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How Galaxies Assemble

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Cold and hot mode distinguished by Tmax, maximum temperature reached by gas ... What about the non-spherical case? Conclusions ... – PowerPoint PPT presentation

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Title: 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.
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