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Early-type galaxy evolution : insights from the rest-frame UV. Sugata Kaviraj Oxford/UCL Collaborators: Sukyoung Yi, Kevin Schawinski, Eric Gawiser, Pieter van Dokkum, Richard Ellis Malaysia 2009. Spheroidal (early-type) galaxies. Smooth and featureless Dominated by old population II stars - PowerPoint PPT Presentation
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Malaysia 2009
Sugata KavirajOxford/UCL
Collaborators: Sukyoung Yi, Kevin Schawinski, Eric Gawiser, Pieter van Dokkum, Richard Ellis
Malaysia 2009Malaysia 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 formationThe ‘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 galaxiesKaviraj et al., 2007, ApJS, 173,
619Schawinski et al., 2007, ApJS, 173, 512Yi et al., 2005, ApJ, 619, L111
Malaysia 2009
Early-type UV sourcesOld or young?
• Old metal-rich HB stars can produce UV (the UV upturn phenomenon)
• Study NUV colour of NGC 4552 - 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 80-90%
• 1-5% in young stars with ages 300-500 Myrs (also Martin, O’Connell et al. 2007)
• Widespread recent star formation in nearby early-types
Kaviraj et al., 2007, ApJS, 173, 619
Malaysia 2009
The high-redshift galaxy populationThe 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 (MV<-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:0711.1493) see also Bezanson et al. 2009 (arXiv0903.2044)
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)
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What drives the recent star formation?
Rest-frame (NUV-g)
COSMOSz=0.6
Malaysia 2009
What drives the recent star formation?
Rest-frame (NUV-g)
COSMOSz=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 reservoirHot gas fraction of the order of total
recent star formation (O'Sullivan et al. 2003)
Major mergersMajor merger rate (e.g. Conselice
2007) does not seem enough to satisfy disturbed ETG fraction (~35-40%)
Minor mergersSeveral factors more frequent than
major mergersConsistent 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
SummaryarXiv:0710.1311
• 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