Stellar mass-to-light ratios from galaxy spectra: how accurate can they be?

Stellar masses play a crucial role in the exploration of galaxy properties and the evolution of the galaxy population. In this paper, we explore the minimum possible uncertainties in stellar mass-to-light (M/L) ratios from the assumed star formation history (SFH) and metallicity distribution, with the goals of providing a minimum set of requirements for observational studies. We use a large Monte Carlo library of SFHs to study as a function of galaxy spectral type and signal-to-noise ratio (S/N) the statistical uncertainties of M/L values using either absorption-line data or broad band colors. The accuracy of M/L estimates can be significantly improved by using metal-sensitive indices in combination with age-sensitive indices, in particular for galaxies with intermediate-age or young stellar populations. While M/L accuracy clearly depends on the spectral S/N ratio, there is no significant gain in improving the S/N much above 50/pix and limiting uncertainties of 0.03 dex are reached. Assuming that dust is accurately corrected or absent and that the redshift is known, color-based M/L estimates are only slightly more uncertain than spectroscopic estimates (at comparable spectroscopic and photometric quality), but are more easily affected by systematic biases. This is the case in particular for galaxies with bursty SFHs (high Hdelta at fixed D4000), the M/L of which cannot be constrained any better than 0.15 dex with any indicators explored here. Finally, we explore the effects of the assumed prior distribution in SFHs and metallicity, finding them to be higher for color-based estimates.Comment: accepted for publication on ApJ

Charting the evolution of the ages and metallicities of massive galaxies since z=0.7

The stellar populations of intermediate-redshift galaxies can shed light onto the growth of massive galaxies in the last 8 billion years. We perform deep, multi-object rest-frame optical spectroscopy with IMACS/Magellan of ~70 galaxies in the E-CDFS with redshift 0.6522.7 and stellar mass >10^{10}Msun. Following the Bayesian approach adopted for previous low-redshift studies, we constrain the stellar mass, mean stellar age and stellar metallicity of individual galaxies from stellar absorption features. We characterize for the first time the dependence of stellar metallicity and age on stellar mass at z~0.7 for all galaxies and for quiescent and star-forming galaxies separately. These relations for the whole sample have a similar shape as the z=0.1 SDSS analog, but are shifted by -0.28 dex in age and by -0.13 dex in metallicity, at odds with simple passive evolution. We find that no additional star formation and chemical enrichment are required for z=0.7 quiescent galaxies to evolve into the present-day quiescent population. However, this must be accompanied by the quenching of a fraction of z=0.7 Mstar>10^{11}Msun star-forming galaxies with metallicities comparable to those of quiescent galaxies, thus increasing the scatter in age without affecting the metallicity distribution. However rapid quenching of the entire population of massive star-forming galaxies at z=0.7 would be inconsistent with the age/metallicity--mass relation for the population as a whole and with the metallicity distribution of star-forming galaxies only, which are on average 0.12 dex less metal-rich than their local counterparts. This indicates chemical enrichment until the present in at least a fraction of the z=0.7 massive star-forming galaxies.[abridged]Comment: accepted for publication on ApJ, 26 pages, 13 figure

Gravitational lens candidates in the E-CDFS

We report ten lens candidates in the E-CDFS from the GEMS survey. Nine of the systems are new detections and only one of the candidates is a known lens system. For the most promising five systems including the known lens system, we present results from preliminary lens mass modelling, which tests if the candidates are plausible lens systems. Photometric redshifts of the candidate lens galaxies are obtained from the COMBO-17 galaxy catalog. Stellar masses of the candidate lens galaxies within the Einstein radius are obtained by using the $z$-band luminosity and the $V-z$ color-based stellar mass-to-light ratios. As expected, the lensing masses are found to be larger than the stellar masses of the candidate lens galaxies. These candidates have similar dark matter fractions as compared to lenses in SLACS and COSMOS. They also roughly follow the halo mass-stellar mass relation predicted by the subhalo abundance matching technique. One of the candidate lens galaxies qualifies as a LIRG and may not be a true lens because the arc-like feature in the system is likely to be an active region of star formation in the candidate lens galaxy. Amongst the five best candidates, one is a confirmed lens system, one is a likely lens system, two are less likely to be lenses and the status of one of the candidates is ambiguous. Spectroscopic follow-up of these systems is still required to confirm lensing and/or for more accurate determination of the lens masses and mass density profiles.Comment: 12 pages, 5 figures, 3 tables, ApJ accepte

SPIDER X - Environmental effects in central and satellite early-type galaxies through the stellar fossil record

