Transcript Less than 15 percent of star formation in massive galaxies is

Less than 10% of star formation in z~0.6
massive galaxies is directly triggered by
galaxy mergers
Aday R. Robaina
&
Eric Bell, Ros Skelton, Dan McIntosh,
Rachel Somerville, Ramin Skibba
+ GEMS & STAGES teams
Kuala Lumpur, April 1st, 2009
Back in the late 90´s…
Le Fèvre et al. (2000)
Madau et al. (1996)
…it was believed that the evolution of the merger rate could drive the
decrease of the SFR density
Star formation in major galaxy
mergers < 35% (Hammer et al. 04,
Bell et al.05, Wolf et al. 05)
Galaxy mergers don’t drive the
evolution in the CSFH, but…
Bell et al. 2005
How much do they contribute to the
stellar-mass growth?
• How enhanced is the SFR by major galaxy interactions?
• What fraction of the SFR is directly triggered by major galaxy
interactions?
The sample:
• COMBO-17 redshifts, colors and stellar masses (ECDFS+A901/2 ~0.5 sqdeg)
• GEMS and STAGES HST/ACS morphologies
• Spitzer/MIPS deep 24 μm
• Stellar-mass cut = 1010 M, 0.4<z<0.8
The method:
• SFR-weighted 2-point correlation functions
• Visual morphologies
Our weighted 2-point correlation function in a nutshell
• Star forming galaxies (blue and/or 24 μm detected)
Two subsamples
defined
• All galaxies
• Mass ratio between 1:4 and 1:1
W(rP) → Weighted correlation function
w(rP) → unweighted correlation function
Weights used: Either SFR of SSFR
Enhancement:
1  W ( rP )
E ( rP ) 
1  w( rP )
SF-SF autocorrelation
Pairing up star forming galaxies with star forming galaxies
SF-All cross-correlation
Pairing up star forming galaxies with all galaxies
Extremely close pairs:
Merger remnants:
Enhancement
SFR / M*
Selected
as a pair
Morphologically
selected
• Clear enhancement out to 40 kpc
• Weak enhancement (ε=1.5-2) on average respect to pairs of isolated
galaxies
• SFR-weighted enhancement agrees within 20% with the SFR/M*
weighted
Comparing with previous results
This work, z = 0.6
SDSS, z = 0.1
• Excellent agreement with Li et al.
(2008) at z=0.1 within 40 kpc
• Excellent agreement with Lin et al.
(2007) at z=1
SFR/M*-weighted SF-All cross-correlation
SFR (or SFR/M*) enhancement seems to scale with the
pre-existing (quiescent) SFR
Then... galaxy mergers don’t trigger intense bursts of star
formation?
Actually they do:
• Process of interaction up to
2.5 Gyr
• Intense burst ~ 100 Myr
Dominated
by isolated
disks
(Di Matteo et al. 2007, Cox et al. 2008)
Increasing
interaction
fraction
Nothing new under the sun...
Barton et al. (2000), Barton et al. (2003), Lambas et al. (2003), Alonso et al.
(2004), Lin et al. (2007), Barton et al. (2007), Li et al. (2008), Ellison et al.
(2008), Jogee et al. (2009)...
• Major interactions mildly enhance the star formation activity
• Interacting systems host < 35% of the SFR density
• Amount of dissipation crucial to understand the properties of present
day massive galaxies.
• Galaxy mergers stop star formation. Do they deplete the cold gas or
something different is going on?
We need to know the amount of extra dissipation
directly triggered by major mergers.
A recipe for directly-triggered SFR
Ingredients:
• Averaged SFR enhancement (ε)
• Number of galaxies undergoing interactions (Ngalfpair, proj)
(from unweighted 2-point correlation function)
• Averaged SFR in galaxy pairs ( SFRtypical,pair≠ 2 × SFRgal )
• Total SFR in merger remnants (SFRrem,tot)
• Number of remnants
N f
0triggered
.5(  1) SFR
 ( SFR =8±3%
N SFR
Directly
SFR

FSFR,trigg.
gal
pair, proj
typical, pair
rem ,tot
N gal 0.5  SFRtypical, pair
rem
typical, pair
)
Confronting the theory and putting in context
• When considering all stages and all kinds of interactions, major mergers
only trigger small enhancements in the SF activity (ε=1.5-1.8)
Di Matteo et al. (2007), Cox et al. (2008):
ε=1.25-1.5 on average during 2-2.5 Gyr of the interaction
• Enhancement independent from pre-existing level of SFR/gas fraction
Di Matteo (2009): Enhancement independent from gas fraction
• Not in conflict with the idea that the strongest starbursts are triggered by
major mergers
• Only a modest fraction (~8%) of the SFR is directly triggered by
major mergers and interactions.
Using mock catalogues from Somerville et al. (2008): Triggered
fraction= 7%
Conclusions:
• SFR (and SFR/M*) in 0.4<z<0.8 massive galaxies enhanced by a
factor of 1.5-1.8 due to major mergers
• Enhancement scales with the pre-existing (non-bursty) SFR
• Directly triggered SFR fraction ~ 8%
• Star formation triggered by major mergers does not
significantly impact the growth of stellar mass at z<1
Noeske et al. 2007
Mquiesc/Mburst
Khochfar 2007
85% of the stars in classic bulges
and ellipticals of M* > 1010 Msun have
to be formed in quiescent mode