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Keck Spectroscopy of Extremely Red Galaxies

Keck Spectroscopy of Extremely Red Galaxies. Michelle Doherty, Institute of Astronomy, Cambridge, UK. Andy Bunker, University of Exeter, UK Richard Ellis, Caltech Pat McCarthy, OCIW. INTRODUCTION. deep Keck spectroscopy of large sample of EROs in the LCIR survey

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Keck Spectroscopy of Extremely Red Galaxies

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  1. Keck Spectroscopy of Extremely Red Galaxies Michelle Doherty,Institute of Astronomy, Cambridge, UK Andy Bunker,University of Exeter, UK Richard Ellis,Caltech Pat McCarthy,OCIW

  2. INTRODUCTION • deep Keck spectroscopy of large sample of EROs in the LCIR survey • Selection criteria: I-H>3, H<20.5 • Individual and composite spectra – what type of objects are we seeing? • Split in spectral types, composite spectra and redshift distribution

  3. SAMPLE SELECTION • Las Campanas Infrared Survey (LCIR) – designed to identify evolved galaxies at z≥1 • I-H>3 selection versus canonical R-K>5 • McCarthy et al. (2001), Firth et al. (2002) predicted selection of evolved massive galaxies • 3 fields – each 170 arcmin2 • Range in no. densities: 0.4 arcmin-2 in CDFS, 0.7 arcmin-2 in SSA22 and 0.8 arcmin-2 in NTT

  4. OBSERVATIONS • LRIS – Low Resolution Imaging spectrograph => Dispersion 1.24Å/pix • DEIMOS – Deep Imaging Multi-Object Spectrograph => Dispersion 0.32Å/pix • Obs by Ellis, Firth, Bunker – Oct 2001 and Jan 2003 • unbiased selection for a subset • Wavelength range ~7000-9000Å

  5. SPECTRA • Extracted over 1.3", sky residuals masked • GDDS (Abraham et al. 2004) Templates

  6. WHAT ARE THEY? • Identify 44/73 redshifts => what have we missed? Too faint or too far • Large field-field variation in split of spectral types:

  7. WHAT ARE THEY? • Large fraction E+A – 30% of absorption line systems BUT =>This ranges from 0-50% across 3 fields • At 0.3<z<1.0 Tran et al. 2004 find 9% ==>EVOLUTION??

  8. COMPOSITE SPECTRA

  9. REDSHIFT DISTRIBUTION • Completeness • Redshift spikes in some fields • Late type uniform but other distributions affected by spikes • Overdensity in CDFS

  10. A Cluster in CDFS? z=1.22 • 5 galaxies over ~5Mpc • Velocity dispersion ~400km s-1 • add in FORS2 z’s – Vanzella et al. (2004) + Vanzella et al. ◊ Doherty et al. => Gravitationally bound cluster? Morphology/density relation at high-z Speculation!

  11. CONCLUSIONS • Cosmic variance a huge effect – large field-field variations in number densities and spectral types • See redshift spikes => old ellipticals tracing the overdensities • ~75% of our sample shows signs of old stellar population (only 28% with pure early-type spectra, i.e. E/S0) • ~70% exhibit signs of ongoing or recent star formation

  12. FUTURE WORK • 2 fields with HST imaging => morphological analysis • SED fits => break age-metallicity degeneracy using metallicities from spectral lines • Will be able to model ages, metallicity and morphology and hence determine nature of these objects in more detail (in particular those showing a mix of spectral features). • Velocity dispersion from G-band => masses

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