A detailed analysis of how environment affects the star formation history of early-type galaxies (ETGs) is undertaken via high signal to noise ratio stacked spectra obtained from a sample of 20,977 ETGs (morphologically selected) from the SDSS-based SPIDER survey. Two major parameters are considered for the study: the central velocity dispersion (sigma), which relates to local drivers of star formation, and the mass of the host halo, which relates to environment-related effects. In addition, we separate the sample between centrals (the most massive galaxy in a halo) and satellites. We derive trends of age, metallicity, and [alpha/Fe] enhancement, with sigma. We confirm that the major driver of stellar population properties in ETGs is velocity dispersion, with a second-order effect associated to the central/satellite nature of the galaxy. No environmental dependence is detected for satellite ETGs, except at low sigma - where satellites in groups or in the outskirts of clusters tend to be younger than those in the central regions of clusters. In contrast, the trends for centrals show a significant dependence on halo mass. Central ETGs in groups (i.e. with a halo mass >10^12.5 M_Sun) have younger ages, lower [alpha/Fe], and higher internal reddening, than "isolated" systems (i.e. centrals residing in low-mass, <10^12.5 M_Sun, halos). Our findings imply that central ETGs in groups formed their stellar component over longer time scales than "isolated" centrals, mainly because of gas-rich interactions with their companion galaxies.Comment: 22 pages, 19 figures, accepted for publication in MNRA

The large molecular gas fraction of post-starburst galaxies at z > 1

Post-starburst galaxies are sources that had the last major episode of star formation about 1 Gyr before the epoch of the observations and are on their way to quiescence. It is important to study such galaxies at redshift z > 1, during their main quenching phase, and estimate their molecular gas content to constrain the processes responsible for the cessation of star formation. We present CO(3-2) ALMA observations of two massive (Mstar ~ 5 x 10^10 Msun) post-starburst galaxies at z > 1. We measure their molecular gas fraction to be f_H2 = M_H2/Mstar ~ 8% - 16%, consistent with z < 1 post-starburst galaxies from the literature. The star formation efficiency of our targets is ~ 10x lower than that of star-forming galaxies at similar redshift, and they are outliers of the f_H2 - specific star formation rate (sSFR) relation of star-forming galaxies, as they have larger f_H2 than expected given their sSFR. The gas fraction of post-starbursts from our sample and the literature correlates with the Dn4000 spectral index, a proxy of the stellar population age. This suggests that their gas content decreases after the last major burst of star formation. Finally, one of our targets is undergoing a major merger phase with two highly star-forming companions. This hints at a picture where a perturber event (e.g., major merger) quenches star formation without completely removing the molecular gas.Comment: Accepted for publication in MNRA

On the connection between galactic downsizing and the most fundamental galactic scaling relations

In their evolution, star-forming galaxies are known to follow scaling relations between some fundamental physical quantities, such as the mass-metallicity and the main sequence relations. We aim at studying the evolution of galaxies that, at a given redshift, lie simultaneously on the mass-metallicity and main sequence relations (MZR, MSR). To this aim, we use the analytical, 'leaky-box' chemical evolution model of Spitoni et al. (2017), in which galaxy evolution is described by an infall timescale $\tau$ and a wind efficiency $\lambda$. We provide a detailed analysis of the temporal evolution of galactic metallicity, stellar mass, mass-weighted age and gas fraction. The evolution of the galaxies lying on the MZR and MSR at $z\sim0.1$ suggests that the average infall time-scale in two different bins of stellar masses ($M_{\star}10^{10} M_{\odot}$) decreases with decreasing redshift. This means that at each redshift, only the youngest galaxies can be assembled on the shortest timescales and still belong to the star-forming MSR. In the lowest mass bin, a decrease of the median $\tau$ is accompanied by an increase of the median $\lambda$ value. This implies that systems which have formed at more recent times will need to eject a larger amount of mass to keep their metallicity at low values. Another important result is that galactic downsizing, as traced by the age-mass relation, is naturally recovered by imposing that local galaxies lie on both the MZR and MSR. Finally, we study the evolution of the hosts of C$_{\rm IV}$ -selected AGN, which at $z\sim 2$ follow a flat MZR, as found by Mignoli et al. (2019). If we impose that these systems lie on the MSR, at lower redshifts we find an 'inverted' MZR, meaning that some additional processes must be at play in their evolution.Comment: Accepted for publication in Astronomy and Astrophysics (A&A), 20 pages, 26 figure

A new population of recently quenched elliptical galaxies in the SDSS

We use the Sloan Digital Sky Survey to investigate the properties of massive elliptical galaxies in the local Universe (z\leq0.08) that have unusually blue optical colors. Through careful inspection, we distinguish elliptical from non-elliptical morphologies among a large sample of similarly blue galaxies with high central light concentrations (c_r\geq2.6). These blue ellipticals comprise 3.7 per cent of all c_r\geq2.6 galaxies with stellar masses between 10^10 and 10^11 h^{-2} {\rm M}_{\sun}. Using published fiber spectra diagnostics, we identify a unique subset of 172 non-star-forming ellipticals with distinctly blue urz colors and young (< 3 Gyr) light-weighted stellar ages. These recently quenched ellipticals (RQEs) have a number density of 2.7-4.7\times 10^{-5}\,h^3\,{\rm Mpc}^{-3} and sufficient numbers above 2.5\times10^{10} h^{-2} {\rm M}_{\sun} to account for more than half of the expected quiescent growth at late cosmic time assuming this phase lasts 0.5 Gyr. RQEs have properties that are consistent with a recent merger origin (i.e., they are strong `first-generation' elliptical candidates), yet few involved a starburst strong enough to produce an E+A signature. The preferred environment of RQEs (90 per cent reside at the centers of < 3\times 10^{12}\,h^{-1}{\rm M}_{\sun} groups) agrees well with the `small group scale' predicted for maximally efficient spiral merging onto their halo center and rules out satellite-specific quenching processes. The high incidence of Seyfert and LINER activity in RQEs and their plausible descendents may heat the atmospheres of small host halos sufficiently to maintain quenching.Comment: 26 pages, 9 figures. Revised version; accepted for publication in MNRA

