galaxy

Galaxy S22 Ul a

Galaxy

A galaxy is a g avi a io ally bou d sys em of s a s, s ella em a s, i e s ella gas, dus , a d da k ma e .12 The wo d is de ived f om he G eek galaxias (γαλαξίας), li e ally ‘milky’, a efe e ce o he Milky Way galaxy ha co ai s he Sola Sys em. Galaxies, ave agi g a es ima ed 100 millio s a s,3 a ge i size f om dwa fs wi h less ha a hu d ed millio (108) s a s, o he la ges galaxies k ow – supe gia s wi h o e hu d ed illio (1014) s a s,4 each o bi i g i s galaxy’s ce e of mass.

Galaxies a e ca ego ized acco di g o hei visual mo phology as ellip ical,5 spi al, o i egula .6 Ma y a e hough o have supe massive black holes a hei ce e s. The Milky Way’s ce al black hole, k ow as Sagi a ius A*, has a mass fou millio imes g ea e ha he Su .7 As of Ma ch 2016, GN-z11 is he oldes a d mos dis a galaxy obse ved. I has a comovi g dis a ce of 32 billio ligh -yea s f om Ea h, a d is see as i exis ed jus 400 millio yea s af e he Big Ba g.

I 2021, da a f om NASA’s New Ho izo s space p obe was used o evise he p evious es ima e o oughly 200 billio galaxies (2×۱۰۱۱),۸ which followed a 2016 es ima e ha he e we e wo illio (2×۱۰۱۲)۹ o mo e1011 galaxies i he obse vable u ive se, ove all, a d as ma y as a es ima ed 1×۱۰۲۴ s a s1213 (mo e s a s ha all he g ai s of sa d o all beaches of he pla e Ea h).14 Mos of he galaxies a e 1,000 o 100,000 pa secs i diame e (app oxima ely 3,000 o 300,000 ligh yea s) a d a e sepa a ed by dis a ces o he o de of millio s of pa secs (o megapa secs). Fo compa iso , he Milky Way has a diame e of a leas 30,000 pa secs (100,000 ly) a d is sepa a ed f om he A d omeda Galaxy (wi h diame e of abou 220,000 ly), i s ea es la ge eighbo , by 780,000 pa secs (2.5 millio ly.)

The space be wee galaxies is filled wi h a e uous gas ( he i e galac ic medium) wi h a ave age de si y of less ha o e a om pe cubic me e . Mos galaxies a e g avi a io ally o ga ized i o g oups, clus e s a d supe clus e s. The Milky Way is pa of he Local G oup, which i domi a es alo g wi h A d omeda Galaxy. The g oup is pa of he Vi go Supe clus e . A he la ges scale, hese associa io s a e ge e ally a a ged i o shee s a d filame s su ou ded by imme se voids.15 Bo h he Local G oup a d he Vi go Supe clus e a e co ai ed i a much la ge cosmic s uc u e amed La iakea.16

Co e s

E ymology

The wo d galaxy was bo owed via F e ch a d Medieval La i f om he G eek e m fo he Milky Way, galaxías (kúklos) γαλαξίας (κύκλος)۱۷۱۸ ‘milky (ci cle)’, amed af e i s appea a ce as a milky ba d of ligh i he sky. I G eek my hology, Zeus places his so bo by a mo al woma , he i fa He acles, o He a’s b eas while she is asleep so he baby will d i k he divi e milk a d hus become immo al. He a wakes up while b eas feedi g a d he ealizes she is u si g a u k ow baby: she pushes he baby away, some of he milk spills, a d i p oduces he ba d of ligh k ow as he Milky Way.1920

I he as o omical li e a u e, he capi alized wo d Galaxy is of e used o efe o ou galaxy, he Milky Way, o dis i guish i f om he o he galaxies i ou u ive se. The E glish e m Milky Way ca be aced back o a s o y by Chauce c. ۱۳۸۰:

See yo de , lo, he Galaxyë Which me clepe h he Milky Wey, Fo hi is why .

Galaxies we e i i ially discove ed elescopically a d we e k ow as spi al ebulae. Mos 18 h- o 19 h-ce u y as o ome s co side ed hem as ei he u esolved s a clus e s o a agalac ic ebulae, a d we e jus hough of as a pa of he Milky Way, bu hei ue composi io a d a u es emai ed a mys e y. Obse va io s usi g la ge elescopes of a few ea by b igh galaxies, like he A d omeda Galaxy, bega esolvi g hem i o huge co glome a io s of s a s, bu based simply o he appa e fai ess a d shee popula io of s a s, he ue dis a ces of hese objec s placed hem well beyo d he Milky Way. Fo his easo hey we e popula ly called isla d u ive ses, bu his e m quickly fell i o disuse, as he wo d u ive se implied he e i e y of exis e ce. I s ead, hey became k ow simply as galaxies.21

Nome cla u e

Te s of housa ds of galaxies have bee ca alogued, bu o ly a few have well-es ablished ames, such as he A d omeda Galaxy, he Magella ic Clouds, he Whi lpool Galaxy, a d he Somb e o Galaxy. As o ome s wo k wi h umbe s f om ce ai ca alogues, such as he Messie ca alogue, he NGC (New Ge e al Ca alogue), he IC (I dex Ca alogue), he CGCG (Ca alogue of Galaxies a d of Clus e s of Galaxies), he MCG (Mo phological Ca alogue of Galaxies), he UGC (Uppsala Ge e al Ca alogue of Galaxies), a d he PGC (Ca alogue of P i cipal Galaxies, also k ow as LEDA). All he well-k ow galaxies appea i o e o mo e of hese ca alogs bu each ime u de a diffe e umbe . Fo example, Messie 109 (o M109) is a spi al galaxy havi g he umbe 109 i he ca alog of Messie . I also has he desig a io s NGC 3992, UGC 6937, CGCG 269-023, MCG 09-20-044, a d PGC 37617 (o LEDA 37617). Millio s of fai e galaxies a e k ow by hei ide ifie s i sky su veys such as he Sloa Digi al Sky Su vey, i which M109 is ca aloged as SDSS J115735.97 532228.9.

