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 T vir, 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 2.5x10 5 K, 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 M halo < 3x10 11 M, virtually independent of redshift. ● This dividing mass is analytically predictable! (Birnboim & Dekel 2003, will discuss later).

Hot particles in green Example: Accretion at z=5 in a 3x10 11 M  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/Mpc 2. ● 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 M halo < few x M. ● 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 <10 11 M) 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.