The cosmic evolution of the spatially-resolved star formation rate and stellar mass of the CALIFA survey

We investigate the cosmic evolution of the absolute and specific star formation rate (SFR, sSFR) of galaxies as derived from a spatially-resolved study of the stellar populations in a set of 366 nearby galaxies from the CALIFA survey. The analysis combines GALEX and SDSS images with the 4000 break, H_beta, and [MgFe] indices measured from the datacubes, to constrain parametric models for the SFH, which are then used to study the cosmic evolution of the star formation rate density (SFRD), the sSFR, the main sequence of star formation (MSSF), and the stellar mass density (SMD). A delayed-tau model, provides the best results, in good agreement with those obtained from cosmological surveys. Our main results from this model are: a) The time since the onset of the star formation is larger in the inner regions than in the outer ones, while tau is similar or smaller in the inner than in the outer regions. b) The sSFR declines rapidly as the Universe evolves, and faster for early than for late type galaxies, and for the inner than for the outer regions of galaxies. c) SFRD and SMD agree well with results from cosmological surveys. At z< 0.5, most star formation takes place in the outer regions of late spiral galaxies, while at z>2 the inner regions of the progenitors of the current E and S0 are the major contributors to SFRD. d) The inner regions of galaxies are the major contributor to SMD at z> 0.5, growing their mass faster than the outer regions, with a lookback time at 50% SMD of 9 and 6 Gyr for the inner and outer regions. e) The MSSF follows a power-law at high redshift, with the slope evolving with time, but always being sub-linear. f) In agreement with galaxy surveys at different redshifts, the average SFH of CALIFA galaxies indicates that galaxies grow their mass mainly in a mode that is well represented by a delayed-tau model, with the peak at z~2 and an e-folding time of 3.9 Gyr.Comment: 23 pages, 16 figures, 6 tables, accepted for publication in Astronomy & Astrophysics. *Abridged abstract

Resolving the age bimodality of galaxy stellar populations on kpc scales

Galaxies in the local Universe are known to follow bimodal distributions in the global stellar populations properties. We analyze the distribution of the local average stellar-population ages of 654,053 sub-galactic regions resolved on ~1-kpc scales in a volume-corrected sample of 394 galaxies, drawn from the CALIFA-DR3 integral-field-spectroscopy survey and complemented by SDSS imaging. We find a bimodal local-age distribution, with an old and a young peak primarily due to regions in early-type galaxies and star-forming regions of spirals, respectively. Within spiral galaxies, the older ages of bulges and inter-arm regions relative to spiral arms support an internal age bimodality. Although regions of higher stellar-mass surface-density, mu*, are typically older, mu* alone does not determine the stellar population age and a bimodal distribution is found at any fixed mu*. We identify an "old ridge" of regions of age ~9 Gyr, independent of mu*, and a "young sequence" of regions with age increasing with mu* from 1-1.5 Gyr to 4-5 Gyr. We interpret the former as regions containing only old stars, and the latter as regions where the relative contamination of old stellar populations by young stars decreases as mu* increases. The reason why this bimodal age distribution is not inconsistent with the unimodal shape of the cosmic-averaged star-formation history is that i) the dominating contribution by young stars biases the age low with respect to the average epoch of star formation, and ii) the use of a single average age per region is unable to represent the full time-extent of the star-formation history of "young-sequence" regions.Comment: 17 pages, 11 figures, MNRAS accepte

The Cosmic Evolution of Metallicity from the SDSS Fossil Record

We present the time evolution of the stellar metallicity for SDSS galaxies, a sample that spans five orders of magnitude in stellar mass (10^7 - 10^{12} Msun). Assuming the BC03 stellar population models, we find that more massive galaxies are more metal-rich than less massive ones at all redshifts; the mass-metallicity relation is imprinted in galaxies from the epoch of formation. For galaxies with present stellar masses > 10^{10} Msun, the time evolution of stellar metallicity is very weak, with at most 0.2-0.3 dex over a Hubble time- for this reason the mass-metallicity relation evolves little with redshift. However, for galaxies with present stellar masses < 10^{10} Msun, the evolution is significant, with metallicity increasing by more than a decade from redshift 3 to the present. By being able to recover the metallicity history, we have managed to identify the origin of a recent discrepancy between the metallicity recovered from nebular lines and absorption lines. As expected, we show that the young population dominates the former while the old population the latter. We have investigated the dependence on the stellar models used and find that older stellar population synthesis codes do not produce a clear result. Finally, we have explored the relationship between cluster environment and metallicity, and find a strong correlation in the sense that galaxies in high density regions have high metallicity.Comment: Submitted to MNRA