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

Heidelberg March 2009 Eric Bell 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 Eric Bell Galaxy merging : dynamical friction Piontek, AIP

Heidelberg March 2009 Eric Bell Dark Matter merger

Heidelberg March 2009 Eric Bell Galaxy merger… Springel, MPA 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 Eric Bell Mergers create spheroids…

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

Heidelberg March 2009 Eric Bell Average effects of major mergers…  Average enhancement in SFR of ~1.6x in major mergers between SF galaxies M>10 10 M sun (averaged over t mrg ~ 2Gyr)  Intense bursts short-lived  Not all mergers produce a burst  <10% of SF directly triggered by a major merger

Heidelberg March 2009 Eric Bell Mergers can drive feedback

Heidelberg March 2009 Eric Bell Correlation between structure and star formation history  A bimodal galaxy population - transition mass of 3e10  Red sequence  Mostly non-star- forming  Bulk of galaxies bulge- dominated  Most massive galaxies  Blue cloud  Star-forming  Bulk of galaxies disk- dominated  Lower mass galaxies Absolute magnitude in i-band Absolute magnitude in i-band Blanton et al. 2003; ApJ, 594, 186 Low mass High mass Blue, forms stars Red, non SF Cessation (quenching) of star formation is empirically correlated with the existence of a prominent spheroid

Heidelberg March 2009 Eric Bell 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 Eric Bell Merger rates/demographics  Morphologies  Close pairs / 2pt correlation functions

Heidelberg March 2009 Eric Bell I. Merger rates  Messed up morphologies?

Heidelberg March 2009 Eric Bell Merger demographics  Some mergers between early-types (hard to recognize)

Heidelberg March 2009 Eric Bell Merger demographics  Many mergers between late-type galaxies  Way to think of it as mergers between central galaxies in ~ halos

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

Heidelberg March 2009 Eric Bell 2pt correlation function w(r p ) = DD/RR - 1 dP (r) = n (1+  (r)) dV  (r) = (r/r 0 ) - 

Heidelberg March 2009 Eric Bell

Heidelberg March 2009 Eric Bell I. Merger rates  Merger rates  2 point correlation function --> fraction of galaxies in close pairs in 3D space (through deprojection) Bell et al M > 2.5x10 10 M  z~0.6 COMBO-17 z~0.1 SDSS/2MASS M B < -20 z~0.6 COMBO-17 z~0.1 2dFGRS

Heidelberg March 2009 Eric Bell II. Assumptions  Assume  Mergers between galaxies 2.5x10 10 M  galaxies  red galaxies with > 5x10 10 M   All r<30kpc pairs merge (limit)  Timescale ~ 2πr / v  r av ~ 15kpc, v ~ 150km/s  timescale ~ 0.4Gyr  Very uncertain  Only way to make z<1 5x10 10 M  galaxy is through merging  Predict rate of growth of number of red galaxies with > 5x10 10 M  Introduction The growth of the red sequence Can mergers drive growth of the red sequence? introduction merger rates assumptions results Summary

Heidelberg March 2009 Eric Bell III. Results  IF all mergers between gals with M > 2.5x10 10 M   red sequence galaxy M>5x10 10 M   There are enough mergers to plausibly feed the growth of red sequence Observed number density of red galaxies with M>5x10 10 M  Predicted rate of growth Introduction The growth of the red sequence Can mergers drive growth of the red sequence? introduction merger rates assumptions results Summary

Heidelberg March 2009 Eric Bell Galaxy Mergers  Galaxy Merging  From dynamical friction (wake of particles behind secondary)   M secondary 2 /v 2  Makes spheroidal structures  If gas, enhances star formation (x2 on average)  Can drive intense stellar and AGN feedback  Early-type galaxies merge, produce very massive elliptical galaxies…  ~ merger per massive galaxy z<1…

24 The influence of halo mass (or galaxy clusters)

Heidelberg March 2009 Eric Bell Historical background  Dressler 1980  Increased E/S0 fraction in denser environments

Heidelberg March 2009 Eric Bell 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 Eric Bell Galaxies in clusters  Redder  Early-type (more spheroid-dominated)  More massive ones + more very low- mass 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 Eric Bell Ram-pressure stripping  Ram pressure stripping  P ~  hot v 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 Eric Bell 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)  Ben Moore, Kenji Bekki… Lake et al.

Heidelberg March 2009 Eric Bell  Moore et al  DM halo of a Milky Way galaxy  DM halo of a large galaxy cluster

Heidelberg March 2009 Eric Bell 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 solar masses  Some minor differences in assembly history (rather more recent assembly for a cluster)  Clusters filled with >10 7 K gas, galaxy-sized halos are likely full of ~10 5 K gas

Heidelberg March 2009 Eric Bell Similarities…  Ram pres. stripping  Tidal disruption

Heidelberg March 2009 Eric Bell What is the effect of cluster mass?  Why are red sequence galaxies red?  Merging + AGN feedback  This happens at solar masses  Environment (ram pressure stripping, harassment)  This happens just in massive halos >10 14

Heidelberg March 2009 Eric Bell 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) Van den Bosch et al. 2008

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

Heidelberg March 2009 Eric Bell 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 <~1Mpc scales matter for galaxy formation  Support for the halo model