Galaxy Assembly: Accretion, Mergers, Feedback, and Galaxy Growth

1. Romeel Davé with: Kristian Finlator, Ben Oppenheimer, Neal Katz, Dusan Keres, Mark Fardal, David Weinberg Galaxy Assembly: Accretion, Mergers, Feedback, and Galaxy Growth

2. Galaxy Formation Simplified • Galaxy formation = Halo growth + Accretion • Halo growth: Cosmology • Accretion: Radiative cooling From Max Tegmark

3. Accretion z=5.5 z=3.2 Rvir Infall from IGM Density (4Rvir)‏ Rcool disk Temperature Temp (Rvir)‏ Keres et al 2005 • Hot mode: Gas heats to ≈Tvir, cools & rains down. – only in massive galaxies! • Cold mode: Gas cools in filaments at ≈104K, enters on dynamical time – dominates accretion.

4. Hot mode Cold mode Spherical models: Dividing mass assuming tcool<txxx Geometry & Overcooling • Filaments cool faster: Spherical models underestimate cold mode. • Cold mode is rapid: Stars overproduced –Overcooling • Don’t ever be surprised by “large” galaxies at high-z! Keres etal 08

5. Mergers vs. Smooth Accretion • Cold smooth accretion dominates galaxy growth at all epochs and masses. • SFR tracks cold accretion rate. • Dry mergers important at z<2, in massive systems. Keres etal 08

6. Smooth Accretion & Star Formation • Galaxy by galaxy: SFR tracks smooth accretion! • True at all epochs, but moreso at high-z. • Even SFR≈100 Mo/yr systems at high-z not merger driven! Keres etal 08

7. Galaxy Formation Complexificated • Need feedback…and lots of it! • Photoheating: Tvir<104K, M*<107. • AGN: Need bulge? • Outflows: Probably dominate in SFGs Halo mass function, scaled by Wb/Wm. Quenching UV Photons Outflows Baldry+ 08

8. SFR Suppression by Outflows • SFR = ACC/(1+h) • h ≈ few needed. • If h is larger in small galaxies, can get right faint-end slope. Data: Bouwens etal 06, z~6 Keres etal 08 RD, Finlator, Oppenheimer 2006

9. Smooth Accretion Prediction: SFR-M* • SFR-M* relation governed by form of SFH. • Smooth accretion predicts tight SFR-M* (regard-less of outflows). • Slope ≈ 0.9, scatter ≈ 0.3 dex. • Evolves down at given M*, indep-endent of M*.

10. Observations: SFR-M* Daddi etal 07 z~1.4-2.5 • Observations of SFGs (z≈0-2): • M*SFR0.7-0.9at all z. • Small scatter (<0.3 dex) on “main sequence” of SFGs. • Evolves downward in SFR (at given M*), independent of M*. Elbaz et al 2007, z~1 Noeske etal 07 z~0.2-1.1

11. Big mergers at high z? • Sub-millimeter galaxies: • Gas-rich major mergers. • Caught in a very brief phase. • In process of becoming ETG. • Growing a SMBH. • Models: • Mergers: brief phase w/SFR >10x quiescent. • Hierarchical: need extreme top-heavy IMF.

12. Cold accretion can explain SMGs? × • New IRAC data: M*~few 1011 Mo. • …but forming stars “normally” for galaxy that large. • i.e. lies on (or a bit above) smooth accretion prediction! Daddi et al. 2007

13. Quiescent SMGs: Forming BCGs Finlator et al 2006 • 4 galaxies with SFR>500 in (100 Mpc)3. • M*~3x1011Mo • n~4x10-6/Mpc3 • Suspiciously like SMGs! • Minor (x2-3) elevation over quiescent SFR.

14. The role of mergers • Mergers still needed for: • morphological transformations • occasional Arp 220-like ULIRG • black hole growth? • Can cold accretion explain galaxy growth in SFGs? Is that part a solved problem? • Well, no. Cold accretion doesn’t quite completely work.

15. SFR-M*: Data vs. Models • Models get right: • Slope • Scatter • General evolution • But amplitude evolution is wrong! • Data has higher SFR at given M*. • But cold accretion sets maximum rate!

16. How can SFR exceed accretion? • Mergers temporarily boost SFR. • But SFR-M* would have large scatter • Gas accumulates in galaxies then is consumed exponentially • Can’t be exluded. But how does gas stay in ISM without forming stars? Kennicutt relation not valid? Not seen locally… • Systematics in SFR or M* at x2-3 level? • Quite possible, but has to operate in a very specific way to keep tight SFR-M* relation and smooth evolution.

17. Or is the IMF evolving? • Mildly top-heavy or bottom-light IMF, increasingly so to high-z, can lower SFR. • Example: Kroupa IMF, with characteristic mass evolving as Mc=0.5(1+z)2. • Also reconciles cosmic M* growth with SFRD

18. Summary • Galaxy growth is driven by smooth, cold, filamentary accretion • Accreted gas gets quickly processed into stars (or ejected in an outflow). • Can achieve high SFRs at early times – need outflow rates >> SFR to regulate. • Mergers are unimportant for feeding gas into galaxies, but have other effects. • But all is not settled – fundamental limit of SFR-M* seems to be exceeded in data: Systematics? Wrong model? IMF evolution?

19. Systematics in SFR or M*? • Need to lower SF / raise M* by ~x3-5. • Raising M* generically hard: Unless stars put out a LOT less red light than locally. [note: Maraston vs. BC03 goes wrong way] • Something else mimicking SF? • AGN: Possible, but would have to be strange to exactly mimic tight M*SFR. • PAH emission: Rest-8m dominated by PAHs, so perhaps PAH emission per unit SF much stronger at high z. • Can’t be ruled out, but would require dramatic differences vs locally calibrated relations. Such differences not seen even in extreme systems.