How do Galaxies Get Their Gas? Dušan Kereš Institute for Theory and Computation - Harvard-Smithsonian CFA Collaborators: Neal Katz, David Weinberg, Romeel Davé, Mark Fardal, Lars Hernquist, T. J. Cox, Phil Hopkins The EVLA Vision: Galaxies Through Cosmic Time Dec. 2008

Hopkins & Beacom 2006 Active star formation occurs during most of the Hubble time What drives the star formation? Why is SFR density so high at early times and decreases at low-z?

What drives this star formation evolution? Stars form from dense/galactic gas Galaxies have very high gas fractions at high redshift and extended gas reservoirs -> can provide high SFRs. Depletion of this gas can then slow down the star formation Could explain some of the observed trends Let’s check if there is enough “galactic” gas at high redshift

The state of dense gas Damped Ly  Absorbers (DLA) are probing column densities typical of the gaseous galactic disks Amount of gas in DLA at high-z is less than the mass locked in stars at z=0. A vast majority (~80%) of stars form after z=2 (e.g. Marchesini et al. ‘08) During the time when most of the stars formed dense HI phase stays constant Dense gas phase needs to be constantly re-supplied. *  * z=0 Prochaska&Wolfe ‘08

Is there an evidence for gas supply in our neighborhood?

Milky Way MW’s SFR 1-3M_sun/yr (Kennicutt ‘01) MW’s gas reservoir is ~5e9M_sun. Without gas supply this is spent in several Gyr. Star formation rate in the Solar neighborhood relatively constant. Not much change in gas density despite large gas depletion (Binney et al. 2000) Supply of gas is needed

HVCs around MW Most direct measure of galactic infall Many clouds with inwards velocities, net infall from known clouds > 0.25M/yr Hard to explain by the galactic fountain Low metallicities Similar net infall rates from clouds around other galaxies Van Woerden et al. 2004

Conclusion: Accretion of gas from the intergalactic medium is ongoing at all epochs.

The Theory

Standard Model ● E.g. White & Rees 1978 ● Gas falling into a dark matter halo, shocks to the virial temperature T vir at the R vir, and continuously forms quasi-hydrostatic equilibrium halo. T vir =10 6 (V circ / 167 km/s) 2 K. ● Hot, virialized gas cools, starting from the central parts, it loses its pressure support and settles into centrifugally supported disk –> the (spiral) galaxy. ● Mergers of disks can later produce spheroids. ● The base for simplified prescriptions used in Semi- Analytic models – SAMs (e. g. White & Frenk 1991). Tvir

Earliest Observation Today How do galaxies form and evolve? NASA/WMAP NOAO Structure formation Gas heating Gas cooling Dynamics-merging Star-formation FEEDBACK Complex and non- linear: Simulations needed

SPH simulations of the galaxy formation We adopt  CDM cosmology. Gadget-2 and 3 (Springel 2005) SPH code: entropy and energy conservation; proper treatment of the “cooling flows” –Cooling (no metals), UV background, a star formation prescription –A simple star formation prescription Star formation happens in the two-phase sub-resolution model: SN pressurize the gas, but does not drive outflows Star formation timescale is selected to match the normalization of the Kennicutt law. –Typically no galactic winds Largest volume 50/h Mpc on a side, with gas particle mass ~1.e8 M ⊙, but most of the findings confirmed with resolution study down to 1.e5 M ⊙ Lagrangian simulations -> we have ability to follow fluid (particles) in time and space.

Global Accretion We define galaxies and follow their growth Galaxies grow through mergers and accretion of gas from the IGM Smooth gas accretion dominates global gas supply Mergers globally important after z=1 Star formation follows smooth gas accretion, owing to short star formation timescales It is within factor of ~2 from the typical Madau plot (e.g. Hopkins & Beacom 2006) Kereš et al. 2008

Temperature history of accretion We utilize the Lagrangian nature of our code We follow each accreted gas particle in time and determine its maximum temperature - T max before the accretion event. In the standard model one expects T max ~ T vir IGM Shocked T max = ? Galactic gas

- Empirical division of 2.e5K, but results are robust for 1.5-3e5K - The gas that was not heated to high temperatures -> COLD MODE ACCRETION - > Accretion from cold filaments - The gas that follows the standard model -> HOT MODE ACCRETION - > Accretion from cooling of the hot halo gas Kereš et al Katz, Keres et al K Evolution of gas properties of accreted particles

How important are these two accretion modes? Cold mode dominates the gas accretion at all times. Hot mode starting to be globally important only at late times

The nature of cold and hot modes

Low mass halos are not virialized Halo gas (excluding galactic gas), within Rvir Low mass halos are not virialized Transition: M h = M ⊙ Approximate description by Birnboim&Dekel 2003 Post shock cooling times are shorter than compression time at Rvir when M h ~1x10 11 M ⊙ More gradual transition in our case because cold filaments enhance the density profile Kereš et al 2008

