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Published August 10, 2012 | Published
Journal Article Open

Deep Near-infrared Spectroscopy of Passively Evolving Galaxies at z ≳ 1.4


We present the results of new near-IR spectroscopic observations of passive galaxies at z ≳ 1.4 in a concentration of BzK-selected galaxies in the COSMOS field. The observations have been conducted with Subaru/MOIRCS, and have resulted in absorption lines and/or continuum detection for 18 out of 34 objects. This allows us to measure spectroscopic redshifts for a sample that is almost complete to K_AB = 21. COSMOS photometric redshifts are found in fair agreement overall with the spectroscopic redshifts, with a standard deviation of ~0.05; however, ~30% of objects have photometric redshifts systematically underestimated by up to ~25%. We show that these systematic offsets in photometric redshifts can be removed by using these objects as a training set. All galaxies fall in four distinct redshift spikes at z = 1.43, 1.53, 1.67, and 1.82, with this latter one including seven galaxies. SED fits to broadband fluxes indicate stellar masses in the range of ~4-40 × 10^10 M_☉ and that star formation was quenched ~1 Gyr before the cosmic epoch at which they are observed. The spectra of several individual galaxies have allowed us to measure their Hδ_F indices and the strengths of the 4000 Å break, which confirms their identification as passive galaxies, as does a composite spectrum resulting from the co-addition of 17 individual spectra. The effective radii of the galaxies have been measured on the COSMOS HST/ACS i_(F814W)-band image, confirming the coexistence at these redshifts of passive galaxies, which are substantially more compact than their local counterparts with others that follow the local effective radius-stellar mass relation. For the galaxy with the best signal-to-noise spectrum we were able to measure a velocity dispersion of 270 ± 105 km s^(–1) (error bar including systematic errors), indicating that this galaxy lies closely on the virial relation given its stellar mass and effective radius.

Additional Information

© 2012 American Astronomical Society. Received 2012 March 4; accepted 2012 June 7; published 2012 July 23. Based on data collected at the Subaru telescope, which is operated by the National Astronomical Observatory of Japan. (Proposal IDs: S09A-043 and S10A-058). We thank the staff of the Subaru telescope, especially Ichi Tanaka, for supporting the observations. We also thank Mariska Kriek for a permission to use FAST, Stephan Charlot for providing the stellar population synthesis models before publication, Alexis Finoguenov for computing the upper limits from XMM-Newton data for the overdense regions, Masayuki Tanaka for checking the existence of red sequence cluster around redshift spikes, and Masafumi Yagi for fruitful discussion.We are grateful to the anonymous referee for her/his comments. This work has been partly supported by the ASI grant "COFIS-Analisi Dati" and by the INAF grant "PRIN-2008" and is supported by a Grant-in-Aid for Science Research (No. 23224005) by the Japanese Ministry of Education, Culture, Sports, Science and Technology. Facility: Subaru (NAOJ)

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