How Do Galaxies Get Their Gas?astro-ph/0407095

Dušan KerešUniversity of Massachusetts

Collaborators:

Neal Katz, UmassDavid Weinberg, Ohio-State

Romeel Davé, University of Arizona

Standard Model

● White and Rees 1978:– Gas infalling in dark matter halo, shock heats to the

virial temperature at the virial radius, and forms quasi-hydrostatic equilibrium halo.

– Shocked, hot, gas slowly cools and travels inwards, forming the central cooled component – the galaxy.

● Basis for Semi-Analytic models (White and Frenk 1991, Cole et al.1994, Somerville and Primack 2000 etc.).

Simulation Properties

● We use PtreeSPH (Davé et al. 1997), with gas physics and star formation based on Katz et al. 1996.

● Main simulation 22h-1Mpc on a side, 1283 SPH particles and 1283DM particles.

● Group (galaxy) resolution: 6.8x109 M⊙.● Higher resolution simulations are used to check

numerical effects.

SFR, Accretion, Mergers

● Galaxies grow via mergers and gas accretion.

● Smooth accretion, not mergers, dominates the mass growth of galaxies! (Murali et al. 2002, Kereš et al. 2004)

● SFR tightly follows smooth accretion rates.

Checking the standard model...

● We track the accreted particles backwards in time and register maximum temperature gas particle had before it was accreted by a galaxy.

● In standard model this Tmax

should be close to

virial temperature.

Results● Distribution of maximum

temperature reached by the gas is clearly bimodal (Katz et al., 2002).

● Cold mode: 104-105K

● Hot mode:106-107K

● Local minimum in the Tmax

distribution is at ~2.5x105K.

● Cold accretion mode is important at high redshifts and hot mode at low redshift.

Global history of the cold and hot accretion● Cold mode dominates at

z>3, while hot mode dominates at z < 2.

● Both modes drop significantly from high to low redshift.

● When integrated over the cosmic time both modes contribute similarly to the total accretion.

Cold/hot fraction dependence on the parent halo mass

● Cold mode dominates in low mass halos

● Hot mode dominates in massive halos.

● Transition between modes ~2 x 1011 M⊙ .

● Similar to the model and 1D sims of Birnboim and Dekel (2003) !

● Virial shock does not develop in low mass halos due to fast cooling.

Cold/hot fraction dependence on galactic mass

● Transition at Mgal

~ 2-3

1010 M⊙.

● Larger dispersion

– Small satellites hot mode dominated in massive halos.

Accretion geometry

ri r

j

ri r

j

cos [ri r

j] -1 1-1 1 cos [r

i r

j]

● Green: accreted gas.

● Left: COLD MODE (z=5.52, M=2.6x1011M⊙)

– All halo gas in the filaments.

– Cold gas, no virial shock

– Directional accretion● Right: HOT MODE halo

(z=3.24, 1.3x1012M⊙)

– Filled with gas, quasi-spherical

– T~Tvir

– Cold filaments penetrating the halo

z=5.52 z=3.24

-2Rvir 0 2Rvir

-0.5Rvir 0 0.5Rvir

~19,000p ~90,000p

rr

TT T

Some Consequences● Filamentary cold mode accretion can have important

consequences on the angular momentum acquisition.● Much of the dissipated energy from the galaxy formation

at high z is emitted in Ly (only some part in X-rays).

– Observational evidence – Matsuda et al. 2004.

● Feedback: fundamental difference in two modes. – In cold mode halos gas is only in the filaments. Winds from

galactic supernovae can easily expel the gas out of the halos, while it is hard to stop the gas falling in.

– Hot mode halos are full of hot gas that moves slowly, hard for winds to expel the gas but much easier to heat up the gas and slow or stop cooling (for example with AGNs)

● Stopping or slowing the hot accretion by some sort of energy input will allow large galaxies to grow mainly via mergers: – much better agreement with observed morphological

types, colors and bright end of LF.– expelling gas out of cold mode galaxies will make

better agreement on the low end of the LF.

● Galactic properties (metallicity, SFR, galaxy type etc.) change at ~1-3x1010M⊙ (Kauffmann et al. 2003, Tremonti et al. 2004) suggesting that existence of the two accretion modes and their different properties may be responsible.

Summary– 1. Smooth accretion is a dominant process of gas supply to

the galaxies (not merging).– 2. Roughly half of the gas accreted onto galaxies does not

shock at virial radius.– 3. Filamentary cold accretion dominates in halos with

Mhalo

< 2x1011 M⊙, while for Mhalo

> 2x1011M⊙ hot mode

dominates.● Similar results from other simulations (1D-Birnboim and Dekel,

Eulerian-Kravtsov et al., GADGET-Springel and Hernquist).

– 4. Existence of separate cold and hot accretion modes can lead to interesting theoretical and observational consequences.

● For SFR-density dependance and other interesting details check Kereš et al. 2004, astro-ph/0407095