1. After decoupling, overdense regions collapse IF Collapse timefor all sizes. More small ripples than large waves. --> Universe dominated by globular clusters (?!) 47 Tuc  Cen Lecture 7. Galaxy Formation

2. Dimensional Analysis --> scaling laws leaving out dimensionless factors (e.g.~10). We ignored: angular momentum -- slows and can halt the collapse --> Spiral Galaxies. cosmological expansion -- delays collapse until expansion time > collapse time. --> Need “Dark Matter halos” (collapsing before decoupling) into which baryon gas falls. Caveats

3. The Dark Ages ( 1100 < z < 20 ) Uniform neutral IGM (Inter-Galactic Medium) Proto-globular clusters. Rare larger objects: proto-galaxies proto-clusters T CMB =2.7(1+z) K No stars! Neutral primordial mix (H, He, D, Li, B) GCs As regions collapse and merge, stars form, and their ultra-violet light can re-ionise the IGM.

4. CMB ~15% Polarised 2003 WMAP discovery. Free electrons have scattered ~15% of CMB photons. --> IGM was re-ionised by redshift z ~ 20. Spergel et al. 2003 ApJ Sup 148,175.

5. CMB (z~1100) First galaxies form (GCs?) Reionisation (z~20) 2 nd phase of galaxy formation Today History of Galaxy Formation

6. Star-Formation Rates (SFR) Consider a condensation of primordial mix [ X=0.75, Y=0.25, Z=0.0 ] Total mass: M gas Star formation: M gas M stars How quickly? With what efficiency?

7. Stellar populations (old vs young stars) reveal SFR histories. Three main galaxy types: Ellipticalexponential SFR Spiralconstant SFR Irregularepisodic SFR Star Formation Histories t E Irr S

8.   Dwarf Elliptical satellite of M31 (Andromeda) Giant Elliptical in Virgo Cluster ~10 4 globular clusters Elliptical Galaxies

9. Ellipticals RedOld stars Few emission linesLow SFR Little dust or gasGas converted to stars. High surface brightnessForm via mergers No net rotation with low net Found in clusters angular momentum. Have many GCs

10.   NGC  NGC 7479 Spiral Galaxies

11. Spirals Red halo, blue discOld and young stars. Emission & absorption linesStar formation + old stars Dust lanes & HIGas available to form stars Moderate surface brightnessForm via collapse with Rotating disk high angular momentum. In clusters & fieldCan survive mergers.

12. Large Magellanic Cloud Small Magellanic Cloud Irregular Galaxies Dwarf Irregulars satellites of the Milky Way

13. Irregulars Blue Young stars Strong emission linesHigh SFR Very dusty Large gas reservoir Low surface brightnessHigh angular momentum RotatingForm via collapse. Have few GCs“ Mainly in field Easily disrupted.

14. M 0 = initial gas mass M G (t) = gas mass at time t M S (t) = mass converted to stars  = fraction of M S returned to gas ( supernovae, stellar winds, PNe )  = 1-  = fraction of M s retained in stars = star formation efficiency In densities: Closed Box Model

15. = fraction of M 0 in gas = fraction of M 0 that has been turned into stars In dimensionless form

16. SFR in Ellipticals Assume ( more gas -> more stars form ) gas stars

17. t * = “e-folding time” = time to turn mass M 0 /e into stars. Typically, Q: If t * = 2 Gyr, how long to turn 90% of gas into stars? A: Star Formation Timescale gas stars

18. Assume SFR = constant = mass converted per year SFR in Spirals gas stars

19. Typically bursts of 100 M yr -1 for 0.5 Gyr at intermittent intervals. Fraction of time spent star-bursting Star formation rate during star bursts. SFR in Irregulars gas stars

20. Star-formation histories For ellipticals most stars form early on. Stars all roughly same age (co-eval). Sabc t E or S0 Irr

21. Main sequence lifetime:... Age Estimates HR diagram L B-V L max L max (top of main sequence) gives age t.

22. Decoupling ~300,000 yrs GC Formation ~10 7 yrs Re-ionisation ~10 7 yrs Galaxy formation ~10 7 yrs Galaxy evolution 10 7 10 10 Gyrs Today 10 10 Gyrs High star-formation rate Low star-formation rate Star-formation over Inert EllipticalsSpiralsIrregulars Slow collapse Star-formation begins Star-formation cont. Delayed collapse Starburst Inert Starburst

23. Summary t * = e-folding time  = star-forming efficiency = mass conversion yr -1 f = fraction of time spent star-bursting...