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

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 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 dont 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 = 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: W(r P ) Weighted correlation function w(r P ) 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:

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 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 dont 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 (N gal f pair, proj ) (from unweighted 2-point correlation function) Averaged SFR in galaxy pairs ( SFR typical,pair 2 × SFR gal ) Total SFR in merger remnants (SFR rem,tot ) Number of remnants Directly triggered SFR = 8±3%

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 = ) Di Matteo et al. (2007), Cox et al. (2008): ε = on average during 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 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 Khochfar % of the stars in classic bulges and ellipticals of M * > M sun have to be formed in quiescent mode M quiesc /M burst