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Title:
An Ultra-Deep Near-Infrared Spectrum of a Compact Quiescent Galaxy at z = 2.2
Authors:
Kriek, Mariska; van Dokkum, Pieter G.; Labbé, Ivo; Franx, Marijn; Illingworth, Garth D.; Marchesini, Danilo; Quadri, Ryan F.
Affiliation:
AA(Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA ), AB(Department of Astronomy, Yale University, New Haven, CT 06520, USA ), AC(Hubble Fellow, Carnegie Observatories, Pasadena, CA 91101, USA ), AD(Leiden Observatory, Leiden University, NL-2300 RA Leiden, Netherlands ), AE(UCO/Lick Observatory, University of California, Santa Cruz, CA 95064, USA), AF(Department of Astronomy, Yale University, New Haven, CT 06520, USA ), AG(Leiden Observatory, Leiden University, NL-2300 RA Leiden, Netherlands )
Publication:
The Astrophysical Journal, Volume 700, Issue 1, pp. 221-231 (2009). (ApJ Homepage)
Publication Date:
07/2009
Origin:
IOP
ApJ Keywords:
galaxies: evolution, galaxies: formation, galaxies: high-redshift
DOI:
10.1088/0004-637X/700/1/221
Bibliographic Code:
2009ApJ...700..221K

Abstract

Several recent studies have shown that about half of the massive galaxies at z ~ 2 are in a quiescent phase. Moreover, these galaxies are commonly found to be ultra-compact with half-light radii of ~1 kpc. We have obtained a ~29 hr spectrum of a typical quiescent, ultra-dense galaxy at z = 2.1865 with the Gemini Near-Infrared Spectrograph. The spectrum exhibits a strong optical break and several absorption features, which have not previously been detected in z > 2 quiescent galaxies. Comparison of the spectral energy distribution with stellar population synthesis models implies a low star formation rate (SFR) of 1-3 M sun yr-1, an age of 1.3-2.2 Gyr, and a stellar mass of ~2 × 1011 M sun. We detect several faint emission lines, with emission-line ratios of [N II]/Hα, [S II]/Hα, and [O II]/[O III] typical of low-ionization nuclear emission-line regions. Thus, neither the stellar continuum nor the nebular emission implies active star formation. The current SFR is <1% of the past average SFR. If this galaxy is representative of compact quiescent galaxies beyond z = 2, it implies that quenching of star formation is extremely efficient and also indicates that low luminosity active galactic nuclei (AGNs) could be common in these objects. Nuclear emission is a potential concern for the size measurement. However, we show that the AGN contributes lsim8% to the rest-frame optical emission. A possible post-starburst population may affect size measurements more strongly; although a 0.5 Gyr old stellar population can make up lsim10% of the total stellar mass, it could account for up to ~40% of the optical light. Nevertheless, this spectrum shows that this compact galaxy is dominated by an evolved stellar population.
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