How Galaxies Assemble

1
How Galaxies Assemble

• Romeel Davé, Univ. of Arizona
• With Duan Kere Neal Katz (U.Mass), and David
  Weinberg (Ohio State)

2
Modes of Gas Accretion

• Hot Mode Gas shock heats to Tvir, cools slowly
  onto disk.
• Cold Mode Gas radiates its potential energy
  away in line emission at TltltTvir.
• Punch line Cold mode dominates during the epoch
  of galaxy assembly, especially in smaller halos
  and (consequently) at earlier times.

3
Phase Diagram of Accretion

• Cold and hot mode distinguished by Tmax, maximum
  temperature reached by gas until it gets into a
  galaxy and forms stars.
• Figure shows example phase paths of 5 particles
  from each case (distinction exaggerated).

4
Differential Accretion in Hot Cold Modes

• Differential smooth accretion rate shows two
  distinct modes.
• Cold mode dominates at high-z, when galaxy
  formation is most vigorous, and becomes
  comparable to hot mode from z2?0.
• Dividing temperature roughly 2.5x105K, but can
  also divide based on individual halos' Tvir.

5
Cumulative Contribution of Hot vs. Cold

• At z0, 70 of accreted baryons never reached
  their halos' Tvir.
• At z3, this number is 95, and 70 never came
  within an order of magnitude of Tvir.

6
Accretion Rates vs. Halo Mass

• Cold accretion dominates for Mhalo lt 3x1011M,
  virtually independent of redshift.
• This dividing mass is analytically predictable!
  (Birnboim Dekel 2003, will discuss later).

7
Example Accretion at z5 in a 3x1011M ?
halo, shown to Rvir.
Hot particles in green
Cold particles in green
8
Accretion Geometry Plays a Role

• Cold accretion is generally more filamentary
  this enhances cooling rate by increasing the
  density.
• Histogram of radius vector dot products shows
  peak in cold mode accretion at cosine1.

9
Merging vs. SmoothAccretion

• Galaxies obtain most of their mass by smooth
  accretion, not merging.
• Sub-resolution merging contributes very little.
• Globally, SFR follows smooth accretion rate.

10
SFR vs. Environment

• Gomez et al SFR begins to shut off well outside
  Rvir, at S1gal/Mpc2.
• Simulations show identical behavior.
• Driven by drop in hot mode accretion rate.

11
Analytic Analysis of Shock Stability

• Birnboim Dekel (2003) Shocks near Virial
  radius are unstable for Mhalo lt few x 1011M.
• Virial shock is not typically formed until late
  times.
• What about the non-spherical case?

12
Conclusions

• Cold mode gas accretion is an important mechanism
  for driving star formation in galaxies.
• Cold mode dominates globally at high-z (zgt2), and
  in smaller halos (Mhalolt1011M) at all times.
• Simulations consistent w/analytic expectations.
• Observational implications
• Line emission from Lya blobs (Fardal etal 01).
• Early growth of stellar mass.
• SFR vs. density/radius from cluster.