How Galaxies Assemble

Romeel Davé, Univ. of Arizona

With: Dušan Kereš & Neal Katz (U.Mass), and David Weinberg (Ohio State)

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 T<<T

vir.

● Punch line: Cold mode dominates during the epoch of galaxy assembly, especially in smaller halos and (consequently) at earlier times.

Phase Diagram of Accretion

● Cold and hot mode distinguished by T

max,

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).

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 z~2→0.

● Dividing temperature roughly 2.5x105K, but can also divide based on individual halos' T

vir.

Cumulative Contribution of Hot vs. Cold

● At z~0, 70% of accreted baryons never reached their halos' T

vir.

● At z~3, this number is 95%, and ~70% never came within an order of magnitude of T

vir.

Accretion Rates vs. Halo Mass

● Cold accretion dominates for

Mhalo

<

3x1011M, virtually independent of redshift.

● This dividing mass is analytically predictable! (Birnboim & Dekel 2003, will discuss later).

Hot particles in green

Example:Accretion at z=5in a 3x1011M halo,shown to R

vir.

Cold particles in green

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 cosine~1.

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.

SFR vs. Environment

● Gomez et al: SFR begins to shut off well outside R

vir, at

~1gal/Mpc2.

● Simulations show identical behavior.

● Driven by drop in hot mode accretion rate.

Analytic Analysis of Shock Stability

● Birnboim & Dekel (2003): Shocks near Virial radius are unstable for

Mhalo

< few x

1011M.

● Virial shock is not typically formed until late times.

● What about the non-spherical case?

Conclusions

● Cold mode gas accretion is an important mechanism for driving star formation in galaxies.

● Cold mode dominates globally at high-z (z>2), and in smaller halos (M

halo<1011M) 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.