Lecture 7. Galaxy Formation

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Patrick Norton
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1 Lecure 7. Galaxy Formaion Afer decoupling, overdense regions collapse IF k T L > L J ~ G m ρ Collapse ime 1/ 2 ~ 50 pc G ~ ( G ρ) 1/ 2 ~ 10 7 yr for all sizes. More small ripples han large waves. --> Universe dominaed by globular clusers (?!) 47 Tuc ω Cen M > M J ~ ρ L J 3 ~ 10 6 M sun

2 Dimensional Analysis --> scaling laws leaving ou dimensionless facors (e.g.~10). We ignored: angular momenum -- slows and can hal he collapse --> Spiral Galaxies. cosmological expansion -- delays collapse unil expansion ime > collapse ime. > ( Gρ) 1/ 2 Caveas --> Need Dark Maer halos (collapsing before decoupling) ino which baryon gas falls.

3 The Dark Ages ( 1100 < z < 20 ) Uniform neural IGM (Iner-Galacic Medium) Proo-globular clusers. Rare larger objecs: proo-galaxies proo-clusers T CMB =2.7(1+z) K No sars! GCs As regions collapse and merge, sars form, and heir ulra-viole ligh can re-ionise he IGM. Neural primordial mix (H, He, D, Li, B)

4 CMB ~15% Polarised 2003 WMAP discovery. Free elecrons have scaered ~15% of CMB phoons. --> IGM was re-ionised by redshif z ~ 20. Spergel e al ApJ Sup 148,175.

5 Hisory of Galaxy Formaion CMB (z~1100) Firs galaxies form (GCs?) Reionisaion (z~20) 2 nd phase of galaxy formaion Today

6 Sar-Formaion Raes (SFR) Consider a condensaion of primordial mix [ X=0.75, Y=0.25, Z=0.0 ] Toal mass: M gas Sar formaion: M gas M sars How quickly? Wih wha efficiency?

7 Sar Formaion Hisories Sellar populaions (old vs young sars) reveal SFR hisories. Three main galaxy ypes: ds d Ellipical Spiral Irregular E exponenial SFR consan SFR episodic SFR Irr S

8 Μ87 Ellipical Galaxies Μ32 Dwarf Ellipical saellie of M31 (Andromeda) Gian Ellipical in Virgo Cluser ~10 4 globular clusers

9 Ellipicals Red Old sars Few emission lines Low SFR Lile dus or gas Gas convered o sars. High surface brighness Form via mergers No ne roaion wih low ne Found in clusers angular momenum. Have many GCs

10 Spiral Galaxies Μ83 Μ51 NGC 891 NGC 7479

11 Spirals Red halo, blue disc Emission & absorpion lines Dus lanes & HI Moderae surface brighness Old and young sars. Sar formaion + old sars Gas available o form sars Form via collapse wih Roaing disk high angular momenum. In clusers & field Can survive mergers.

12 Irregular Galaxies Large Magellanic Cloud Small Magellanic Cloud Dwarf Irregulars saellies of he Milky Way

13 Irregulars Blue Srong emission lines Very dusy Low surface brighness Roaing Have few GCs Young sars High SFR Large gas reservoir High angular momenum Form via collapse. Mainly in field Easily disruped.

14 Closed Box Model M 0 = iniial gas mass M G () = gas mass a ime M S () = mass convered o sars β = fracion of M S reurned o gas ( supernovae, sellar winds, PNe ) M G = M 0 M S + β M S = M 0 (1 β)m S = M 0 α M S α = 1- β = fracion of M s reained in sars = sar formaion efficiency In densiies: ρ G = ρ 0 α ρ S

15 In dimensionless form µ() M G () M 0 S() M S () M 0 = fracion of M 0 in gas = fracion of M 0 ha has been urned ino sars µ() M G = M 0 α M S µ =1 α S 1 Since α <1, S() S >1 S S()

16 gas µ() =1 α S() dµ d = α ds d = α C µ dµ µ ds d = C µ = α C d = d C = 1 α lnµ = + A A = 0 gives µ(0) =1 µ() = e SFR in Ellipicals Assume ds /d µ ( more gas -> more sars form ) sars α S() = 1- e

17 Sar Formaion Timescale * = e-folding ime = ime o urn mass M 0 /e ino sars. Typically, ~ 1-5 Gyr 1 µ() = e / gas Q: If * = 2 Gyr, how long o urn 90% of gas ino sars? A: µ() = e / = 0.1 = ln µ = 2ln 0.1 ( ) ( ) = 4.6 Gyr α S() =1 e / 1 sars

18 SFR in Spirals Assume SFR = consan ds d = M M 0 dµ ds = α d d = α µ() =1 α M M 0 M M 0 1 M = mass convered per year µ() =1 α S() S() = M M 0 sars gas

19 SFR in Irregulars Typically burss of 100 M. yr -1 for 0.5 Gyr a inermien inervals ds d = f dµ d = α f µ() =1 α f M M 0 M M 0 M M 0 Sar formaion rae during sar burss. Fracion of ime spen sar-bursing 1 µ() =1 α S() sars gas

20 Sar-formaion hisories ds d E or S0 Irr Sabc For ellipicals mos sars form early on. Sars all roughly same age (co-eval).

21 Age Esimaes Main sequence lifeime: M τ MS = M. L L. 1 yr L L max HR diagram τ MS = L L 3 4 yr (since L M 4 M L 1/ 4 ). B-V L max (op of main sequence) gives age.

22 Decoupling ~300,000 yrs GC Formaion ~10 7 yrs Re-ionisaion ~10 7 yrs Galaxy formaion ~10 7 yrs Galaxy evoluion Gyrs Today Gyrs High sar-formaion rae Low sar-formaion rae Sar-formaion over Iner Slow collapse Sar-formaion begins Sar-formaion con. Delayed collapse Sarburs Iner Sarburs Ellipicals Spirals Irregulars

23 Summary µ Ell = e * = e-folding ime µ Sp =1 α M M 0 α = sar-forming efficiency M = mass conversion yr -1 µ Irr =1 α f M M 0 f = fracion of ime spen sar-bursing τ MS = M M L L 1 L = L yr

Lecture 7. Galaxy Formation After decoupling, overdense regions collapse IF. Caveats. The Dark Ages ( 1100 < z < 20 ) Redshift of Galaxy Formation

Lecture 7. Galaxy Formation After decoupling, overdense regions collapse IF. Caveats. The Dark Ages ( 1100 < z < 20 ) Redshift of Galaxy Formation Lecure 7 Galaxy Formaion Afer decoupling, overdense regions collapse IF # k T L > L J ~ % ( $ G m "' / 2 ~ 50 pc Collapse ime G ~ ( G " #/ 2 ~ 0 7 yr for all sizes More small ripples han large waves -->