Obse va io his o y

The ealiza io ha we live i a galaxy ha is o e amo g ma y pa allels majo discove ies abou he Milky Way a d o he ebulae.

Milky Way

G eek philosophe Democ i us (450–۳۷۰ BCE) p oposed ha he b igh ba d o he igh sky k ow as he Milky Way migh co sis of dis a s a s.22 A is o le (384–۳۲۲ BCE), howeve , believed he Milky Way was caused by he ig i io of he fie y exhala io of some s a s ha we e la ge, ume ous a d close oge he a d ha he ig i io akes place i he uppe pa of he a mosphe e, i he egio of he Wo ld ha is co i uous wi h he heave ly mo io s.23 Neopla o is philosophe Olympiodo us he You ge (c. ۴۹۵–۵۷۰ CE) was c i ical of his view, a gui g ha if he Milky Way was sublu a y (si ua ed be wee Ea h a d he Moo ) i should appea diffe e a diffe e imes a d places o Ea h, a d ha i should have pa allax, which i did o . I his view, he Milky Way was celes ial.24

Acco di g o Moha i Mohamed, A abia as o ome Alhaze (965–۱۰۳۷) made he fi s a emp a obse vi g a d measu i g he Milky Way’s pa allax,25 a d he hus de e mi ed ha because he Milky Way had o pa allax, i mus be emo e f om he Ea h, o belo gi g o he a mosphe e.26 Pe sia as o ome al-Bī ū ī (۹۷۳–۱۰۴۸) p oposed he Milky Way galaxy was a collec io of cou less f agme s of he a u e of ebulous s a s.27 A dalusia as o ome Ib Bâjjah (Avempace, d. 1138) p oposed ha i was composed of ma y s a s ha almos ouched o e a o he , a d appea ed o be a co i uous image due o he effec of ef ac io f om sublu a y ma e ial,2328 ci i g his obse va io of he co ju c io of Jupi e a d Ma s as evide ce of his occu i g whe wo objec s we e ea .23 I he 14 h ce u y, Sy ia -bo Ib Qayyim p oposed he Milky Way galaxy was a my iad of i y s a s packed oge he i he sphe e of he fixed s a s.29

Ac ual p oof of he Milky Way co sis i g of ma y s a s came i 1610 whe he I alia as o ome Galileo Galilei used a elescope o s udy i a d discove ed i was composed of a huge umbe of fai s a s.3031 I 1750, E glish as o ome Thomas W igh , i his A O igi al Theo y o New Hypo hesis of he U ive se, co ec ly specula ed ha i migh be a o a i g body of a huge umbe of s a s held oge he by g avi a io al fo ces, aki o he Sola Sys em bu o a much la ge scale, a d ha he esul i g disk of s a s could be see as a ba d o he sky f om ou pe spec ive i side i .3233 I his 1755 ea ise, Imma uel Ka elabo a ed o W igh ‘s idea abou he Milky Way’s s uc u e.34

The fi s p ojec o desc ibe he shape of he Milky Way a d he posi io of he Su was u de ake by William He schel i 1785 by cou i g he umbe of s a s i diffe e egio s of he sky. He p oduced a diag am of he shape of he galaxy wi h he Sola Sys em close o he ce e .3536 Usi g a efi ed app oach, Kap ey i 1920 a ived a he pic u e of a small (diame e abou 15 kilopa secs) ellipsoid galaxy wi h he Su close o he ce e . A diffe e me hod by Ha low Shapley based o he ca alogui g of globula clus e s led o a adically diffe e pic u e: a fla disk wi h diame e app oxima ely 70 kilopa secs a d he Su fa f om he ce e .33 Bo h a alyses failed o ake i o accou he abso p io of ligh by i e s ella dus p ese i he galac ic pla e; bu af e Robe Julius T umple qua ified his effec i 1930 by s udyi g ope clus e s, he p ese pic u e of ou hos galaxy eme ged.37

Dis i c io f om o he ebulae

A few galaxies ou side he Milky Way a e visible o a da k igh o he u aided eye, i cludi g he A d omeda Galaxy, La ge Magella ic Cloud, he Small Magella ic Cloud, a d he T ia gulum Galaxy. I he 10 h ce u y, Pe sia as o ome Al-Sufi made he ea lies eco ded ide ifica io of he A d omeda Galaxy, desc ibi g i as a small cloud.38 I 964, he p obably me io ed he La ge Magella ic Cloud i his Book of Fixed S a s ( efe i g o Al Bak of he sou he A abs,39 si ce a a decli a io of abou 70° sou h i was o visible whe e he lived); i was o well k ow o Eu opea s u il Magella ‘s voyage i he 16 h ce u y.4039 The A d omeda Galaxy was la e i depe de ly o ed by Simo Ma ius i 1612.38 I 1734, philosophe Ema uel Swede bo g i his P i cipia specula ed ha he e migh be galaxies ou side ou ow ha we e fo med i o galac ic clus e s ha we e mi uscule pa s of he u ive se ha ex e ded fa beyo d wha we could see. These views a e ema kably close o he p ese -day views of he cosmos.41 I 1745, Pie e Louis Maupe uis co jec u ed ha some ebula-like objec s we e collec io s of s a s wi h u ique p ope ies, i cludi g a glow exceedi g he ligh i s s a s p oduced o hei ow , a d epea ed Joha es Hevelius’s view ha he b igh spo s we e massive a d fla e ed due o hei o a io .42 I 1750, Thomas W igh co ec ly specula ed ha he Milky Way was a fla e ed disk of s a s, a d ha some of he ebulae visible i he igh sky migh be sepa a e Milky Ways.3343

