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Aday R. Robaina & Eric Bell, Ros Skelton, Dan McIntosh, Rachel Somerville, Ramin Skibba

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….

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Aday R. Robaina & Eric Bell, Ros Skelton, Dan McIntosh, Rachel Somerville, Ramin Skibba

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  1. 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

  2. Back in the late 90´s… Le Fèvre et al. (2000) Madau et al. (1996) …it was believed that the evolutionof the merger rate could drive the decrease of the SFR density

  3. 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… How much do they contribute to the stellar-mass growth? Bell et al. 2005 • How enhanced is the SFRby major galaxy interactions? • What fraction of the SFR is directly triggered by major galaxy interactions?

  4. The sample: • COMBO-17 redshifts, colors and stellar masses (ECDFS+A901/2 ~0.5 sqdeg) • GEMS and STAGES HST/ACS morphologies • Spitzer/MIPSdeep 24μm • Stellar-mass cut = 1010 M, 0.4<z<0.8 The method: • SFR-weighted 2-point correlation functions • Visual morphologies

  5. Our weighted 2-point correlation function in a nutshell • Star forming galaxies (blue and/or 24 μm detected) • All galaxies Two subsamples defined • 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:

  6. SF-SF autocorrelation Pairing up star forming galaxies with star forming galaxies

  7. SF-All cross-correlation Pairing up star forming galaxies with all galaxies

  8. Extremely close pairs: Merger remnants:

  9. Selected as a pair Morphologically selected Enhancement SFR / M* • Clear enhancement out to 40 kpc • Weak enhancement (ε=1.5-2) on averagerespect to pairs of isolated galaxies • SFR-weighted enhancement agrees within 20% with the SFR/M* weighted

  10. Comparing with previous results • Excellent agreement with Li et al. (2008) at z=0.1 within 40 kpc • Excellent agreement with Lin et al. (2007) at z=1 This work, z = 0.6 SDSS, z = 0.1 SFR/M*-weighted SF-All cross-correlation SFR (or SFR/M*) enhancement seems to scale with the pre-existing (quiescent) SFR

  11. 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)

  12. Nothing new under thesun... 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.

  13. 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 Directly triggered SFR = 8±3%

  14. 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%

  15. 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

  16. 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 mode Mquiesc/Mburst

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