Galaxy Mergers: major/minor wet/dry Hans-Walter Rix MPIA, Heidelberg … which galaxies are most shaped by mergers? … when do mergers matter for the SFR? … inevitable minor merger damage? White 1978

Predicting the rate of galaxy mergers Issues: merger definition DM halo merger rates DM halo vs. galaxy merger Stewart et al 2009; Hopkins et al 2010, Khochfar et al 2008,

For what galaxies are (major) mergers most expected to shape the present-day structure? Stewart et al 2009; Hopkins et al 2010, Khochfar et al 2008, Moster, Maccio & Somerville 2010 i.p., predicted halo merger rate halo mass stellar mass Which galaxies sit in which halo? For M halo > , M * varies little all mergers are major (… and likely dry after z~1) Major mergers z<1 only likely in massive halos

(Late) Merging: the only way to make a very massive galaxy SDSS study of shape distribution of passive (=early type) galaxies: Most M sun <M*< M sun are disk-like BUT: beyond M*~10 11 M sun, no disks Formation only through major (dry-ish) mergers Stellar mass blue color Van der Wel et al 2009; Bernardi et al 2008, All galaxies non-star-forming early types Observed shape distribution = intrinsic shape x viewing angle stellar mass flat (axial ratio) obs round van der Wel et al. (2009) (Dry) major merging: the best way to destroy stellar disks and end-up round If thats only way to build >10 11 M sun, all should be close to round

Measuring merger rates Need to estimate: –fraction of galaxies in (different phases of) mergers morphology close pairs as function of redshift, galaxy mass, mass ratio, etc.. –duration (or recognizeable time interval) of these phases observational estimate of merger rate Bell et al 2006; Lotz et al 2008; Jogee et al 2009, Conselice et al 2009; Robaina et al 2010, De Propris et al 2007,

Merger Fraction over the last 8 Gyrs HST imaging has been key 3% - 10% depending on –Definition –Mass/luminosity ratio Robaina et al 2008 pairs <30kpc Jogee et al 2009 morphologically selected

Duration of Recognizeability project simulations into space of observables t recognizeable ~ Gyrs e.g Bell et al 2006; Lotz et al 2008

Merger Rate and the Growth of the Massive Red Sequence >10 11 M o ; Robaina et al 2010

Mergers and Star-formation The most intense SF events are almost always in mergers –Merger speeds up consumption of the available cold gas. Galaxies involved in such ULIRG-ish mergers would have formed stars vigorously anyway Dominated by isolated disks Increasing interaction fraction e.g. Wolf et al 2005; Robaina et al 2009, Engel et al 2010 fraction of global SFR triggered by mergers is modest ( 8%+-3% at z<1) Mergers matter little for (z) Extreme SBs generally are mergers SFR/M * -weighted cross-correlation Robaina et al 10, z = 0.6 SDSS, z = 0.1 SFR Enhancement ( ε )

Major mergers (~1:1) (in simulations) lead to elliptical galaxies In observed samples (SAURON): division between slow and fast rotators Mergers and Galaxy Structure Can major mergers result in spheroids that are fast rotators?

From Moster et al 2010

Moster, Maccio et al 2010) We find that fast rotators can be created (even for orbits which lead to slow rotators otherwise). Major mergers of disc galaxies in dissipationless simulations slow rotators The role of the cooling gas halo Stars Gas For very high gas fractions in disc fast rotators can be created (only for some merger orbits). In nature: galaxies have a hot gas halo from which gas can cool (reservoir)

The Incessant Drumbeat of Minor Mergers M31s Outskirts (PAndas, McConnachie et al 2010)

Martinez-Delgado et al 2010

Thin small-bulge galaxy disks How can galaxy disks form as large as observed? –very high-resolution simulations with feed-back are getting there… How can they stay so thin? –Ostriker & Toth 93; e.g. Katzantzidis et al 08, Moster et al 10 How can there be galaxies that have no bulge? –Kormendy 2007; Springel & Hernquist 2005; Kautsch 2006/9, Jun et al 2009, Hopkins et al 2010 –the role of cold gas in suppressing bulge formation? predicted halo merger rate

How well can disk-dominated galaxies handle mergers of satellite (halos)? Major mergers almost always destroy disk Significant gas component (>20%) in disk and in satellite greatly aid the presence of a post-merger cold disk no disk-thickening crisis Initial Post-merger No gas Post-merger 20% gas Disk thickening no, 20%, 40% gas from Moster et al 2010 Katzanzidis et al 08, Moster et al 2009)

Mergers: some take home messages (Major) mergers are expected/and seen to drastically shape the most massive galaxies: Merger-rate vs build-up of the red sequence No disks in early types >10 11 M o Mergers are not central to the global SFR=f(z) … but they cause most of the most intense SFR events Minor mergers constantly affect big disk galaxies … but disk dominated galaxies can exist