Towa d he e d of he 18 h ce u y, Cha les Messie compiled a ca alog co ai i g he 109 b igh es celes ial objec s havi g ebulous appea a ce. Subseque ly, William He schel assembled a ca alog of 5,000 ebulae.33 I 1845, Lo d Rosse co s uc ed a ew elescope a d was able o dis i guish be wee ellip ical a d spi al ebulae. He also ma aged o make ou i dividual poi sou ces i some of hese ebulae, le di g c ede ce o Ka ‘s ea lie co jec u e.44

I 1912, Ves o Sliphe made spec og aphic s udies of he b igh es spi al ebulae o de e mi e hei composi io . Sliphe discove ed ha he spi al ebulae have high Dopple shif s, i dica i g ha hey a e movi g a a a e exceedi g he veloci y of he s a s he had measu ed. He fou d ha he majo i y of hese ebulae a e movi g away f om us.4546

I 1917, Hebe Cu is obse ved ova S A d omedae wi hi he G ea A d omeda Nebula (as he A d omeda Galaxy, Messie objec M31, was he k ow ). Sea chi g he pho og aphic eco d, he fou d 11 mo e ovae. Cu is o iced ha hese ovae we e, o ave age, 10 mag i udes fai e ha hose ha occu ed wi hi ou galaxy. As a esul , he was able o come up wi h a dis a ce es ima e of 150,000 pa secs. He became a p opo e of he so-called isla d u ive ses hypo hesis, which holds ha spi al ebulae a e ac ually i depe de galaxies.47

I 1920 a deba e ook place be wee Ha low Shapley a d Hebe Cu is ( he G ea Deba e), co ce i g he a u e of he Milky Way, spi al ebulae, a d he dime sio s of he u ive se. To suppo his claim ha he G ea A d omeda Nebula is a ex e al galaxy, Cu is o ed he appea a ce of da k la es esembli g he dus clouds i he Milky Way, as well as he sig ifica Dopple shif .48

I 1922, he Es o ia as o ome E s Öpik gave a dis a ce de e mi a io ha suppo ed he heo y ha he A d omeda Nebula is i deed a dis a ex a-galac ic objec .49 Usi g he ew 100-i ch M . Wilso elescope, Edwi Hubble was able o esolve he ou e pa s of some spi al ebulae as collec io s of i dividual s a s a d ide ified some Cepheid va iables, hus allowi g him o es ima e he dis a ce o he ebulae: hey we e fa oo dis a o be pa of he Milky Way.50 I 1936 Hubble p oduced a classifica io of galac ic mo phology ha is used o his day.51

Mode esea ch

I 1944, He d ik va de Huls p edic ed ha mic owave adia io wi h wavele g h of 21 cm would be de ec able f om i e s ella a omic hyd oge gas;52 a d i 1951 i was obse ved. This adia io is o affec ed by dus abso p io , a d so i s Dopple shif ca be used o map he mo io of he gas i ou galaxy. These obse va io s led o he hypo hesis of a o a i g ba s uc u e i he ce e of ou galaxy.53 Wi h imp oved adio elescopes, hyd oge gas could also be aced i o he galaxies. I he 1970s, Ve a Rubi u cove ed a disc epa cy be wee obse ved galac ic o a io speed a d ha p edic ed by he visible mass of s a s a d gas. Today, he galaxy o a io p oblem is hough o be explai ed by he p ese ce of la ge qua i ies of u see da k ma e .5455

Begi i g i he 1990s, he Hubble Space Telescope yielded imp oved obse va io s. Amo g o he hi gs, i s da a helped es ablish ha he missi g da k ma e i ou galaxy could o co sis solely of i he e ly fai a d small s a s.57 The Hubble Deep Field, a ex emely lo g exposu e of a ela ively emp y pa of he sky, p ovided evide ce ha he e a e abou 125 billio (1.25×۱۰۱۱) galaxies i he obse vable u ive se.58 Imp oved ech ology i de ec i g he spec a i visible o huma s ( adio elescopes, i f a ed came as, a d x- ay elescopes) allows de ec io of o he galaxies ha a e o de ec ed by Hubble. Pa icula ly, su veys i he Zo e of Avoida ce ( he egio of sky blocked a visible-ligh wavele g hs by he Milky Way) have evealed a umbe of ew galaxies.59

A 2016 s udy published i The As ophysical Jou al, led by Ch is ophe Co selice of he U ive si y of No i gham, used 20 yea s of Hubble images o es ima e ha he obse vable u ive se co ai ed a leas wo illio (2×۱۰۱۲) galaxies.1011 Howeve , la e obse va io s wi h he New Ho izo s space p obe f om ou side he zodiacal ligh educed his o oughly 200 billio (2×۱۰۱۱).۶۰۶۱

Types a d mo phology

Galaxies come i h ee mai ypes: ellip icals, spi als, a d i egula s. A sligh ly mo e ex e sive desc ip io of galaxy ypes based o hei appea a ce is give by he Hubble seque ce. Si ce he Hubble seque ce is e i ely based upo visual mo phological ype (shape), i may miss ce ai impo a cha ac e is ics of galaxies such as s a fo ma io a e i s a bu s galaxies a d ac ivi y i he co es of ac ive galaxies.6