Low mass halos High-z well defined filamentary accretion Low-z -> Tvir is lower, filaments are warmer and larger than the halo size Low mass halos

High-z accretion Strong mass dependence of accretion Cold mode infalls on a roughly free fall timescale. It follows the growth of DM halos in the lower mass halos and still is within a factor of ~3 in massive halos High accretion rates result in high SFRs of high redshift galaxies Even in massive halos cold mode filaments supply galaxies with gas Cooling from the hot atmosphere not important. Satellites accrete with the same rates as central galaxies Kereš et al. 2008

< 1kpc resolution, m_p ~1.e6 M_sun Kereš et al 2008

Zoom to 0.8Rvir z~>2 cold mode can supply the central galaxy and satellite galaxies efficiency to supply galaxies directly declines with time z~1 cold mode less efficient in massive halos; lower density contrast -> easier to disrupt It can supply satellites directly but supply to the center is limited and often goes trough cold clumps

Low-z accretion Drastic accretion change over time, factor of ~30 from z=4 to 0. At low mass this roughly follows the drop in the dark matter halo accretion Some hot mode accretion around the transition mass. Halos more massive than ~10 12 M ⊙ (an order of magnitude above transitions) stop cooling hot gas At z~0, a fraction of massive halos, around MW mass is able to cool the hot gas Red - hot mode Blue - cold mode Kereš et al. 2008

Recent & future simulations

1.2/h Mpc cmv. 300/h kpc ~ 2Rvir Z~ /h pc physical res. M_p~1.e5M_sun M_h~1.3e11M_sun Yellow Tvir Blue ~ 1e4K 5kpc/h physical New simulations

z~2.6 25/h kpc (physical)

Halo mass at z=0, 7x10 11 M_sun Gas particle mass ~10 5 M_sun

Zoom onto a ~Rvir region, with the lowest overdensity moving from 200 to 1500

Can we detect the smoothly accreting gas with EVLA? (very preliminary …)

Log n_HI z=2.5 z=2.0 z= kpc region, 1pixel=1kpc, approx 5” at 40Mpc

High-z Filaments have n_HI ~ 1.e18-1.e20, and around 1.e11Msun halos widths of several tens of kpc. Massive halos, around MW size are good targets to constrain the accretion models This continues to about z=1 Lower mass galaxies are embedded in thick filaments Need to work on kinematic signatures

z=1.0 z=0.5 z=0 400kpc region, 1pixel=1kpc, approx 5” at 40Mpc

EVLA range z=0-0.5 Hard to detect filaments Their physical size and typical temperature larger at low redshift: 20 to 100 kpc scale. This results in low columns. Galaxies in MW size halos are surrounded by the cold clouds with mixed origin No more clear distinction of the cold and hot modes. Many clouds form from cooling instabilities in warm ~1.e5K gas in the outskirts of halos. Similar process occurs when filaments approach the virial radius. Clouds are easily detectable once inside ~50kpc. A large number of clouds should be visible around external galaxies with n_HI >1.e18 cm -2 Clouds form in much larger numbers if there are galactic winds operating. Observations could place some constraints on the level of such feedback, cloud formation radii and cloud survival. Clouds should be common around MW size galaxies that are central in halos. Less common in lower mass halos. More work is needed for more massive objects.

Where should an idealized HI telescope look? Higher redshift -> more coherent structure Higher mass galaxies -> higher density contrast Low mass galaxies, relatively more HI gas compared to the galaxy size – no clouds far from the galaxy, harder to detect extended halo z=1 is already an interesting regime.

Summary Low mass halos do not undergo classic virialization –In more massive halos gas is heated to Tvir –The transition into the hot halos is gradual because of the filaments that enhance the density structure in a halo At high redshift (z > 1.5) smooth gas accretion is completely dominated by the cold mode accretion, even in massive halos Cooling from the hot atmosphere is inefficient at all times At low redshift hot mode is important in a fraction of objects but the most massive halos accrete very little from the hot atmosphere Accretion of cold clouds with mixed hot and cold mode origin dominates the late time accretion in MW size halos, but more work is needed to understand the physics of this process. EVLA could probe the last epoch where coherent structures feed galaxies and should be great for studies of cold (HVC like) clouds.

Suggestions of cloud infall in external galaxies NGC 6946 Boomsma 2007 Velocity wiggles Holes punctured In the HI disk (out of the SF region)

Extraplanar gas, warps, streams, filaments Modeling extra- planar gas kinematics Fraternali&Binney: –Cannot be explained by fountain –Suggests net inflow of gas Similar for other galaxies NGC 891 Osterloo et al. 2007