1 / 23

Lecture 7. Galaxy Formation

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

howell
Download Presentation

Lecture 7. Galaxy Formation

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

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

  2. Caveats 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.

  3. GCs Neutral primordial mix (H, He, D, Li, B) The Dark Ages ( 1100 < z < 20 ) Uniform neutral IGM (Inter-Galactic Medium) Proto-globular clusters. Rare larger objects: proto-galaxies proto-clusters TCMB=2.7(1+z) K No stars! 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. History of Galaxy Formation CMB (z~1100) First galaxies form (GCs?) Reionisation (z~20) 2nd phase of galaxy formation Today

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

  7. E Irr S t Star Formation Histories Stellar populations (old vs young stars) reveal SFR histories. Three main galaxy types: Elliptical exponential SFR Spiral constant SFR Irregular episodic SFR

  8. M87 M32 Elliptical Galaxies Dwarf Elliptical satellite of M31 (Andromeda) Giant Elliptical in Virgo Cluster ~104 globular clusters

  9. Ellipticals Red Old stars Few emission lines Low SFR Little dust or gas Gas converted to stars. High surface brightness Form via mergers No net rotation with low net Found in clusters angular momentum. Have many GCs

  10. M51 M83 NGC 7479 NGC 891 Spiral Galaxies

  11. Spirals Red halo, blue disc Old and young stars. Emission & absorption lines Star formation + old stars Dust lanes & HI Gas available to form stars Moderate surface brightness Form via collapse with Rotating disk high angular momentum. In clusters & field Can 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 lines High SFR Very dusty Large gas reservoir Low surface brightness High angular momentum Rotating Form via collapse. Have few GCs “ Mainly in field Easily disrupted.

  14. Closed Box Model M0 = initial gas mass MG(t) = gas mass at time t MS(t) = mass converted to stars b = fraction of MS returned to gas ( supernovae, stellar winds, PNe ) a = 1- b = fraction of Ms retained in stars = star formation efficiency In densities:

  15. In dimensionless form = fraction of M0 in gas = fraction of M0 that has beenturned into stars

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

  17. gas stars Star Formation Timescale t*= “e-folding time” = time to turn mass M0 /e into stars. Typically, Q: If t*= 2 Gyr, how long to turn 90% of gas into stars? A:

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

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

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

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

  22. Decoupling ~300,000 yrs GC Formation ~107 yrs Re-ionisation ~107 yrs Galaxy formation ~107 yrs Galaxy evolution 107 1010 Gyrs Today 1010 Gyrs High star-formation rate Delayed collapse Slow collapse Low star-formation rate Starburst Star-formation begins Star-formation over Inert Inert Star-formation cont. Starburst Ellipticals Spirals Irregulars

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

More Related