zCOSMOS – 10k-bright spectroscopic sample

Abstract

We present the galaxy stellar mass function (GSMF) to redshift z 1, based on the analysis of about 8500 galaxies with I < 22.5 (AB mag) over 1.4 deg 2 , which are part of the zCOSMOS-bright 10k spectroscopic sample. We investigate the total GSMF, as well as the contributions of early-and late-type galaxies (ETGs and LTGs, respectively), defined by different criteria (broad-band spectral energy distribution, morphology, spectral properties, or star formation activities). We unveil a galaxy bimodality in the global GSMF, whose shape is more accurately represented by 2 Schechter functions, one linked to the ETG and the other to the LTG populations. For the global population, we confirm a mass-dependent evolution ("mass-assembly downsizing"), i.e., galaxy number density increases with cosmic time by a factor of two between z = 1 and z = 0 for intermediate-to-low mass (log(M/M ) ∼ 10.5) galaxies but less than 15% for log(M/M ) > 11. We find that the GSMF evolution at intermediateto-low values of M (log(M/M ) < 10.6) is mostly explained by the growth in stellar mass driven by smoothly decreasing star formation activities, despite the redder colours predicted in particular at low redshift. The low residual evolution is consistent, on average, with ∼0.16 merger per galaxy per Gyr (of which fewer than 0.1 are major), with a hint of a decrease with cosmic time but not a clear dependence on the mass. From the analysis of different galaxy types, we find that ETGs, regardless of the classification method, increase in number density with cosmic time more rapidly with decreasing M, i.e., follow a top-down building history, with a median "building redshift" increasing with mass (z > 1 for log(M/M ) > 11), in contrast to hierarchical model predictions. For LTGs, we find that the number density of blue or spiral galaxies with log(M/M ) > 10 remains almost constant with cosmic time from z ∼ 1. Instead, the most extreme population of star-forming galaxies (with high specific star formation), at intermediate/high-mass, rapidly decreases in number density with cosmic time. Our data can be interpreted as a combination of different effects. Firstly, we suggest a transformation, driven mainly by SFH, from blue, active, spiral galaxies of intermediate mass to blue quiescent and subsequently (1−2 Gyr after) red, passive types of low specific star formation. We find an indication that the complete morphological transformation, probably driven by dynamical processes, into red spheroidal galaxies, occurred on longer timescales or followed after 1−2 Gyr. A continuous replacement of blue galaxies is expected to be accomplished by low-mass active spirals increasing their stellar mass. We estimate the growth rate in number and mass density of the red galaxies at different redshifts and masses. The corresponding fraction of blue galaxies that, at any given time, is transforming into red galaxies per Gyr, due to the quenching of their SFR, is on average ∼25% for log(M/M ) < 11. We conclude that the build-up of galaxies and in particular of...

Improved constraints on the expansion rate of the Universe up to z ∼ 1.1 from the spectroscopic evolution of cosmic chronometers

“…with similar redshift of formation [15]), which means that they represent an homogeneous sample in terms of ages of formation, and can be used as "cosmic chornometers". Moreover, basing on the stellar mass function [16] and clustering [63], it has been found that this population of galaxies experienced negligible major merging events in a large fraction of the redshift range considered in our study [64], i.e. they did not increase their mass significantly in the time comprised between the redshifts adopted for the differential approach, meaning that the contamination to the initial population (with a given age of formation) is minimal.…”

“…Based on a plethora of observational results, there is general agreement that these are typically massive (M stars ∼ 10 11 M ⊙ ) early-type galaxies (ETGs hereafter) which formed the vast majority (>90%) of their stellar mass at high-redshifts (z > 2 − 3) very rapidly (∼0.1-0.3 Gyr) and have experienced only minor subsequent episode of star formation, therefore being the oldest objects at all redshifts (e.g. [13][14][15][16]). Thus, a differential dating of their stellar populations provides dt in Eq.…”

“…see [13][14][15][16][25][26][27][28]). With the described selection criteria, thus, we have considered the reddest, oldest, passive envelope of ETGs in the entire redshift range, i.e.…”

“…However, this fact does not pose a major concern for the analysis, since different techniques recover very similar stellar masses for passively evolving galaxies (e.g. see [16,42]), and the primary parameter which may significantly bias the estimate is the IMF, which has been corrected for. Moreover it is worth emphasize that the masses estimated do not directly affect the scientific results, but are only used to select the most massive galaxies in all the surveys, independently on their absolute value.…”

“…[28], who used a wide library of BC03 models (with exponentially decaying SFHs, SF R(t) ∝ 1/τ exp(−t/τ ), with 0.1 < τ < 15 Gyr, ages in the range 10 7 < t < 10 10.2 yr, dust reddening 0 < E B−V < 1 modeled with SMC law, metallicities in the range 0.02 < Z/Z ⊙ < 2.5, and a Salpeter IMF). The stellar masses were rescaled to a Chabrier IMF by subtracting 0.23 dex from logM (see [16,42]), and selected to have log(M/M ⊙ ) > 10.6. In conclusion, the K20 ETGs sample contains 50 galaxies in the redshift range 0.26 < z < 1.16.…”