Mechanisms of Galaxy Evolution Things that happen to

Mechanisms of Galaxy Evolution Things that happen to galaxies… Galaxy merging 1

Galaxy merging : basics § Same physics as gravitational slingshot, just backwards…. § M = secondary mass, going at speed v. M § = local density § C depends on how v. M compares with velocity dispersion of the matter around it… § Dark matter is important contributor to dynamical friction for galaxy mergers… Heidelberg March 2009 2

Galaxy merging : dynamical friction Piontek, AIP Heidelberg March 2009 3

Dark Matter merger Heidelberg March 2009 4

Galaxy merger… i) Mergers create spheroids ii) Can lead to enhanced star formation iii) Merger initiates feedback which quenches SF (recall spheroids empirically associated with quenched SF) Heidelberg March 2009 Springel, MPA 5

Mergers create spheroids… Heidelberg March 2009 6

Except when they don’t… § Minor mergers § Puff up disks (how much - active debate) § Toth/Ostriker, Velasquez/White, Hopkins et al! § Major mergers with lots of gas § May end up producing a disk-dominated remnant (Robertson et al. 2006). Heidelberg March 2009 7

Mergers are responsible for the largest starbursts § The most intensely-SF galaxies are merging… Borne et al. 1999 Heidelberg March 2009 8

Average effects of major mergers… § Average enhancement in SFR of ~1. 6 x in major mergers between SF galaxies M>1010 Msun (averaged over tmrg ~ 2 Gyr) § Intense bursts short-lived § Not all mergers produce a burst § <10% of SF directly triggered by a major merger Heidelberg March 2009 9

Mergers can drive feedback Heidelberg March 2009 10

Correlation between structure and star formation history Blanton et al. 2003; Ap. J, 594, 186 Red, non SF § A bimodal galaxy population - transition mass of 3 e 10 § Red sequence Blue, forms stars § Mostly non-star-forming § Bulk of galaxies bulgedominated § Most massive galaxies § Blue cloud § Star-forming § Bulk of galaxies diskdominated § Lower mass galaxies Low mass High mass -18 -20 -22 Absolute magnitude in i-band Cessation (quenching) of star formation is empirically correlated with the existence of a prominent spheroid Heidelberg March 2009 11

Merger effects: § Spheroid creation in many major mergers § Minor mergers likely to leave a disk § Major mergers with high >50% gas fraction may give disk § Enhancement of SFR § Modest, some very intense short-lived events § Feedback § Observed winds § Correlation spheroids with quenching Heidelberg March 2009 12

Merger rates/demographics § Morphologies § Close pairs / 2 pt correlation functions Heidelberg March 2009 13

I. Merger rates § Messed up morphologies? Heidelberg March 2009 14

Merger demographics § Some mergers between early -types (hard to recognize) Heidelberg March 2009 15

Merger demographics § Many mergers between late-type galaxies § Way to think of it as mergers between central galaxies in ~1012 -1013. 5 halos Heidelberg March 2009 16

Close pairs… § Projected close pairs § Galaxies with separations < xxkpc on sky § Projected close pairs with spectra § Spectra of both galaxies, + separation<xxkpc § 2 pt correlation function § Formalizing projected close pairs, can infer 3 d close pair fraction… Heidelberg March 2009 17

2 pt correlation function d. P (r) = n (1+ (r)) d. V (r) = (r/r 0)- Heidelberg March 2009 w(rp) = DD/RR - 1 18

Heidelberg March 2009 19

I. Merger rates § 2 point correlation function --> fraction of galaxies in close pairs in 3 D space (through deprojection) MB < -20 M > 2. 5 x 1010 M z~0. 6 COMBO-17 z~0. 1 2 d. FGRS z~0. 6 COMBO-17 z~0. 1 SDSS/2 MASS Heidelberg March 2009 20 Bell et al. 2006

