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)

Malaysia 2009

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