Ellip icals

The Hubble classifica io sys em a es ellip ical galaxies o he basis of hei ellip ici y, a gi g f om E0, bei g ea ly sphe ical, up o E7, which is highly elo ga ed. These galaxies have a ellipsoidal p ofile, givi g hem a ellip ical appea a ce ega dless of he viewi g a gle. Thei appea a ce shows li le s uc u e a d hey ypically have ela ively li le i e s ella ma e . Co seque ly, hese galaxies also have a low po io of ope clus e s a d a educed a e of ew s a fo ma io . I s ead, hey a e domi a ed by ge e ally olde , mo e evolved s a s ha a e o bi i g he commo ce e of g avi y i a dom di ec io s. The s a s co ai low abu da ces of heavy eleme s because s a fo ma io ceases af e he i i ial bu s . I his se se hey have some simila i y o he much smalle globula clus e s.62

The la ges galaxies a e supe gia ellip icals, o ype-cD galaxies. Ma y ellip ical galaxies a e believed o fo m due o he i e ac io of galaxies, esul i g i a collisio a d me ge . They ca g ow o e o mous sizes (compa ed o spi al galaxies, fo example), a d gia ellip ical galaxies a e of e fou d ea he co e of la ge galaxy clus e s.63

Shell galaxy

A shell galaxy is a ype of ellip ical galaxy whe e he s a s i i s halo a e a a ged i co ce ic shells. Abou o e- e h of ellip ical galaxies have a shell-like s uc u e, which has eve bee obse ved i spi al galaxies. These s uc u es a e hough o develop whe a la ge galaxy abso bs a smalle compa io galaxy— ha as he wo galaxy ce e s app oach, hey s a o oscilla e a ou d a ce e poi , a d he oscilla io c ea es g avi a io al ipples fo mi g he shells of s a s, simila o ipples sp eadi g o wa e . Fo example, galaxy NGC 3923 has ove 20 shells.64

Spi als

Spi al galaxies esemble spi ali g pi wheels. Though he s a s a d o he visible ma e ial co ai ed i such a galaxy lie mos ly o a pla e, he majo i y of mass i spi al galaxies exis s i a oughly sphe ical halo of da k ma e which ex e ds beyo d he visible compo e , as demo s a ed by he u ive sal o a io cu ve co cep .65

Spi al galaxies co sis of a o a i g disk of s a s a d i e s ella medium, alo g wi h a ce al bulge of ge e ally olde s a s. Ex e di g ou wa d f om he bulge a e ela ively b igh a ms. I he Hubble classifica io scheme, spi al galaxies a e lis ed as ype S, followed by a le e (a, b, o c) which i dica es he deg ee of igh ess of he spi al a ms a d he size of he ce al bulge. A Sa galaxy has igh ly wou d, poo ly defi ed a ms a d possesses a ela ively la ge co e egio . A he o he ex eme, a Sc galaxy has ope , well-defi ed a ms a d a small co e egio .66 A galaxy wi h poo ly defi ed a ms is some imes efe ed o as a floccule spi al galaxy; i co as o he g a d desig spi al galaxy ha has p omi e a d well-defi ed spi al a ms.67 The speed i which a galaxy o a es is hough o co ela e wi h he fla ess of he disc as some spi al galaxies have hick bulges, while o he s a e hi a d de se.68

I spi al galaxies, he spi al a ms do have he shape of app oxima e loga i hmic spi als, a pa e ha ca be heo e ically show o esul f om a dis u ba ce i a u ifo mly o a i g mass of s a s. Like he s a s, he spi al a ms o a e a ou d he ce e , bu hey do so wi h co s a a gula veloci y. The spi al a ms a e hough o be a eas of high-de si y ma e , o de si y waves.69 As s a s move h ough a a m, he space veloci y of each s ella sys em is modified by he g avi a io al fo ce of he highe de si y. (The veloci y e u s o o mal af e he s a s depa o he o he side of he a m.) This effec is aki o a wave of slowdow s movi g alo g a highway full of movi g ca s. The a ms a e visible because he high de si y facili a es s a fo ma io , a d he efo e hey ha bo ma y b igh a d you g s a s.70

Ba ed spi al galaxy

A majo i y of spi al galaxies, i cludi g ou ow Milky Way galaxy, have a li ea , ba -shaped ba d of s a s ha ex e ds ou wa d o ei he side of he co e, he me ges i o he spi al a m s uc u e.71 I he Hubble classifica io scheme, hese a e desig a ed by a SB, followed by a lowe -case le e (a, b o c) which i dica es he fo m of he spi al a ms (i he same ma e as he ca ego iza io of o mal spi al galaxies). Ba s a e hough o be empo a y s uc u es ha ca occu as a esul of a de si y wave adia i g ou wa d f om he co e, o else due o a idal i e ac io wi h a o he galaxy.72 Ma y ba ed spi al galaxies a e ac ive, possibly as a esul of gas bei g cha eled i o he co e alo g he a ms.73

Ou ow galaxy, he Milky Way, is a la ge disk-shaped ba ed-spi al galaxy74 abou 30 kilopa secs i diame e a d a kilopa sec hick. I co ai s abou wo hu d ed billio (2×۱۰۱۱)۷۵ s a s a d has a o al mass of abou six hu d ed billio (6×۱۰۱۱) imes he mass of he Su .76

Supe -lumi ous spi al

Rece ly, esea che s desc ibed galaxies called supe -lumi ous spi als. They a e ve y la ge wi h a upwa d diame e of 437,000 ligh -yea s (compa ed o he Milky Way’s 100,000 ligh -yea diame e ). Wi h a mass of 340 billio sola masses, hey ge e a e a sig ifica amou of ul aviole a d mid-i f a ed ligh . They a e hough o have a i c eased s a fo ma io a e a ou d 30 imes fas e ha he Milky Way.7778

O he mo phologies

Dwa fs

Despi e he p omi e ce of la ge ellip ical a d spi al galaxies, mos galaxies a e dwa f galaxies. They a e ela ively small whe compa ed wi h o he galac ic fo ma io s, bei g abou o e hu d ed h he size of he Milky Way, wi h o ly a few billio s a s. Ul a-compac dwa f galaxies have ece ly bee discove ed ha a e o ly 100 pa secs ac oss.83

Ma y dwa f galaxies may o bi a si gle la ge galaxy; he Milky Way has a leas a doze such sa elli es, wi h a es ima ed 300–۵۰۰ ye o be discove ed.84 Dwa f galaxies may also be classified as ellip ical, spi al, o i egula . Si ce small dwa f ellip icals bea li le esembla ce o la ge ellip icals, hey a e of e called dwa f sphe oidal galaxies i s ead.

