Galaxy Formation: An Overview

Our Universe,Age 380,000 years

Galaxy Formation

Cosmology StarFormation

Stellar Evolution

ChemicalEnrichment

Dark Matter

BlackHoles

?

Hubble GOODS field

Hierarchical Galaxy

Formation• Galaxy formation

= Halo growth + Gas accretion +Feedback

• Halo growth: Cosmology

• Gas Accretion: Gravity + cooling

• Feedback: ???Gravitational instability: Cosmic Web

From Max Tegmark

InfallfromIGM

disk

Rvir

Rcool

Dekel+ 09 z=2, AMR

How Does Gas Accrete Into Galaxies?

• Hot mode: Heated to Tvir; ~spherical, slow.• Cold mode: Filamentary, rapid, smooth, T ~ 104 K.• Transition mass: Hot halo when Mhalo >~ 1012 M.

White & Rees 1978Gabor, RD+11

Feedback Regulates SFFeedback Regulates SF

Baldry+ 08

Halo mass function, scaled by b/m.

Baldry+ 08

Milky Way Schematic

Multiwavelength M31

Multiwavelength Antennae

Gas Processing Factories

★

Mgrav

Mgrav

SFR = Mgrav/(1+)(1-Z)

Equilibrium Relations

Finlator+RD 08

Scaling Relations & Scatter

• First order: Scaling rel’ns– Mgrav fb Mhalo

1.1 (1+z)2.25

That gas comes in “lumps” is, to first order, irrelevant.

• Second order: Scatter– Mergers, environment, satel-

lites, etc are 2nd order effects.– Accreting a lump higher fgas

& SFR, lower Z.

Mannucci+10

Preventive Feedback:

Photo-ionization, AGN, gravity, winds, …?

Stars can form

RD+11

Intuition from the Equilibrium Scenario

Main Sequence Galaxies as “Gas Processing

Engines”• Obtain gas via cold, smooth accretion

– Creates tight evolving M*-SFR relation.• Process some gas into stars

– Produces cosmic evolution of SF, main seq.

• Ejects most gas into outflows– SFR, gas content, metallicity set by

balance of inflow vs outflow.• Ejected material recycles

– Critical at low-z, sets e.g. stellar mass fcn shape.

Cold accretion dominates

• Star formation is supply-limited.• Mergers are a small contribution to accretion.

Keres+09

Fundamental driver: Halo growth

e.g. 1012 M halo

… at z=0, Min = 6 M/yr

… at z=2, Min = 80 M/yr

Prediction: A given mass galaxy forms stars faster at high redshifts.

Dekel et al 2009

from D. Elbaz

Observations

Quenching star formation

• Use SAM intuition to make “red and dead” galaxies:

Heat halo gas when fhot>0.6

• Produces correct:

– Red sequence

– Bright-end luminosity/mass fcn

– Does not change faint end

• Physics uncertain, but it likely has something to do with hot halo gas!

Gabor, RD+11

“Holistic” Approaches: SAMs and Sims• Semi-analytic models (SAMs)

parameterize baryon dynamics, constrain via observables.+Fast; tunable; builds on N-body.- Non-unique; builds in assumptions.

• Hydrodynamic simulations directly track gas.+Physics more robust; convergence tests possible- Slow; limited dynamic range; subgrid physics

parameters

• Compare to widest possible range of data: “holistic”.

• Sims develop physical insight; SAMs explore, synthesize.

Our Cosmological Hydro Code

N-body plus hydro:- Gravity using Tree-PM- Gas dynamics using EC-SPH- Cooling (H,He,metal)

Parameterized subgrid physics:- Star formation- Galactic outflows- (Quenching feedback)

Typical simulation parameters:- Spatial resolution: ~1-5 kpc (Kennicutt Law

scale)- Mass resolution: ~106-8 M.- Box size: ~10-100 Mpc.- Evolve from linear regime (z>>100) to today.

IGM enrichment: Outflow TracerIGM enrichment: Outflow Tracerw

ind

spee

d

mass loading

Too fewmetals in IGM

IGM too hot

Diffuse IGM unenriched

Too fewmetalsproduced

Momentum-driven wind scalings!

Oppenheimer & RD 2006Oppenheimer & RD 2008Oppenheimer & RD 2009a,b

Lyman alpha forest Metal absorbers(mostly CIV)