![Page 1: How Galaxies Assemble Romeel Davé, Univ. of Arizona With: Dušan Kereš & Neal Katz (U.Mass), and David Weinberg (Ohio State)](https://reader035.vdocuments.site/reader035/viewer/2022062315/56649d435503460f94a1f281/html5/thumbnails/1.jpg)
How Galaxies Assemble
Romeel Davé, Univ. of Arizona
With: Dušan Kereš & Neal Katz (U.Mass), and David Weinberg (Ohio State)
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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.
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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).
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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.
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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.
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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).
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Hot particles in green
Example:Accretion at z=5in a 3x1011M halo,shown to R
vir.
Cold particles in green
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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.
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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.
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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.
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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?
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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.