2009ApJ...700..221K -
Astrophys. J., 700, 221-231 (2009/July-3)
An ultra-deep near-infrared spectrum of a compact quiescent galaxy at z = 2.2.
KRIEK M., VAN DOKKUM P.G., LABBE I., FRANX M., ILLINGWORTH G.D., MARCHESINI D. and QUADRI R.F.
Abstract (from CDS):
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☉/yr, an age of 1.3-2.2 Gyr, and a stellar mass of ∼2x1011 M☉. 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 ≲8% 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 ≲10% 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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Journal keyword(s):
galaxies: evolution - galaxies: formation - galaxies: high-redshift
Simbad objects:
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