Galaxy Mergers: major/minor

wet/dry

Hans-Walter RixMPIA, 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

s

tella

r m

ass

Which galaxies sit in which halo?

For Mhalo > 10 12.5, 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 1010Msun<M*<1011Msun are disk-like

BUT:• beyond M*~1011Msun, no disks

•Formation only through major (dry-ish) mergers

Stellar mass

b

lue c

olo

r

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 massflat

(

axia

l ra

tio) o

bs

ro

und

van der Wel et al. (2009)

(Dry) major merging: the best way to destroy stellar disks and end-up round

If that’s only way to build >1011 Msun, 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

• trecognizeable ~ 0.5+-0.2 Gyrs

e.g Bell et al 2006; Lotz et al 2008

Merger Rate and the Growth of the Massive Red Sequence

>1011 Mo; 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 <SFR>(z)

Extreme SBs generally are mergers SFR/M*-weighted cross-correlation

Robaina et al 10, z = 0.6

SDSS, z = 0.1

SF

R E

nh

ance

men

t (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 rotatorscan be created(even for orbits whichlead to slow rotatorsotherwise).

• Major mergers of disc galaxies in dissipationless simulations ➙ slow rotators

The role of the cooling gas haloStars

Gas

• For very high gas fractions in disc ➙ fast rotators can be created (only for some merger orbits).

• In nature: galaxies have ahot gas halo from whichgas can cool (reservoir)

The Incessant Drumbeat of Minor Mergers

M31’s 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-mergerNo gas

Post-merger20% gas

Disk thickeningno, 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 >1011 Mo

• 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