Galaxy properties as revealed by MaNGA – I. Constraints on IMF and M*/L gradients in ellipticals

Abstract

We estimate ages, metallicities, α-element abundance ratios and stellar initial mass functions of elliptical (E) and S0 galaxies from the MaNGA-DR15 survey. We stack spectra and use a variety of single stellar population synthesis models to interpret the absorption line strengths in these spectra. We quantify how these properties vary across the population, as well as with galactocentric distance. This paper is the first of a series and is based on a sample of pure elliptical galaxies at z ≤ 0.08. We confirm previous work showing that IMFs in Es with the largest luminosity (L r ) and central velocity dispersion (σ 0 ) appear to be increasingly bottom heavy towards their centres. For these galaxies the stellar mass-to-light ratio decreases at most by a factor of 2 from the central regions to R e . In contrast, for lower L r and σ 0 galaxies, the IMF is shallower and M * /L r in the central regions is similar to the outskirts, although quantitative estimates depend on assumptions about element abundance gradients.Accounting self-consistently for these gradients when estimating both M * and M dyn brings the two into good agreement: gradients reduce M dyn by ∼ 0.2 dex while only slightly increasing the M * inferred using a Kroupa IMF. This is a different resolution of the M * -M dyn discrepancy than has been followed in the recent literature where M * of massive galaxies is increased by adopting a Salpeter IMF throughout the galaxy while leaving M dyn unchanged. A companion paper discusses how stellar population differences are even more pronounced if one separates slow from fast rotators. Graves et al. 2009). We now know that the mix of stars in galaxies varies across the galaxy population, over and above the obvious variations with morphology across the Hubble sequence.In this and the following papers of this series, we focus almost exclusively on early-type galaxies (Es and S0s) since late-type galaxies (Spirals) have gas and dust which complicate the spectral analysis. In fact, in this paper, we only study Es (we study S0s in paper III -in prep.). However, even amongst Es, the stellar populations depend on other global properties such as velocity dispersion, luminosity, size, etc. (Trager et al. 1998;Thomas et al. 2005;Bernardi et al. 2005Bernardi et al. , 2006Bernardi et al. , 2011.In addition to varying across the early-type galaxy population, there are stellar population gradients even within does. These two estimates are thought to provide two distinct routes -albeit with rather different systematic biases -to the same underlying physical quantity.

A deep learning approach to test the small-scale galaxy morphology and its relationship with star formation activity in hydrodynamical simulations

“…Given the relatively low amount of star forming gas in the simulated passive galaxies, most objects in this population are modelled using simple stellar populations evolving on the 'Padova 1994' evolutionary tracks and a Chabrier (2003) Initial Mass Function (IMF, see Rodriguez-Gomez et al 2019 for more details). However, several observational studies have also reported IMF gradients in passive galaxies (e.g., La Barbera et al 2016;Conroy et al 2017;Domínguez Sánchez et al 2019 only to name a few), which are not modelled here. Since all stars are formed according to a Chabrier (2003) IMF in our simulations (Vogelsberger et al 2013;Pillepich et al 2018a), we are unable to quantify how the assumption of a universal IMF affects our results.…”

A precise benchmark for cluster scaling relations: Fundamental Plane, Mass Plane, and IMF in the Coma cluster from dynamical models

“…Hence the effect of M/L gradients on the measured IMF-σ relation is unlikely to be larger than the uncertainties presented in the relationship here. However, the test on the effect of M/L gradients is simplistic, and a complete understanding should take into account the variation of the gradient with other galaxy properties (Domínguez Sánchez et al 2019). Such an analysis is beyond the scope of this study.…”

“…In order to study the effect of these gradients, we introduced a M/L gradient and recreated the dynamical models for the two samples of galaxies. The M/L gradient used in this test was based on the results of Domínguez Sánchez et al (2019) where the authors studied gradients of various stellar population properties within elliptical and S0 galaxies of the MaNGA-DR15 survey (Aguado et al 2019). Within their sample, the authors find that galaxies with the highest central velocity dispersions and absolute r -band magnitudes demonstrate the strongest but modest gradient, with M/L never varying more than a factor of two out to 0.8Re.…”

The Central Dark Matter Fraction of Massive Early-Type Galaxies

“…This can strongly affect the conclusions on the central DM fraction in these extreme cases. On the other hand, "non-universality" of the IMF, i.e., the systematic variation of the IMF with mass (or velocity dispersion), from a bottom-lighter (i.e., "lower-mass") IMF for low mass ETGs to a bottom-heavier (i.e., "higher-mass") IMF in massive galaxies, has accumulated convincing evidences in the last decade (e.g., Treu et al, 2010;Cappellari et al, 2012;Conroy and van Dokkum 2012;Spiniello et al, 2012;Goudfrooij and Kruijssen 2013;La Barbera et al, 2013;Tortora et al, 2013;Martín-Navarro et al, 2015;Li et al, 2017;Domínguez Sánchez et al, 2019), despite, also in this case, contradicting evidences pointing to bottom-light IMFs for massive systems have been reported (Smith et al, 2015). This nonuniversal IMF can, in principle, incorporate most (if not all) of the "apparent" DM fraction trend with mass (e.g., Thomas et al, 2011;Tortora et al, 2013).…”