Cool Halo Gas in a Cosmological Context Kyle Stewart “Team Irvine” UC Santa Cruz Galaxy Formation Workshop Kyle Stewart “Team Irvine” UC Santa Cruz Galaxy Formation Workshop Collaborators: James Bullock, Betsy Barton (UCI) Tobias Kaufman, Lucio Mayer (UZH) Jürg Diemand, Piero Madau (UCSC) James Wadsley (McMaster), Ari Maller (NYCCT)

Outline Theoretical Motivations –Baryonic content of DM halos –Gas accretion via gas-rich mergers Observing Cool Halo Gas –Unresolved / open questions The Simulation: VL2 + GASOLINE –Covering Fraction –Kinematics: Halo Gas vs. Galaxy

Motivations How do galaxies acquire their cool gas? –Cold flows? Cloud Fragmentation? (e.g. Keres et al. ‘09, Dekel & Birnboim ‘06, Maller & Bullock ’04, most of Tuesday’s talks…) Gas rich mergers? –Stewart et al. 09 3

Small halos have a lot of gas and few stars (especially at z~1) 4 Stewart 2009 Abundance matching (Conroy & Wechsler ‘09) + baryonic TF

Gas-rich mergers & galaxy assembly 5 Stewart et al ~30% of an L* galaxy’s baryons accreted in Major, gas-rich mergers over it’s history (since z=2). ~20% of bright galaxies at z~1 have had a Major, gas-rich merger in last Gyr (not based on this plot)

Motivations How do galaxy acquire their cool gas? How can we test ideas? Absorption-systems as probes of cool halo gas… 6

Observing Gas Around Galaxies: QSO (Mg II) D ~ 100 kpc (or less) Image from Tripp & Bowen (2005) 1)Covering Fraction 2) Cloud vs. Galaxy Kinematics

Observing Gas Around Galaxies: 1)Covering Fraction 2) Cloud vs. Galaxy Kinematics But what ARE they? Spherical halo gas? Cold Filaments? Pressure-confined gas clouds? Outflowing winds? Tidal Streams? Mg II C f ~20-80% e.g. Tripp & Bowen ’05; Tinker & Chen ‘08; Kackprzak et al. '08

Observing Gas Around Galaxies: 1)Covering Fraction 2) Cloud vs. Galaxy Kinematics Kacprzak et al. ‘09 (submitted) 7/10 Mg II absorbers show velocities that co-rotate with galaxy

Galaxies Probing Galaxies 10 z~0.5 z~0.7 Rubin et al. ‘09 Keck/LRIS absorption spectrum Cool gas ejected from host galaxy during past merger? Spatially-extended complex of cool clouds at d>17kpc from galaxy (with high velocity width)

Our Simulation: VL2 (initial conditions) + GASOLINE (sph code) Diemand et al. ‘08 Wadsley et al. ‘04 Some stats: WMAP3 cosmo:   =0.24,  =0.76, h=0.73, σ 8 =0.77,  b =0.042 m DM, m gas, m star ~3e5, 4e5, 1e5 M sun, Np~4 million. Sph smooth len: 332 pc. Final halo mass M vir ~2.e12 M sun ‘Blast-wave’ feedback of Stinson et al. ‘06; Haardt & Madau ‘96 UV field; NOTE: no strong blow-out winds Log  HI [M sun /pc 3 ]= [-8, -1] Log  stars [M sun /pc 3 ] = [-7, 1]

Results: Covering Fraction R outer ~ 50 kpc (comoving) N grid ~ 1000 R inner ~ 5 kpc (comoving) LOS “covered” if N(HI) >10 16,18,20 atoms/cm 2

Results: Covering Fraction Note: VL2 chosen to be quiescent at late times Fragmented Flows + Mergers Cold flows (and mergers) (averaged over 3 projections) Covering Fraction Depends on Recent Gas Accretion!

Gas and Galaxy Kinematics: Log  HI = [-7, 1] LOS velocity [-250 to +250 km/s]

Gas and Galaxy Kinematics: Log  HI = [-7, 1] LOS velocity [-250 to +250 km/s]

Summary: High-res SPH simulation of VL2 halo with gas + stars Extended cool halo gas betrays a complex assembly history: – Gas-rich & star-poor mergers are common and responsible for much of the halo gas (especially at z<2) –These mergers would be invisible to pair-counts at fixed luminosity Cool halo gas tends to co-rotate with the galaxy, as indicated by observations. This gas includes clouds, streams, and other complex structures – the gas that will build the galaxy itself. Covering Fraction for cool gas depends on recent gas accretion: smooth (or fragmented) filaments, mergers, etc. Covering fraction in VL2 remains high well past the time associated with the canonical cold flow epoch, as a result of mergers and infalling fragments. 16

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