Sugata Kaviraj

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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

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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

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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 (zgt2)
Star formation timescale lt 1 Gyr
Early-types already in place at high redshift?
Short high-redshift starburst then mainly passive
ageing (monolithic evolution)
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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 lt 3)
  mergers (e.g. Cole et al. 2000, Hatton et al.
  2003)
• Continuous star formation over a Hubble time
• Compatible with early-type properties?

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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!

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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!

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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) fracdevgt0.95
• Cross-match with GALEX, visually inspect for
  contaminants and remove AGN (which could
  contaminate UV)

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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
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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

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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, OConnell et al. 2007)
• Widespread recent star formation in nearby
  early-types

Kaviraj et al., 2007, ApJS, 173, 619
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The high-redshift galaxy populationThe Chandra
Deep Field South (CDFS)

• Optical (U,B,V) photometry traces rest-frame UV
  beyond z0.5
• 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
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Rest-frame UV colours at high redshift(0.5ltzlt1)
Low z
Kaviraj et al., 2008, MNRAS, 388, 67
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Rest-frame UV colours at high redshift(0.5ltzlt1)

• 0.2 of early-types consistent with purely
  passive evolution since z3
• 1.2 of early-types consistent with purely
  passive evolution since z2
• SSP assumption, result is robust using either
  BC2003 or Yi (2003) stellar models

Kaviraj et al. 2008, MNRAS, 388, 67
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Recent star formation in high-z early-types

• Luminous galaxies (MVlt-21) form up to 10-15 of
  their mass after z1 (consistent with SAM
  predictions)
• Low-mass galaxies form 30-60 of their mass after
  z1

Kaviraj et al. 2008, MNRAS, 388, 67
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What drives the recent star formation?Indirect
evidence for minor mergers

• Numerical simulations of minor mergers (mass
  ratios 13 - 110)
• Satellite gas fractions gt 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
(arXiv0711.1493) see also Bezanson et al. 2009
(arXiv0903.2044)
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What drives the recent star formation?
Relaxed ETGs
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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?
COSMOS z0.6
Rest-frame (NUV-g)
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What drives the recent star formation?
COSMOS z0.6
Rest-frame (NUV-g)
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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
• Several 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
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SummaryarXiv0710.1311

• 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 z2
• Recent star formation mass fraction 1-5 at
  z0.1, 7-10 at z0.7
• Massive early-types form up to 10-15 of their
  mass after z1, low-mass early-types form 30-60
  of their mass after z1
• Most likely mechanism driving recent star
  formation is minor merging only mechanism that
  is consistent with all observational data