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Galaxy Formation: An Overview
This document discusses galaxy formation and evolution from cosmological simulations and models. It summarizes that galaxy formation is driven by the hierarchical growth of dark matter halos, gas accretion via cold filamentary streams or hot spherical halos, and feedback regulating star formation. Galaxy properties like star formation rates and metallicities are set by the balance between gas inflow and outflow.
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Galaxy Formation: An Overview
- 1. Our Universe, Age 380,000years Galaxy Formation Cosmology Star Formation Stellar Evolution Chemical Enrichment Dark Matter Black Holes ? Hubble GOODS field
- 2. Hierarchical Galaxy Formation • Galaxyformation = Halo growth + Gas accretion + Feedback • Halo growth: Cosmology • Gas Accretion: Gravity + cooling • Feedback: ??? Gravitational instability: Cosmic Web From Max Tegmark
- 3. Infall from IGM 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
- 4. Feedback Regulates SFFeedbackRegulates SF Baldry+ 08 Halo mass function, scaled by Ωb/Ωm. Baldry+ 08
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- 9. Gas Processing Factories η/(1+η) ★ 1/(1+η) αΖ Mgrav ζMgrav SFR= ζMgrav/(1+η)(1-αZ)
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- 11. 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
- 12. Preventive Feedback: Photo-ionization, AGN, gravity,winds, …? Stars can form RD+11
- 13. Intuition from theEquilibrium Scenario
- 14. Main Sequence Galaxiesas “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.
- 15. Cold accretion dominates •Star formation is supply-limited. • Mergers are a small contribution to accretion. Keres+09
- 16. Fundamental driver: Halogrowth 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
- 17. Quenching star formation • UseSAM 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
- 18. “Holistic” Approaches: SAMs andSims• 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,
- 19. Our Cosmological HydroCode 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.
- 20. IGM enrichment: OutflowTracerIGM enrichment: Outflow Tracerwindspeed mass loading Too few metals in IGM IGM too hot Diffuse IGM unenriched Too few metals produced Momentum-driven wind scalings! Oppenheimer & RD 2006 Oppenheimer & RD 2008 Oppenheimer & RD 2009a,b Lyman alpha forest Metal absorbers (mostly CIV)