Malaysia 2009 Sugata Kaviraj Oxford/UCL Collaborators: Sukyoung Yi, Kevin Schawinski, Eric Gawiser, Pieter van Dokkum, Richard Ellis Malaysia 2009 Early-type galaxy evolution : insights from the rest-frame UV

Malaysia 2009 Spheroidal (early-type) galaxies Smooth and featureless Dominated by old population II stars Dominate the high-mass end of the galaxy LF Dominate the stellar mass density at low redshift

Malaysia 2009 Early-type galaxy formation The classical model Optical colours are red with small scatter (e.g. Bower et al. 1992) High [Mg/Fe] values (e.g. Thomas 1999 but see Smith et al. 2009!) Little or no (optical) luminosity evolution (e.g. Scarlata et al. 2007) Bulk of star formation at high redshift (z>2) Star formation timescale < 1 Gyr Early-types already in place at high redshift? Short high-redshift starburst then mainly passive ageing (monolithic evolution)

Malaysia 2009 Making galaxies in the standard LCDM model Build DM backbone (N-body sims) Seed DM halos with gas Baryonic evolution (gas to star conversion, feedback) using recipes DM halo mass function galaxy luminosity function Early-types form through major (mass ratio < 3) mergers (e.g. Cole et al. 2000, Hatton et al. 2003) Continuous star formation over a Hubble time Compatible with early-type properties?

Malaysia 2009 The effect of young stars 99.9% of stars form 10 Gyrs ago 0.1% of stars form 0.1 Gyrs in the past Optical spectrum indistinguishable from purely old population But a strong UV signal!

Malaysia 2009 The effect of young stars 99.5% of stars form 10 Gyrs ago 0.5% of stars form 0.1 Gyrs in the past Optical spectrum indistinguishable from purely old population But a strong UV signal!

Malaysia 2009 UV studies of nearby early-type galaxies UV data requires space-based telescope GALEX –Far UV (1500 A) and Near UV (2300 A) filters –Unprecedented resolution and FOV Pre-select spheroidal galaxies using SDSS fracdev parameter (crude bulge/disk decomposition): fracdev>0.95 Cross-match with GALEX, visually inspect for contaminants and remove AGN (which could contaminate UV)

Malaysia 2009 Early-type UV colours: GALEX & SDSS Expected tight relation in optical (g-r) CMR But NUV CMR shows a spread of 6 mags - strong UV sources present in nearby early-type galaxies Kaviraj et al., 2007, ApJS, 173, 619 Schawinski et al., 2007, ApJS, 173, 512 Yi et al., 2005, ApJ, 619, L111

Malaysia 2009 Early-type UV sources Old or young? Old metal-rich HB stars can produce UV (the UV upturn phenomenon) Study NUV colour of NGC any UV left over likely to be from young stars Yi et al. (2005) find only 4/62 galaxies with the UV spectral shape of UV upturn galaxies

Malaysia 2009 The star-forming early-type fraction Low-redshift star-forming fraction is at least 30% Could be as high as % 1-5% in young stars with ages Myrs (also Martin, OConnell et al. 2007) Widespread recent star formation in nearby early-types Kaviraj et al., 2007, ApJS, 173, 619

Malaysia 2009 The high-redshift galaxy population The Chandra Deep Field South (CDFS) Optical (U,B,V…) photometry traces rest- frame UV beyond z~0.5 No old stars! Need depth (in U and B bands especially), redshifts and morphologies MUSYC (UBVRIzJK) + COMBO-17 (photo-z) + VVDS (spec-z) + GEMS (HST images) HST ACS images

Malaysia 2009 Rest-frame UV colours at high redshift (0.5<z<1) Kaviraj et al., 2008, MNRAS, 388, 67 Low z

Malaysia 2009 Rest-frame UV colours at high redshift (0.5<z<1) 0.2% of early-types consistent with purely passive evolution since z=3 1.2% of early-types consistent with purely passive evolution since z=2 SSP assumption, result is robust using either BC2003 or Yi (2003) stellar models Kaviraj et al. 2008, MNRAS, 388, 67

Malaysia 2009 Luminous galaxies (M V <-21) form up to 10-15% of their mass after z=1 (consistent with SAM predictions) Low-mass galaxies form 30-60% of their mass after z=1 Recent star formation in high-z early-types Kaviraj et al. 2008, MNRAS, 388, 67

Malaysia 2009 What drives the recent star formation? Indirect evidence for minor mergers Numerical simulations of minor mergers (mass ratios 1:3 - 1:10) Satellite gas fractions > 20% Monte-Carlo simulation drived by LCDM minor mergers statistics Good agreement with observed UV and optical CMRs Kaviraj et al. 2009, MNRAS in press (arXiv: ) see also Bezanson et al (arXiv )

Malaysia 2009 What drives the recent star formation? Relaxed ETGs

Malaysia 2009 What drives the recent star formation? Relaxed ETGs Disturbed ETGs (30% of the ETG population)

Malaysia 2009 What drives the recent star formation? Rest-frame (NUV-g) COSMOS z=0.6

Malaysia 2009 What drives the recent star formation? Rest-frame (NUV-g) COSMOS z=0.6

Malaysia 2009 What drives the recent star formation? Internal mass loss Not enough gas at low redshift (Kaviraj et al. 2007) Condensation from hot gas reservoir Hot gas fraction of the order of total recent star formation (O'Sullivan et al. 2003) Major mergers Major merger rate (e.g. Conselice 2007) does not seem enough to satisfy disturbed ETG fraction (~35-40%) Minor mergers S everal factors more frequent than major mergers Consistent with disturbed ETGs dominating the blue cloud and mass fractions forming in young stars Not expected to perturb the morphology of the galaxy

Malaysia 2009 Summary arXiv: Early-types of all luminosities form stars over the last 10 Gyrs, although bulk of the stars do form at high redshift Negligible fraction of early-types consistent with purely passive ageing since z=2 Recent star formation mass fraction ~ 1-5% at z~0.1, ~ 7-10% at z~0.7 Massive early-types form up to 10-15% of their mass after z~1, low-mass early-types form 30-60% of their mass after z~1 Most likely mechanism driving recent star formation is minor merging – only mechanism that is consistent with all observational data