zCOSMOS 10k-bright spectroscopic sample

Abstract

Aims. We present the analysis of the U − V rest-frame color distribution and some spectral features as a function of mass and environment for a sample of early-type galaxies up to z = 1 extracted from the zCOSMOS spectroscopic survey. This analysis is used to place constraints on the relative importance of these two properties in controlling galaxy evolution. Methods. We used the zCOSMOS 10k-bright sample, limited to the AB magnitude range 15 < I < 22.5, from which we extracted two different subsamples of early-type galaxies. The first sample ("red galaxies") was selected using a photometric classification (2098 galaxies), while in the second case ("ETGs") we combined morphological, photometric, and spectroscopic properties to obtain a more reliable sample of elliptical, red, passive, early-type galaxies (981 galaxies). The analysis is performed at fixed mass to search for any dependence of the color distribution on environment, and at fixed environment to search for any mass dependence. Results. In agreement with the low redshift results of the SDSS, we find that the color distribution of red galaxies is not strongly dependent on environment for all mass bins, exhibiting only a weak trend such that galaxies in overdense regions (log 10 (1 + δ) ∼ 1.2) are redder than galaxies in underdense regions (log 10 (1 + δ) ∼ 0.1), with a difference of Δ(U − V) rest = 0.027 ± 0.008 mag. On the other hand, the dependence on mass is far more significant, and we find that the average colors of massive galaxies (log 10 (M/M ) ∼ 10.8) are redder by Δ(U − V) rest = 0.093 ± 0.007 mag than low-mass galaxies (log 10 (M/M ) ∼ 10) throughout the entire redshift range. We study the color-mass (U − V) rest ∝ S M · log 10 (M/M ) relation, finding a mean slope S M = 0.12 ± 0.005, while the color-environment (U − V) rest ∝ S δ · log 10 (1 + δ) relation is flatter, with a slope always smaller than S δ ≈ 0.04. The spectral analysis that we perform on our ETGs sample is in good agreement with our photometric results: we study the 4000 Å break and the equivalent width of the Hδ Balmer line, finding for D4000 a dependence on mass ( ΔD4000 = 0.11 ± 0.02 between log 10 (M/M ) ∼ 10.2 and log 10 (M/M ) ∼ 10.8), and a much weaker dependence on environment ( ΔD4000 = 0.05 ± 0.02 between high and low environment quartiles). The same is true for the equivalent width of Hδ, for which we measure a difference of ΔEW 0 (Hδ) = 0.28 ± 0.08 Å across the same mass range and no significant dependence on environment. By analyzing the lookback time of early-type galaxies, we support the possibility of a downsizing scenario, in which massive galaxies with a stronger D4000 and an almost constant equivalent width of Hδ formed their mass at higher redshift than lower mass ones. We also conclude that the main driver of galaxy evolution is the galaxy mass, the environment playing a subdominant role.

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

“…(i) extraction of the reddest galaxies with multi-band photometric spectral energy distributions (SEDs) compatible with the template SEDs of ETGs at z ∼ 0 or with old passive stellar populations [25];…”

“…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.…”

“…Passive ETGs have been selected by combining photometric, morphological and optical spectroscopic criteria, following the approach of Ref. [25]. Galaxies have been chosen with a reliable redshift measurement, a best-fit to the SED matching a local E-S0 template, weak/no emission lines (EW< 5Å), spheroidal morphology, and a K − 24µm color typical of E/S0 local galaxies (i.e.…”

“…K − 24µm< −0.5); for further details about the sample selection, see Ref. [25]. The stellar mass has been estimated from SED fitting of those galaxies, using a wide library built with BC03 models, exponentially delayed SFHs with SF R(t) ∝ t/τ 2 exp(−t/τ ) with 0.05 < τ < 1 Gyr, ages with 0 < t < 20 Gyr, dust reddening 0 < A V < 1 modeled with a Calzetti's extinction law (0 < A V < 0.6 in the case of values of age/τ > 4), solar metallicity, and a Chabrier IMF.…”

The dependence of AGN activity on environment in SDSS

“…Overdensity δ, for example, has been used as an efficient indicator of local environment which can be easily defined in observation (e.g. Bolzonella et al 2010;Moresco et al 2010;Peng et al 2012;Jian et al 2012;Kovač et al 2014;Darvish et al 2016). As argued in Muldrew et al (2012), the local environment which corresponds to scales internal to a halo is best probed using the nearest-neighbour methods.…”