In and out star formation in z ~ 1.5 quiescent galaxies from rest-frame UV spectroscopy and the far-infrared^⋆

¹ School of Physics, Korea Institute for Advanced Study, Hoegiro 85, Dongdaemun-gu, 02455 Seoul, Republic of Korea
e-mail: rgobat@kias.re.kr
² Laboratoire AIM-Paris-Saclay, CEA/DSM-CNRS–Université Paris Diderot, Irfu/Service d’Astrophysique, CEA Saclay, Orme des Merisiers, 91191 Gif-sur-Yvette, France
³ Department of Physics, Ludwig-Maximilians-Universität, Scheinerstr. 1, 81679 München, Germany
⁴ INAF–IASF, via Bassini 15, 20133 Milano, Italy
⁵ INAF–Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁶ Department of Astronomy, University of Science and Technology of China, Hefei, 230026, PR China
⁷ Key Laboratory of Modern Astronomy and Astrophysics in Ministry of Education, School of Astronomy and Space Science, Nanjing University, 210093 Nanjing, PR China
⁸ Institute of Cosmology and Gravitation, Dennis Sciama Building, Burnaby Road, Portsmouth PO1 3FX, UK
⁹ Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Mariesvej 30, 2100 Copenhagen, Denmark
¹⁰ Institute for Astronomy, Astrophysics, Space Applications and Remote Sensing, National Observatory of Athens, 15236 Athens, Greece
¹¹ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany
¹² Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK
¹³ Institute for Astronomy, ETH Zurich, 8093 Zurich, Switzerland
¹⁴ Department of Physics and Astronomy, University of Bologna, Viale Berti Pichat 6/2, 30127 Bologna, Italy
¹⁵ INAF–Osservatorio Astronomico di Trieste, via G.B. Tiepolo 11, 34143 Trieste, Italy
¹⁶ Subaru Telescope, National Astronomical Observatory of Japan, National Institutes of Natural Sciences (NINS), 650 North A’ohoku Place, Hilo, HI 96720, USA
¹⁷ Department of Astronomical Science, SOKENDAI (The Graduate University for Advanced Studies), 650 North A’ohoku Place, Hilo, HI 96720, USA
¹⁸ INAF–Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
¹⁹ Astronomy Centre, Department of Physics and Astronomy, University of Sussex, Brighton, BN1 9QH, UK

Received: 5 October 2016
Accepted: 20 December 2016

Abstract

We present a sample of 34 spectroscopically confirmed BzK-selected ~10¹¹M_⊙ quiescent galaxies (pBzK) in the COSMOS field. The targets were initially observed with VIMOS on the VLT to facilitate the calibration of the photometric redshifts of massive galaxies at z ≳ 1.5. Here we describe the reduction and analysis of the data, and the spectrophotometric properties of these pBzK galaxies. In particular, using a spatially resolved median 2D spectrum, we find that the fraction of stellar populations with ages <1 Gyr is at least 3 times higher in the outer regions of the pBzK galaxies than in their cores. This results in a mild age gradient of Δage ≤ 0.4 Gyr over ~6 kpc and suggests either the occurrence of widespread rejuvenation episodes or that inside-out quenching played a role in the passivization of this galaxy population. We also report on low-level star formation rates derived from the [OII]3727 Å emission line, with SFR_OII ~ 3.7−4.5M_⊙ yr^-1. This estimate is confirmed by an independent measurement on a separate sample of similarly-selected quiescent galaxies in the COSMOS field, using stacked far-infrared data (SFR_FIR ~ 2−4M_⊙ yr^-1). This second, photometric sample also displays significant excess at 1.4 GHz, suggestive of the presence of radio-mode AGN activity.