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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 × SFRgal ) • 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