have missed. This is a challenging correction to make, so the authors do so by adopting an agnostic approach, seeing how their SFRD looks depending on how complete their sample might be.

As shown by the red bars in Figure 1, it is precisely this population of highly dust-obscured galaxies at Z > 3, invisible to optical and infrared surveys, that may indeed constitute a significant portion of the star formation rate density in the early universe compared to other less-dusty samples!

These findings highlight the surprising extent of our missing knowledge of the first galaxies, and they encourage investment in future radio surveys with ALMA and follow-up with JWST.

Original astrobite edited by William Saunders with Lukas Zalesky.

About the author, John Weaver:

I am a second year PhD student at the Cosmic Dawn Center at the University of Copenhagen, where I study the formation and evolution of galaxies across cosmic time with incredibly deep observations in the optical and infrared. I got my start at a little planetarium, and I’ve been doing lots of public outreach and citizen science ever since.


Astronomers observe how two suns collect matter in a binary system

Cosmic delivery room: This picture shows Barnard 59, part of a vast dark cloud of interstellar dust called the Pipe Nebula. The proto-binary systems [BHB2007] 11 studied with high-resolution images is embedded in dense clouds, but can be observed at longer wavelengths with the radio telescope ALMA (Atacama Large Millimeter/submillimeter Array). Krediet: ESO

Stars are born in the midst of large clouds of gas and dust. Local densifications first form "embryos," which then collect matter and grow. But how exactly does this accretion process work? And what happens when two stars form in a disk of matter? High-resolution images of a young stellar binary system for the first time reveal a complex network of accretion filaments nurturing two protostars at the center of the circumbinary disk. With these observations, an international team of astronomers led by the Max Planck Institute for Extraterrestrial Physics was able to identify a two-level accretion process, circumbinary disk to circumstellar disk to stars, constraining the conditions leading to the formation and evolution of binary star systems.

Most stars in the universe come in the form of pairs—binaries—or even multiple star systems. Now, the formation of such a binary star system has been observed for the first time with high-resolution ALMA (Atacama Large Millimetre/submillimeter Array) images. An international team of astronomers led by the Max Planck Institute for Extraterrestrial Physics targeted the system [BHB2007] 11, the youngest member of a small cluster of young stellar objects in the Barnard 59 core in the Pipe nebula molecular cloud. While previous observations showed an accretion envelope surrounding a circumbinary disk, the new observations now also reveal its inner structure.

"We see two compact sources, that we interpret as circumstellar disks around the two young stars," explains Felipe Alves from MPE, who led the study. "The size of each of these disks is similar to the asteroid belt in our Solar System, and their mutual distance is about 28 times the distance between the Earth and the Sun." Both protostars are surrounded by a circumbinary disk with a total mass of about 80 Jupiter masses, which shows a complex network of dust structures distributed in spiral shapes. The shape of the filaments suggest streamers of in-falling material, which is confirmed by the observation of molecular emission lines.

  • A zoom into the shared disk: this observation of ALMA shows that the proto-binary system [BHB2007] 11 is surrounded by dust filaments, where the southern (brighter) young star accretes more material. Credit: MPE
  • The Atacama Large Millimeter/submillimeter Array (ALMA) captured this unprecedented image of two circumstellar disks, in which baby stars are growing, feeding with material from their surrounding birth disk. The complex network of dust structures distributed in spiral shapes remind of the loops of a pretzel. These observations shed new light on the earliest phases of the lives of stars and help astronomers determine the conditions in which binary stars are born. Credit: ALMA (ESO/NAOJ/NRAO), Alves et al.

"This is a really important result," says Paola Caselli, director and MPE and head of the center of Astrochemical Studies. "We have finally imaged the complex structure of young binary stars, with their "feeding filaments" connecting them to the circumbinary disk. This provides important constraints for current models of star formation."

The astronomers interpret the filaments as inflow streamers from the extended circumbinary disk, where the circumstellar disk around the less massive of the two protostars receives more input, consistent with theoretical predictions. The estimated accretion rate is only about 0.01 Jupiter masses per year, which agrees with rates estimated for other protostellar systems. In a similar way as the circumbinary disk feeds the circumstellar disks, each circumstellar disk feeds the protostar in its center. At the disk-star level though, the accretion rate inferred from the observations is higher for the more massive object. The observation of emission from an extended radio jet for the northern object confirms this result, which is an independent indication that this protostar is indeed accreting more material.

"We expect this two-level accretion process to drive the dynamics of the binary system during its mass accretion phase," says Alves. "While the good agreement of these observations with theory is already very promising, we will need to study more young binary systems in detail to further constrain the conditions that lead to stellar multiplicity."


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