A mass threshold in the number density of passive galaxies at z ~ 2

¹ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00040 Monteporzio, Italy
e-mail: veronica.sommariva@oa-roma.inaf.it
² INAF – Osservatorio Astronomico di Trieste, Via G.B. Tiepolo 11, 34131 Trieste, Italy
³ SUPA, Institute for Astronomy, University of Edinburgh, Royal Observatory, Edinburgh, EH9 3HJ, UK
⁴ Space Telescope Science Institute, 3700 San Martin Dr., Baltimore, MD 21218, USA
⁵ Astronomy Department, University of Massachusetts, Amherst, MA 01003, USA
⁶ Kavli Institute for Particle Astrophysics and Cosmology, 452 Lomita Mall, Stanford, CA 94305-4085  
⁷ Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK
⁸ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA

Received: 17 July 2013
Accepted: 1 September 2014

Abstract

The process that quenched star formation in galaxies at intermediate and high redshifts is still the subject of considerable debate. One way to investigate this puzzling issue is to study the number density of quiescent galaxies at z ≃ 2 and its dependence on mass. Here we present the results of a new study based on very deep K_s-band imaging (with the HAWK-I instrument on the VLT) of two HST CANDELS fields (the UKIDSS Ultra-deep survey (UDS) field and GOODS-South). The new HAWK-I data (taken as part of the HUGS VLT Large Program) reach detection limits of K_s> 26 (AB mag). We have combined this imaging with the other ground-based and HST data in the CANDELS fields to select a sample of passively-evolving galaxies in the redshift range 1.4 <z< 2.5 (via the pBzK color-based selection criterion). Thanks to the depth and wide area coverage of our imaging, we have been able to extend the selection of quiescent galaxies to a magnitude fainter than previous analyses. Through extensive simulations we demonstrate, for the first time, that the observed turnover in the number of quiescent galaxies at K ≥ 22 is not due to incompleteness, but is real. This has enabled us to establish unambiguously that the number counts of quiescent galaxies at z ≃ 2 flatten and slightly decline at magnitudes fainter than K_s ~ 22 (AB mag.), in contrast to the number density of star-forming galaxies, which continues to rise to fainter magnitudes. We show that this trend corresponds to a stellar mass threshold M_∗ ≃ 10^10.8M_⊙ below which the mechanism that halts the star formation in high-redshift galaxies seems to be inefficient. We also show that, while pBzK galaxies at K< 23 are in the redshift range 1.4 <z< 2.5, as expected, at K> 23 a higher redshift population of z ≃ 3pBzK galaxies is detected and dominates the counts at the faintest magnitudes. Finally, we compare the observed pBzK number counts with those of quiescent galaxies extracted from four different semi-analytic models. We find that only two of these models reproduce the observed trend in the number counts, even qualitatively, and that none of the models provides a statistically acceptable description of the number density of quiescent galaxies at these redshifts. We conclude that the mass function of quiescent galaxies as a function of redshift continues to present a key and demanding challenge for proposed models of galaxy formation and evolution.