galaxy mergers: major/minor wet/dry hans-walter rix mpia, heidelberg … which galaxies are most...
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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
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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 ,
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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
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(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
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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 ,
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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
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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
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Merger Rate and the Growth of the Massive Red Sequence
>1011 Mo; Robaina et al 2010
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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
)
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• 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’?
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From Moster et al 2010
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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)
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The Incessant Drumbeat of Minor Mergers
M31’s Outskirts (PAndas, McConnachie et al 2010)
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Martinez-Delgado et al 2010
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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
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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)
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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