In, Out, and Around: An Overview from SimulationsDavid Weinberg, Ohio State University

Collaborators: Amanda Brady Ford, Romeel Davé, Mark Fardal, Neal Katz, Dusan Keres, Juna Kollmeier, Ben Oppenheimer, Molly Peeples, Vimal Simha, Jason Tumlinson

Why is galaxy formation so inefficient?

How do galaxies get their gas? (In)

Why do galaxies lose their gas? (Out)

Where does ejected gas go? (Around)

Observables

Li & White 2009Guo, White, Li, & Boylan-Kolchin 2010

Conversion of halo baryons to stars of central galaxy peaks at ~20%.

Global fraction of baryons in stars is ~ 3.5%

Why is galaxy formation so inefficient?

Why is galaxy formation so inefficient?

Global fraction of baryons in stars is ~ 3.5%.

Conversion of halo baryons to stars of central galaxy peaks at ~ 20%.

Probable answer: Feedback.Open issues:

How much feedback is “ejective” vs. “preventive”?

What are the mechanisms of ejective and preventive feedback?

How are the remaining baryons distributed, spatially, and in temperature-density-metallicity? How can they be mapped?

Why is galaxy formation so inefficient?

Why is galaxy formation so inefficient?

Open issues:

How are the remaining baryons distributed, spatially, and in temperature-density-metallicity? How can they be mapped?

Shull, Smith, & Danforth 2011

How do galaxies get their gas? (In)

How do galaxies get their gas?

Keres, Katz, Weinberg, Davé 2005Yellow: T ≈ 106 KMagenta: T ≈ 104 KGreen: Will accrete in next 0.2 Gyr

Answer 1: Gas cools from the dense central regions of a hot gas corona, shock-heated to T ~ Tvir .

Answer 2: Gas accretes along cold (T~104 K) filamentary streams, never reaching Tvir .

Keres et al. 2005

Roughly half of gas accreting onto galaxies in SPH simulation is never shock heated close to virial temperature.

Keres et al. 2005

Cold mode dominates in low mass halos, hot mode in high mass halos. Transition at ~1011.3 Msun.

Can be understood in terms of ratio of cooling time to compression time: rapidly cooling gas can’t support a virial shock (Binney 1977; Birnboim & Dekel 2003; Dekel & Birnboim 2006).

Fraction of recent galaxy accretion in cold / hot mode

Luminosity

Color

Tempting to associate hot/cold transition with transition from star-forming galaxies to passive high-mass galaxies.

Kauffmann et al. 2004

Blanton, Hogg, et al. 2003

Z. Zheng

Near 1012 Msun, cold filaments penetrate shocked gas halos.Similar behavior with different cosmological hydro codes.

Keres et al. 2009Gadget-2 SPH

Brooks et al. 2009Gasoline SPH

Dekel et al. 2009Ramses AMR

Amount of hot accretion is code sensitive, more so than cold accretion.

Accretion: Open IssuesWhat is the dynamics of cold accretion?• Where and how does infalling gas shed its acquired gravitational energy? Steadily, or in a shock at the disk?• Does infalling gas accelerate, slow down, or go at constant speed?

Goerdt et al. 2010

• How clumpy is the filamentary flow?

Closely connected to the question of observability via Lyα cooling radiation.

Rosdahl & Blaizot 2012

Accretion: Open IssuesWhat is the dynamics of cold accretion?• Where and how does infalling gas shed its acquired gravitational energy? Steadily, or in a shock at the disk?• Does infalling gas accelerate, slow down, or go at constant speed?

Lyα luminosity function from cooling radiation only.

Fardal et al. 2001

• How clumpy is the filamentary flow?

Closely connected to the question of observability via Lyα cooling radiation.

Accretion: Open IssuesWhat is the role of cold accretion in determining galaxy morphology and angular momentum?Filamentary accretion hits edge of disk. Origin of angular momentum may lie in dynamics of filament formation at hundreds of kpc.Rapid accretion at high-z can produce unstable, clumpy disk.