A s udy of 27 Milky Way eighbo s fou d ha i all dwa f galaxies, he ce al mass is app oxima ely 10 millio sola masses, ega dless of whe he i has housa ds o millio s of s a s. This sugges s ha galaxies a e la gely fo med by da k ma e , a d ha he mi imum size may i dica e a fo m of wa m da k ma e i capable of g avi a io al coalesce ce o a smalle scale.85

O he ypes of galaxies

I e ac i g

I e ac io s be wee galaxies a e ela ively f eque , a d hey ca play a impo a ole i galac ic evolu io . Nea misses be wee galaxies esul i wa pi g dis o io s due o idal i e ac io s, a d may cause some excha ge of gas a d dus .8687 Collisio s occu whe wo galaxies pass di ec ly h ough each o he a d have sufficie ela ive mome um o o me ge. The s a s of i e ac i g galaxies usually do o collide, bu he gas a d dus wi hi he wo fo ms i e ac s, some imes igge i g s a fo ma io . A collisio ca seve ely dis o he galaxies’ shapes, fo mi g ba s, i gs o ail-like s uc u es.8687

A he ex eme of i e ac io s a e galac ic me ge s, whe e he galaxies’ ela ive mome ums a e i sufficie o allow hem o pass h ough each o he . I s ead, hey g adually me ge o fo m a si gle, la ge galaxy. Me ge s ca esul i sig ifica cha ges o he galaxies’ o igi al mo phology. If o e of he galaxies is much mo e massive ha he o he , he esul is k ow as ca ibalism, whe e he mo e massive la ge galaxy emai s ela ively u dis u bed, a d he smalle o e is o apa . The Milky Way galaxy is cu e ly i he p ocess of ca ibalizi g he Sagi a ius Dwa f Ellip ical Galaxy a d he Ca is Majo Dwa f Galaxy.8687

S a bu s

S a s a e c ea ed wi hi galaxies f om a ese ve of cold gas ha fo ms gia molecula clouds. Some galaxies have bee obse ved o fo m s a s a a excep io al a e, which is k ow as a s a bu s . If hey co i ue o do so, hey would co sume hei ese ve of gas i a ime spa less ha he galaxy’s lifespa . He ce s a bu s ac ivi y usually las s o ly abou e millio yea s, a ela ively b ief pe iod i a galaxy’s his o y. S a bu s galaxies we e mo e commo du i g he u ive se’s ea ly his o y,89 bu s ill co ibu e a es ima ed 15% o o al s a p oduc io .90

S a bu s galaxies a e cha ac e ized by dus y co ce a io s of gas a d he appea a ce of ewly fo med s a s, i cludi g massive s a s ha io ize he su ou di g clouds o c ea e H II egio s.91 These s a s p oduce supe ova explosio s, c ea i g expa di g em a s ha i e ac powe fully wi h he su ou di g gas. These ou bu s s igge a chai eac io of s a -buildi g ha sp eads h oughou he gaseous egio . O ly whe he available gas is ea ly co sumed o dispe sed does he ac ivi y e d.89

S a bu s s a e of e associa ed wi h me gi g o i e ac i g galaxies. The p o o ype example of such a s a bu s -fo mi g i e ac io is M82, which expe ie ced a close e cou e wi h he la ge M81. I egula galaxies of e exhibi spaced k o s of s a bu s ac ivi y.92

Ac ive galaxy

Some obse vable galaxies a e classified as ac ive if hey co ai a ac ive galac ic ucleus (AGN). A sig ifica po io of he galaxy’s o al e e gy ou pu is emi ed by he ac ive ucleus i s ead of i s s a s, dus a d i e s ella medium. The e a e mul iple classifica io a d ami g schemes fo AGNs, bu hose i he lowe a ges of lumi osi y a e called Seyfe galaxies, while hose wi h lumi osi ies much g ea e ha ha of he hos galaxy a e k ow as quasi-s ella objec s o quasa s. AGNs emi adia io h oughou he elec omag e ic spec um f om adio wavele g hs o X- ays, hough some of i may be abso bed by dus o gas associa ed wi h he AGN i self o wi h he hos galaxy.

The s a da d model fo a ac ive galac ic ucleus is based o a acc e io disc ha fo ms a ou d a supe massive black hole (SMBH) a he galaxy’s co e egio . The adia io f om a ac ive galac ic ucleus esul s f om he g avi a io al e e gy of ma e as i falls owa d he black hole f om he disc.93 The AGN’s lumi osi y depe ds o he SMBH’s mass a d he a e a which ma e falls o o i . I abou 10% of hese galaxies, a diame ically opposed pai of e e ge ic je s ejec s pa icles f om he galaxy co e a veloci ies close o he speed of ligh . The mecha ism fo p oduci g hese je s is o well u de s ood.94

Blaza s

Blaza s a e believed o be ac ive galaxies wi h a ela ivis ic je poi ed i he di ec io of Ea h. A adio galaxy emi s adio f eque cies f om ela ivis ic je s. A u ified model of hese ypes of ac ive galaxies explai s hei diffe e ces based o he obse ve ‘s posi io .94

