How Do Galaxies Get Their Gas? astro-ph/ Dušan Kereš University of Massachusetts Collaborators: Neal Katz, Umass David 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.).

● Binney (1977) showed that only small part of the gas in his simulation reached T~T vir ● Katz and Gunn (1991): most of the gas in simulation of a single galaxy formation never reached high temperatures ( radiated mostly in Lya). ● Kay et al (2000): only small fraction of the accreted gas in their simulated galaxies was heated to T vir. ● Fardal et al. (2001): most of the cooling radiation from the halos is radiated in Lya with T < 20,000K. (implying that gas never reaches virial temperature). Less Standard Results

Simulation Properties ● We use PtreeSPH (Davé et al. 1997), with gas physics and star formation based on Katz et al ● Main simulation 22h -1 Mpc on a side, SPH particles and DM particles. ● Group (galaxy) resolution: 6.8x10 9 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 T max should be close to virial temperature.

Results ● Distribution of maximum temperature reached by the gas is clearly bimodal (Katz et al., 2002). ● Cold mode: K ● Hot mode: K ● Local minimum in the T max distribution is at ~2.5x10 5 K. ● Cold accretion mode is important at high redshifts and hot mode at low redshift.

Tmax/Tvir ratio also show clear bimodality

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.

Dependence of the accretion rate on the environment ● High “z”: both modes important in all environments ● Low “z” hot mode dominates in high density regions, cold in low density regions. ● Accretion (SFR) drops in high density regions.

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 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 M gal ~ M ⊙. ● Larger dispersion – Small satellites hot mode dominated in massive halos.

Comparison with observations ● We closely mimic observational selection. ● Sharp drop in SFR happens at S~1Mpc -2 or out of virial radius. ● SFR high in sims. but shape of the relation is in qualitative agreement. ● No need for gas stripping from the disc, truncation (strangulation) of gas supply qualitatively re- produce observations. Goméz et al. 2003

Robustness ● Test with higher resolution simulations: bimodal distribution of T max is not a consequence of the limited resolution (details in Kereš et al. 2004) ● High resolution 1D simulations of Birnboim and Dekel shows qualitatively the same result! ● A. Kravtsov Eulerian simulation (much better shock capturing) show qualitatively same result! ● Initial tests of GADGET simulations (Hernquist and Springel 2002), show similar bimodal gas accretion! ● Existence of cold and hot accretion is real physical result not just numerical artifact!

Accretion geometry riri rjrj riri rjrj cos [r i r j ] 1 1 cos [r i r j ]

● Green: accreted gas. ● Left: COLD MODE (z=5.52, M=2.6x10 11 M ⊙ ) – All halo gas in the filaments. – Cold gas, no virial shock – Directional accretion ● Right: HOT MODE halo (z=3.24, 1.3x10 12 M ⊙ ) – Filled with gas, quasi- spherical – T~Tvir – Cold filaments penetrating the halo z=5.52z= Rvir 0 2Rvir -0.5Rvir 0 0.5Rvir ~19,000p~90,000p r r TTT

Hot Mode ● Temperature much different from the virial temperature at the virial radius, grows inwards. ● More “standard” at lower redshift (more massive halos). ● T_vir reached only in the inner 0.5 R vir. ● Large dispersion around T max /T vir =1: – Filaments penetrate deeply inside hot mode dominated halos (see also Nagai & Kravtsov 2003) – Temperature profile – Numerical effects

Cold Mode ● Virial shock never develops in halos with M < M ⊙ (Birnboim and Dekel) due to the short cooling time (unstable shock), instead cold gas gets to galaxy in free fall time! ● Cold mode is highly filamentary. – All the halo gas is in galaxies or filaments ● Slowing in a series of small shocks, v rad drops inwards. ● Small hot halo around the galaxy – > some shocking at the disk.

Why bimodality? ● In small halos virial shock fails to develop, due to short cooling time (Birnboim & Dekel 2003). There is no virial shock, since there is no hot halo gas accumulated in the halo ! – No hydrostatic equilibrium, collapse on the dynamical time scale (hints also in White & Frenk 1991). ● Big halos: developed virial shock, quasi- hydrostatic equilibrium, gas spends some time in the halo before it cools. – Much more complicated, temperature gradients, filaments in the halos!

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 ● 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 ~1-3x10 10 M ⊙ (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 M halo 2x10 11 M ⊙ 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/