Agertz, Moore, & Teyssier 2009

B = densityR = temperatureG = metallicity

Pichon et al. 2011

Accretion: Open IssuesHow much hot accretion is there? Is preventive feedback needed?What is the interaction between accretion and outflows?

Accretion rate vs. Tmax

Keres et al. 2009

Si IV column density mapShen et al. 2012

How do galaxies get their gas?

Answer 1: Gas cools from the dense central regions of a hot gas corona, shock-heated to T ~ Tvir .

Answer 2: Gas accretes along cold (T~104 K) filamentary streams, never reaching Tvir .

Answer 1.5: Disk galaxies extract gas from the hot corona by entraining it with cool gas ejected by supernovae (Fraternali & Binney 2008).

Marinacci et al. 2010

Why do galaxies lose their gas? (Out)

Why do galaxies lose their gas?

Possible answers:• Gas heating via supernovae and stellar winds. “Energy driven winds.”• Radiation pressure on dust, entraining gas. “Momentum driven winds.”• Cosmic ray pressure.• Gas ejection from AGN-driven outflow, details uncertain (disk wind driven by radiation pressure?).

AGN are the prime suspect for preventive feedback, injecting energy that balances the cooling of the hot gas corona.

Keres et al. 2009

Simulations with minimal feedback greatly overproduce stellar mass function. Eliminating hot accretion (e.g., via AGN feedback) can make massive galaxies red, but still too massive.

Hopkins, Quataert, Murray 2012

Very high resolution simulations of isolated disks with variety of masses and gas fractions.

Include heating mechanisms and radiation pressure.

Different mechanisms dominate in different situations.

Sometimes the combined effects are critical; supernovae keep the ISM puffed up, and radiation pressure drives it out.

Hopkins, Quataert, Murray 2012

Very high resolution simulations of isolated disks with variety of masses and gas fractions.

Include heating mechanisms (supernovae, stellar winds, HI regions), and radiation pressure.

Different mechanisms dominate in different situations.

Sometimes the combined effects are critical; supernovae keep the ISM puffed up, and radiation pressure drives it out.

Blue = no ISM heatingRed = no radiation pressure

Hopkins, Quataert, Murray 2012

Substantial time variation.

Time-averaged behavior of mass loading factor η = (dM/dt) / SFR well described by

η ~ (Vesc)-1.1 × (Σgas)-0.6

For momentum driven wind, simple scaling argument predicts η ~ (Vesc)-1 and Vwind ~ Vesc.

For momentum driven wind, simple scaling argument predicts η ~ (Vesc)-1 and Vwind ~ Vesc.

Oppenheimer, Davé, and various collaborators show that these scalings give better agreement than constant η and Vwind for a variety of observables: IGM enrichment, mass-metallicity, galaxy mass function.

But metallicity differences are subtle.

Oppenheimer & Davé 2006

Pontzen & Governato 2011

Governato, Quinn, Wadsley, and collaborators produce impressively realistic disk galaxies with SN feedback only in high resolution SPH simulations.

Episodic star formation and violent fluctuations in central baryon mass can erase dark matter cusp.

Outflows: Open Issues

• What is the relative importance of the different stellar feedback mechanisms (SNe, stellar winds, ionization, radiation pressure, cosmic rays)?

• Are outflows steady or episodic?

• Are outflows metal-enhanced relative to the ISM?

• Do AGN drive significant outflows from galaxies? If so, what is the mechanism?

• Are AGN the main cause of preventive feedback? If so, how does it work?

Where does ejected gas go? (Around)

Probable answer:

Low mass galaxies eject gas to the IGM.

High mass galaxies retain ejected gas in the halo.

The simple expectation is that much of the ejected gas should be recycled.

But this makes big galaxies too massive and too gas rich.

Where does ejected gas go?

Oppenheimer et al. 2010

What’s needed to fit observations:• Winds to suppress galaxy masses at low M*.