LINERS

Possibly ela ed o ac ive galac ic uclei (as well as s a bu s egio s) a e low-io iza io uclea emissio -li e egio s (LINERs). The emissio f om LINER- ype galaxies is domi a ed by weakly io ized eleme s. The exci a io sou ces fo he weakly io ized li es i clude pos -AGB s a s, AGN, a d shocks.95 App oxima ely o e- hi d of ea by galaxies a e classified as co ai i g LINER uclei.939596

Seyfe galaxy

Seyfe galaxies a e o e of he wo la ges g oups of ac ive galaxies, alo g wi h quasa s. They have quasa -like uclei (ve y lumi ous, dis a a d b igh sou ces of elec omag e ic adia io ) wi h ve y high su face b igh esses; bu u like quasa s, hei hos galaxies a e clea ly de ec able.97 Seyfe galaxies accou fo abou 10% of all galaxies. See i visible ligh , mos look like o mal spi al galaxies; bu whe s udied u de o he wavele g hs, hei co es’ lumi osi y is equivale o he lumi osi y of whole galaxies he size of he Milky Way.

Quasa

Quasa s (ˈkweɪzɑ ) o quasi-s ella adio sou ces, a e he mos e e ge ic a d dis a membe s of ac ive galac ic uclei. Ex emely lumi ous, hey we e fi s ide ified as high edshif sou ces of elec omag e ic e e gy, i cludi g adio waves a d visible ligh , ha appea ed mo e simila o s a s ha o ex e ded sou ces simila o galaxies. Thei lumi osi y ca be 100 imes ha of he Milky Way.

Lumi ous i f a ed galaxy

Lumi ous i f a ed galaxies (LIRGs) a e galaxies wi h lumi osi ies— he measu eme of elec omag e ic powe ou pu —above 1011 L☉ (sola lumi osi ies). I mos cases, mos of hei e e gy comes f om la ge umbe s of you g s a s which hea su ou di g dus , which e adia es he e e gy i he i f a ed. Lumi osi y high e ough o be a LIRG equi es a s a fo ma io a e of a leas 18 M☉ y −۱٫ Ul a-lumi ous i f a ed galaxies (ULIRGs) a e a leas e imes mo e lumi ous s ill a d fo m s a s a a es 180 M☉ y −۱٫ Ma y LIRGs also emi adia io f om a AGN. I f a ed galaxies emi mo e e e gy i he i f a ed ha all o he wavele g hs combi ed, wi h peak emissio ypically a wavele g hs of 60 o 100 mic o s. LIRGs a e u commo i he local u ive se bu we e much mo e commo whe he u ive se was you ge .

P ope ies

Mag e ic fields

Galaxies have mag e ic fields of hei ow .98 They a e s o g e ough o be dy amically impo a , as hey:

The ypical ave age equipa i io s e g h fo spi al galaxies is abou 10 μG (mic ogauss) o 1  T ( a o esla). By compa iso , he Ea h’s mag e ic field has a ave age s e g h of abou 0.3 G (Gauss o 30 μT (mic o esla). Radio-fai galaxies like M 31 a d M33, ou Milky Way’s eighbo s, have weake fields (abou 5 μG), while gas- ich galaxies wi h high s a -fo ma io a es, like M 51, M 83 a d NGC 6946, have 15 μG o ave age. I p omi e spi al a ms, he field s e g h ca be up o 25 μG, i egio s whe e cold gas a d dus a e also co ce a ed. The s o ges o al equipa i io fields (50–۱۰۰ μG) we e fou d i s a bu s galaxies—fo example, i M 82 a d he A e ae; a d i uclea s a bu s egio s, such as he ce e s of NGC 1097 a d o he ba ed galaxies.98

Fo ma io a d evolu io

Galac ic fo ma io a d evolu io is a ac ive a ea of esea ch i as ophysics.

Fo ma io

Cu e models of he fo ma io of galaxies i he ea ly u ive se a e based o he ΛCDM model. Abou 300,000 yea s af e he big ba g, a oms of hyd oge a d helium bega o fo m, i a eve called ecombi a io . Nea ly all he hyd oge was eu al ( o -io ized) a d eadily abso bed ligh , a d o s a s had ye fo med. As a esul , his pe iod has bee called he da k ages. I was f om de si y fluc ua io s (o a iso opic i egula i ies) i his p imo dial ma e ha la ge s uc u es bega o appea . As a esul , masses of ba yo ic ma e s a ed o co de se wi hi cold da k ma e halos.100101 These p imo dial s uc u es eve ually became he galaxies we see oday.

Ea ly galaxy fo ma io

Evide ce fo he appea a ce of galaxies ve y ea ly i he U ive se’s his o y was fou d i 2006, whe i was discove ed ha he galaxy IOK-1 has a u usually high edshif of 6.96, co espo di g o jus 750 millio yea s af e he Big Ba g a d maki g i he mos dis a a d ea lies – o-fo m galaxy see a ha ime.102 While some scie is s have claimed o he objec s (such as Abell 1835 IR1916) have highe edshif s (a d he efo e a e see i a ea lie s age of he u ive se’s evolu io ), IOK-1’s age a d composi io have bee mo e eliably es ablished. I Decembe 2012, as o ome s epo ed ha UDFj-39546284 is he mos dis a objec k ow a d has a edshif value of 11.9. The objec , es ima ed o have exis ed a ou d 380 millio yea s103 af e he Big Ba g (which was abou 13.8 billio yea s ago),104 is abou 13.42 billio ligh avel dis a ce yea s away. The exis e ce of galaxies so soo af e he Big Ba g sugges s ha p o ogalaxies mus have g ow i he so-called da k ages.100 As of May 5, 2015, he galaxy EGS-zs8-1 is he mos dis a a d ea lies galaxy measu ed, fo mi g 670 millio yea s af e he Big Ba g. The ligh f om EGS-zs8-1 has ake 13 billio yea s o each Ea h, a d is ow 30 billio ligh -yea s away, because of he expa sio of he u ive se du i g 13 billio yea s.105106107108109

