Less than 10% of star formation in zLess than 10% of star formation in z~~0.6 0.6 massive galaxies is directly triggered by massive galaxies is directly triggered by

galaxy mergersgalaxy mergers

Aday R. Robaina&

Eric Bell, Ros Skelton, Dan McIntosh, Rachel Somerville, Ramin Skibba

+ GEMS & STAGES teams

Kuala Lumpur, April 1st, 2009

Le Fèvre et al. (2000) Madau et al. (1996)

Back in the late 90´s…

…it was believed that the evolution of the merger rate could drive the

decrease of the SFR density

Bell et al. 2005

Star formation in major galaxy mergers < 35% (Hammer et al. 04, Bell et al.05, Wolf et al. 05)

• How enhanced is the SFR by major galaxy interactions?

• What fraction of the SFR is directly triggered by major galaxy interactions?

Galaxy mergers don’t drive the evolution in the CSFH, but…

How much do they contribute to the stellar-mass growth?

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

Two subsamples defined

• Star forming galaxies (blue and/or 24 μm detected)

• All galaxies

• Mass ratio between 1:4 and 1:1

Weights used: Either SFR of SSFR

Enhancement:)(1

)(1)(

P

PP rw

rWrE

W(rP) → Weighted correlation function

w(rP) → unweighted correlation function

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:

EnhancementSFR / 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

SFR/M*-weighted SF-All cross-correlation

• 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 (or SFR/M*) enhancement seems to scale with the pre-existing (quiescent) SFR

This work, z = 0.6

SDSS, z = 0.1

Dominated by isolated

disks

Increasing interaction fraction

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

(Di Matteo et al. 2007, Cox et al. 2008)

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 × SFRgal )

• Total SFR in merger remnants (SFRrem,tot)

• Number of remnants

Directly triggered SFR = 8±3%pairtypicalgal

pairtypicalremtotrempairtypicalprojpairgaltriggSFR SFRN

SFRNSFRSFRfNF

,

,,,,., 5.0

)()1(5.0

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

• 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%

• Enhancement independent from pre-existing level of SFR/gas fraction

Di Matteo (2009): Enhancement independent from gas fraction

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

Khochfar 2007

85% of the stars in classic bulges and ellipticals of M* > 1010 Msun have to be formed in quiescent modeM

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