Introduction The growth of the red sequence Can mergers drive growth of the red sequence? introduction merger rates assumptions results Summary II. Assumptions § Assume § Mergers between galaxies 2. 5 x 1010 M galaxies red galaxies with > 5 x 1010 M § All r<30 kpc pairs merge (limit) § Timescale ~ 2πr / v § rav ~ 15 kpc, v ~ 150 km/s timescale ~ 0. 4 Gyr § Very uncertain § Only way to make z<1 5 x 1010 M galaxy is through merging § Predict rate of growth of number of red galaxies with > 5 x 1010 M Heidelberg March 2009 21

Introduction The growth of the red sequence Can mergers drive growth of the red sequence? introduction merger rates assumptions results Summary III. Results § IF all mergers between gals with M > 2. 5 x 1010 M red sequence galaxy M>5 x 1010 M § There are enough mergers to plausibly feed the growth of red sequence Heidelberg March 2009 Observed number density of red galaxies with M>5 x 1010 M Predicted rate of growth 22

Galaxy Mergers § Galaxy Merging § From dynamical friction (wake of particles behind secondary) § Msecondary 2/v 2 § § Makes spheroidal structures If gas, enhances star formation (x 2 on average) Can drive intense stellar and AGN feedback Early-type galaxies merge, produce very massive elliptical galaxies… § ~ 0. 5 -1 merger per massive galaxy z<1… Heidelberg March 2009 23

The influence of halo mass (or galaxy clusters) 24

Historical background § Dressler 1980 § Increased E/S 0 fraction in denser environments Heidelberg March 2009 25

More background § Blanton+ 05 § Environment a strong function of color § @ given color, environment not a strong function of sersic index (structure) § SFH depends on environment § Structure (morphology)-density relation is a secondary effect Heidelberg March 2009 26

Galaxies in clusters § Redder § Early-type (more spheroid-dominated) § More massive ones + more very lowmass ones § Question was : do galaxies form different in clusters or do environmental processes make them different? § E. g. , ram pressure stripping or tidal interactions? Heidelberg March 2009 27

Ram-pressure stripping § Ram pressure stripping § P ~ hotv 2 § Restoring force 2 G g * § Stripping if ram pressure > restoring force… § When of outer gas envelope called strangulation § Gunn & Gott 1972 Heidelberg March 2009 28

Tidal processes… § Galaxy tidal interactions / harassment § Interactions with dark matter halo / individual galaxies in the cluster § Tidal interactions § Drive gas to middle § Thicken disk (increase vel. disp) Lake et al. § Ben Moore, Kenji Bekki… Heidelberg March 2009 29

§ Moore et al. 1999 § DM halo of a Milky Way galaxy § DM halo of a large galaxy cluster Heidelberg March 2009 30

What is different between a galaxy cluster and a galaxy (with little satellites)? § The behaviour of the baryons is the main thing that is different… § Efficiency of galaxy formation low for low-mass halos/subhalos § Efficiency of galaxy formation maximal at 1012 solar masses § Some minor differences in assembly history (rather more recent assembly for a cluster) § Clusters filled with >107 K gas, galaxy-sized halos are likely full of ~105 K gas Heidelberg March 2009 31

Similarities… § Ram pres. stripping § Tidal disruption Heidelberg March 2009 32

What is the effect of cluster mass? § Why are red sequence galaxies red? § Merging + AGN feedback § This happens at 1012 -1013 solar masses § Environment (ram pressure stripping, harassment) § This happens just in massive halos >1014 Heidelberg March 2009 33

Another view… § g-r vs. stellar mass § Weakly dependent on halo mass (bottom panel) § Weakly dependent on radius (centre panel) § Stellar mass is a much more important driver of properties than halo mass § Weak residual trend towards redness for more massive clusters (small radii) Heidelberg March 2009 Van den Bosch et al. 2008 34

What scales matter? Small scales (< 1 Mpc) matter Large scales (~ 6 Mpc) do not blue fraction as a function of density Blanton et al. 2005

Summary § Dark matter scale-free § Behaviour baryons very scale-dependent § Galaxy clusters § Lots of ~L* galaxies --> tides / harassment § Lots of hot gas --> ram pressure § Appears that properties of L* galaxies determined before fall into a cluster § Clusters are a second-order effect for L* galaxies § Decisive for low-mass galaxies § Only <~1 Mpc scales matter for galaxy formation § Support for the halo model Heidelberg March 2009 36