The de ailed p ocess by which he ea lies galaxies fo med is a ope ques io i as ophysics. Theo ies ca be divided i o wo ca ego ies: op-dow a d bo om-up. I op-dow co ela io s (such as he Egge –Ly de -Bell–Sa dage ELS model), p o ogalaxies fo m i a la ge-scale simul a eous collapse las i g abou o e hu d ed millio yea s.111 I bo om-up heo ies (such as he Sea le-Zi SZ model), small s uc u es such as globula clus e s fo m fi s , a d he a umbe of such bodies acc e e o fo m a la ge galaxy.112 O ce p o ogalaxies bega o fo m a d co ac , he fi s halo s a s (called Popula io III s a s) appea ed wi hi hem. These we e composed almos e i ely of hyd oge a d helium a d may have bee mo e massive ha 100 imes he Su ‘s mass. If so, hese huge s a s would have quickly co sumed hei supply of fuel a d became supe ovae, eleasi g heavy eleme s i o he i e s ella medium.113 This fi s ge e a io of s a s e-io ized he su ou di g eu al hyd oge , c ea i g expa di g bubbles of space h ough which ligh could eadily avel.114

I Ju e 2015, as o ome s epo ed evide ce fo Popula io III s a s i he Cosmos Redshif 7 galaxy a z = 6.60. Such s a s a e likely o have exis ed i he ve y ea ly u ive se (i.e., a high edshif ), a d may have s a ed he p oduc io of chemical eleme s heavie ha hyd oge ha a e eeded fo he la e fo ma io of pla e s a d life as we k ow i .115116

Evolu io

Wi hi a billio yea s of a galaxy’s fo ma io , key s uc u es begi o appea . Globula clus e s, he ce al supe massive black hole, a d a galac ic bulge of me al-poo Popula io II s a s fo m. The c ea io of a supe massive black hole appea s o play a key ole i ac ively egula i g he g ow h of galaxies by limi i g he o al amou of addi io al ma e added.117 Du i g his ea ly epoch, galaxies u de go a majo bu s of s a fo ma io .118

Du i g he followi g wo billio yea s, he accumula ed ma e se les i o a galac ic disc.119 A galaxy will co i ue o abso b i falli g ma e ial f om high-veloci y clouds a d dwa f galaxies h oughou i s life.120 This ma e is mos ly hyd oge a d helium. The cycle of s ella bi h a d dea h slowly i c eases he abu da ce of heavy eleme s, eve ually allowi g he fo ma io of pla e s.121

The evolu io of galaxies ca be sig ifica ly affec ed by i e ac io s a d collisio s. Me ge s of galaxies we e commo du i g he ea ly epoch, a d he majo i y of galaxies we e peculia i mo phology.123 Give he dis a ces be wee he s a s, he g ea majo i y of s ella sys ems i collidi g galaxies will be u affec ed. Howeve , g avi a io al s ippi g of he i e s ella gas a d dus ha makes up he spi al a ms p oduces a lo g ai of s a s k ow as idal ails. Examples of hese fo ma io s ca be see i NGC 4676124 o he A e ae Galaxies.125

The Milky Way galaxy a d he ea by A d omeda Galaxy a e movi g owa d each o he a abou 130 kms, a d—depe di g upo he la e al moveme s— he wo migh collide i abou five o six billio yea s. Al hough he Milky Way has eve collided wi h a galaxy as la ge as A d omeda befo e, evide ce of pas collisio s of he Milky Way wi h smalle dwa f galaxies is i c easi g.126

Such la ge-scale i e ac io s a e a e. As ime passes, me ge s of wo sys ems of equal size become less commo . Mos b igh galaxies have emai ed fu dame ally u cha ged fo he las few billio yea s, a d he e a e of s a fo ma io p obably also peaked abou e billio yea s ago.127

Fu u e e ds

Spi al galaxies, like he Milky Way, p oduce ew ge e a io s of s a s as lo g as hey have de se molecula clouds of i e s ella hyd oge i hei spi al a ms.128 Ellip ical galaxies a e la gely devoid of his gas, a d so fo m few ew s a s.129 The supply of s a -fo mi g ma e ial is fi i e; o ce s a s have co ve ed he available supply of hyd oge i o heavie eleme s, ew s a fo ma io will come o a e d.130131

The cu e e a of s a fo ma io is expec ed o co i ue fo up o o e hu d ed billio yea s, a d he he s ella age will wi d dow af e abou e illio o o e hu d ed illio yea s (1013–۱۰۱۴ yea s), as he smalles , lo ges -lived s a s i ou u ive se, i y ed dwa fs, begi o fade. A he e d of he s ella age, galaxies will be composed of compac objec s: b ow dwa fs, whi e dwa fs ha a e cooli g o cold (black dwa fs), eu o s a s, a d black holes. Eve ually, as a esul of g avi a io al elaxa io , all s a s will ei he fall i o ce al supe massive black holes o be flu g i o i e galac ic space as a esul of collisio s.130132

La ge -scale s uc u es

Deep-sky su veys show ha galaxies a e of e fou d i g oups a d clus e s. Soli a y galaxies ha have o sig ifica ly i e ac ed wi h o he galaxies of compa able mass i he pas billio yea s a e ela ively sca ce. O ly abou 5% of he galaxies su veyed a e uly isola ed; howeve , hey may have i e ac ed a d eve me ged wi h o he galaxies i he pas , a d may s ill be o bi ed by smalle sa elli e galaxies. Isola ed galaxies o e 2 ca p oduce s a s a a highe a e ha o mal, as hei gas is o bei g s ipped by o he ea by galaxies.133