• Suppression of wind recycling, but only at high M*.• Gradual shut-off of accretion in satellites.

Circumgalactic Gas: Open Issues

• Are the numerical treatments reliable?

• How efficiently are ejected metals mixed into halo gas?

• What is the interaction between outflows and accretion?

• Is recycling of ejected material suppressed? If so, how?

• What is the density-temperature structure of gaseous halos?

Shen, Guedes, Madau, Mayer, Prochaska 2012Metallicity of circumgalactic gas at z=2.8 in high resolution SPH simulation of MW-like disk galaxy.Box is 600 kpc across, circles mark virial radii of galaxy halos; central has Rvir = 50 kpc.

Observables

Observables• Galaxy stellar mass functions.

• Joint distribution of (M* , SFR , fgas , Zgas , Z* , redshift), correlation with morphology, environment.

• Lyα emission from cold accretion: cooling radiation or fluorescence. Maybe 21cm emission at low redshift?

• Direct observations of outflows: emission from hot, warm, and cool gas.

• Outflow diagnostics in galaxy and AGN spectra.

• Global statistics of IGM metals in quasar absorption lines.

• Circumgalactic absorption towards background quasars.

Simulation results for star-forming galaxies are generally well characterized by “equilibrium” relations between gas supply and gas consumption.

Davé, Finlator, & Oppenheimer 2012MNRAS 421, 98

η ejective feedback Mgrav accretion rate

ζ preventive feedback tdep ISM depletion time

αZ reaccretion y nucleosynthetic yield

See also Peeples & Shankar 2011, MNRAS 417, 2962

Observables• Galaxy stellar mass functions.

• Joint distribution of (M* , SFR , fgas , Zgas , Z* , redshift), correlation with morphology, environment.

• Lyα emission from cold accretion: cooling radiation or fluorescence. Maybe 21cm emission at low redshift?

• Direct observations of outflows: emission from hot, warm, and cool gas.

• Outflow diagnostics in galaxy and AGN spectra.

• Global statistics of IGM metals in quasar absorption lines.

• Circumgalactic absorption towards background quasars.

Tumlinson et al. 2011OVI absorption around z ~ 0.2 star-forming galaxies (ubiquitous, strong) and passive galaxies (weaker).

Steidel et al. 2010HI, CII, SiII, CIV, and SiIV absorption around z ~ 2.5 galaxies. Cool gas with high covering fraction, outflow.

Rudie et al. 2012HI at z ~ 2.5High column density HI absorption increases sharply close to galaxies.

Amanda Ford et al., in prep. (see poster)

Median absorption along lines of sight to central galaxies of 1011 or 1012 Msun halos.

Simulation of 48 h-1 Mpc box, N = 2×3843

mgas = 3.6×107 Msun

mdm = 1.8×108 Msun

Images 600 kpc across

Ford et al., physical conditions of absorbers at different impact parameters.

Shen et al. 2012, “Eris” simulation at z=2.8, mgas = 2.0×104 Msun mdm = 9.8×104 Msun

Cool gasT < 105 K

Warm gasT > 105 K

Shen et al. 2012

Inflowing gas

Outflowing gas

Shen et al. 2012

Why is galaxy formation so inefficient?• What is the balance of ejective vs. preventive feedback? • What are the primary feedback mechanisms?• Where are the remaining baryons?

How do galaxies get their gas?• What is the dynamics of cold accretion? Smooth or clumpy? Where does the gravitational energy go?• What role does filamentary accretion play in galaxy angular momentum and morphology?• How much hot accretion is there? What governs it?

Why do galaxies lose their gas?• What drives galactic winds: heating, radiation pressure, AGN?• Are outflows episodic or steady? Are they metal-enhanced?• How do outflows affect accretion, and vice versa?

Where does ejected gas go?• How much recycling occurs? What, if anything suppresses it?• What is the phase structure of the CGM? Are ejected metals mixed? • Are numerical treatments of gas ejection and the CGM reliable?