O he la ges scale, he u ive se is co i ually expa di g, esul i g i a ave age i c ease i he sepa a io be wee i dividual galaxies (see Hubble’s law). Associa io s of galaxies ca ove come his expa sio o a local scale h ough hei mu ual g avi a io al a ac io . These associa io s fo med ea ly, as clumps of da k ma e pulled hei espec ive galaxies oge he . Nea by g oups la e me ged o fo m la ge -scale clus e s. This o goi g me gi g p ocess (as well as a i flux of i falli g gas) hea s he i e galac ic gas i a clus e o ve y high empe a u es of 30–۱۰۰ megakelvi s.134 Abou 70–۸۰% of a clus e ‘s mass is i he fo m of da k ma e , wi h 10–۳۰% co sis i g of his hea ed gas a d he emai i g few pe ce i he fo m of galaxies.135

Mos galaxies a e g avi a io ally bou d o a umbe of o he galaxies. These fo m a f ac al-like hie a chical dis ibu io of clus e ed s uc u es, wi h he smalles such associa io s bei g e med g oups. A g oup of galaxies is he mos commo ype of galac ic clus e ; hese fo ma io s co ai he majo i y of galaxies (as well as mos of he ba yo ic mass) i he u ive se.136137 To emai g avi a io ally bou d o such a g oup, each membe galaxy mus have a sufficie ly low veloci y o p eve i f om escapi g (see Vi ial heo em). If he e is i sufficie ki e ic e e gy, howeve , he g oup may evolve i o a smalle umbe of galaxies h ough me ge s.138

Clus e s of galaxies co sis of hu d eds o housa ds of galaxies bou d oge he by g avi y.139 Clus e s of galaxies a e of e domi a ed by a si gle gia ellip ical galaxy, k ow as he b igh es clus e galaxy, which, ove ime, idally des oys i s sa elli e galaxies a d adds hei mass o i s ow .140

Supe clus e s co ai e s of housa ds of galaxies, which a e fou d i clus e s, g oups a d some imes i dividually. A he supe clus e scale, galaxies a e a a ged i o shee s a d filame s su ou di g vas emp y voids.142 Above his scale, he u ive se appea s o be he same i all di ec io s (iso opic a d homoge eous).,143 hough his o io has bee challe ged i ece yea s by ume ous fi di gs of la ge-scale s uc u es ha appea o be exceedi g his scale. The He cules-Co o a Bo ealis G ea Wall, cu e ly he la ges s uc u e i he u ive se fou d so fa , is 10 billio ligh -yea s ( h ee gigapa secs) i le g h.144145146

The Milky Way galaxy is a membe of a associa io amed he Local G oup, a ela ively small g oup of galaxies ha has a diame e of app oxima ely o e megapa sec. The Milky Way a d he A d omeda Galaxy a e he wo b igh es galaxies wi hi he g oup; ma y of he o he membe galaxies a e dwa f compa io s of hese wo.147 The Local G oup i self is a pa of a cloud-like s uc u e wi hi he Vi go Supe clus e , a la ge, ex e ded s uc u e of g oups a d clus e s of galaxies ce e ed o he Vi go Clus e .148 A d he Vi go Supe clus e i self is a pa of he Pisces-Ce us Supe clus e Complex, a gia galaxy filame .

Mul i-wavele g h obse va io

The peak adia io of mos s a s lies i he visible spec um, so he obse va io of he s a s ha fo m galaxies has bee a majo compo e of op ical as o omy. I is also a favo able po io of he spec um fo obse vi g io ized H II egio s, a d fo exami i g he dis ibu io of dus y a ms.

The dus p ese i he i e s ella medium is opaque o visual ligh . I is mo e a spa e o fa -i f a ed, which ca be used o obse ve he i e io egio s of gia molecula clouds a d galac ic co es i g ea de ail.149 I f a ed is also used o obse ve dis a , ed-shif ed galaxies ha we e fo med much ea lie . Wa e vapo a d ca bo dioxide abso b a umbe of useful po io s of he i f a ed spec um, so high-al i ude o space-based elescopes a e used fo i f a ed as o omy.

The fi s o -visual s udy of galaxies, pa icula ly ac ive galaxies, was made usi g adio f eque cies. The Ea h’s a mosphe e is ea ly a spa e o adio be wee 5 MHz a d 30 GHz. (The io osphe e blocks sig als below his a ge.)150 La ge adio i e fe ome e s have bee used o map he ac ive je s emi ed f om ac ive uclei. Radio elescopes ca also be used o obse ve eu al hyd oge (via 21 cm adia io ), i cludi g, po e ially, he o -io ized ma e i he ea ly u ive se ha la e collapsed o fo m galaxies.151

Ul aviole a d X- ay elescopes ca obse ve highly e e ge ic galac ic phe ome a. Ul aviole fla es a e some imes obse ved whe a s a i a dis a galaxy is o apa f om he idal fo ces of a ea by black hole.152 The dis ibu io of ho gas i galac ic clus e s ca be mapped by X- ays. The exis e ce of supe massive black holes a he co es of galaxies was co fi med h ough X- ay as o omy.153

Galle y

Squabbli g Galac ic Sibli gs154

LEFT: ARP-MADORE2115-273 is a a e example of a i e ac i g galaxy pai i he sou he hemisphe e. RIGHT: ARP-MADORE0002-503 is a la ge spi al galaxy wi h u usual, ex e ded spi al a ms, a a dis a ce of 490 millio ligh -yea s.155