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Register a new userConnection between dynamically derived IMF normalisation and stellar population parameters
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We report on empirical trends between the dynamically determined stellar initial mass function (IMF) and stellar population properties for a complete, volume-limited sample of 260 early-type galaxies from the Atlas3D project. We study trends between our dynamically-derived IMF normalisation and absorption line strengths, and interpret these via single stellar population- (SSP-) equivalent ages, abundance ratios (measured as [alpha/Fe]), and total metallicity, [Z/H]. We find that old and alpha-enhanced galaxies tend to have on average heavier (Salpeter-like) mass normalisation of the IMF, but stellar population does not appear to be a good predictor of the IMF, with a large range of normalisation at a given population parameter. As a result, we find weak IMF-[alpha/Fe] and IMF-age correlations, and no significant IMF-[Z/H] correlation. The observed trends appear significantly weaker than those reported in studies that measure the IMF normalisation via low-mass star demographics inferred through stellar spectral analysis.
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In this contributed talk I present recent results on the connection between stellar population properties and the normalisation of the stellar initial mass function (IMF) measured using stellar dynamics, based on a large sample of 260 early-type galaxies observed as part of the Atlas3D project. This measure of the IMF normalisation is found to vary non-uniformly with age- and metallicity-sensitive absorption line strengths. Applying single stellar population models, there are weak but measurable trends of the IMF with age and abundance ratio. Accounting for the dependence of stellar population parameters on velocity dispersion effectively removes these trends, but subsequently introduces a trend with metallicity, such that `heavy IMFs favour lower metallicities. The correlations are weaker than those found from previous studies directly detecting low-mass stars, suggesting some degree of tension between the different approaches of measuring the IMF. Resolving these discrepancies will be the focus of future work.
The stellar initial mass function (IMF) is central to our interpretation of astronomical observables and to our understanding of most baryonic processes within galaxies. The universality of the IMF, suggested by observations in our own Milky Way, has been thoroughly revisited due to the apparent excess of low-mass stars in the central regions of massive quiescent galaxies. As part of the efforts within the Fornax 3D project, we aim to characterize the two-dimensional IMF variations in a sample of 23 quiescent galaxies within the Fornax cluster. For each galaxy in the sample, we measured the mean age, metallicity, [Mg/Fe], and IMF slope maps from spatially resolved integrated spectra. The IMF maps show a variety of behaviors and internal substructures, roughly following metallicity variations. However, metallicity alone is not able to fully explain the complexity exhibited by the IMF maps. In particular, for relatively metal-poor stellar populations, the slope of the IMF seems to depend on the (specific) star formation rate at which stars were formed. Moreover, metallicity maps have systematically higher ellipticities than IMF slope ones. At the same time, both metallicity and IMF slope maps have at the same time higher ellipticities than the stellar light distribution in our sample of galaxies. In addition we find that, regardless of the stellar mass, every galaxy in our sample shows a positive radial [Mg/Fe] gradient. This results in a strong [Fe/H]-[Mg/Fe] relation, similar to what is observed in nearby, resolved galaxies. Since the formation history and chemical enrichment of galaxies are causally driven by changes in the IMF, our findings call for a physically motivated interpretation of stellar population measurements based on integrated spectra that take into account any possible time evolution of the stellar populations.
Some ultra-compact dwarf galaxies have large dynamical mass to light (M/L) ratios and also appear to contain an overabundance of LMXB sources, and some Milky Way globular clusters have a low concentration and appear to have a deficit of low-mass stars. These observations can be explained if the stellar IMF becomes increasingly top-heavy with decreasing metallicity and increasing gas density of the forming object. The thus constrained stellar IMF then accounts for the observed trend of metallicity and M/L ratio found amongst M31 globular star clusters. It also accounts for the overall shift of the observationally deduced galaxy-wide IMF from top-light to top-heavy with increasing star formation rate amongst galaxies. If the IMF varies similarly to deduced here, then extremely young very massive star-burst clusters observed at a high redshift would appear quasar-like (Jerabkova et al. 2017) .
We study the history from $zsim2$ to $zsim0$ of the stellar mass assembly of quiescent and star-forming galaxies in a spatially resolved fashion. For this purpose we use multi-wavelength imaging data from the Hubble Space Telescope (HST) over the GOODS fields and the Sloan Digital Sky Survey (SDSS) for the local population. We present the radial stellar mass surface density profiles of galaxies with $M_{ast}>10^{10} M_{odot}$, corrected for mass-to-light ratio ($M_{ast}/L$) variations, and derive the half-mass radius ($R_{m}$), central stellar mass surface density within 1 kpc ($Sigma_{1}$) and surface density at $R_{m}$ ($Sigma_{m}$) for star-forming and quiescent galaxies and study their evolution with redshift. At fixed stellar mass, the half-mass sizes of quiescent galaxies increase from $zsim2$ to $zsim0$ by a factor of $sim3-5$, whereas the half-mass sizes of star-forming galaxies increase only slightly, by a factor of $sim2$. The central densities $Sigma_{1}$ of quiescent galaxies decline slightly (by a factor of $lesssim1.7$) from $zsim2$ to $zsim0$, while for star-forming galaxies $Sigma_{1}$ increases with time, at fixed mass. We show that the central density $Sigma_{1}$ has a tighter correlation with specific star-formation rate (sSFR) than $Sigma_{m}$ and for all masses and redshifts galaxies with higher central density are more prone to be quenched. Reaching a high central density ($Sigma_{1} gtrsim 10^{10} M_{odot} mathrm{kpc}^2$) seems to be a prerequisite for the cessation of star formation, though a causal link between high $Sigma_{1}$ and quenching is difficult to prove and their correlation can have a different origin.
Spatially resolved kinematics of nearby galaxies has shown that the ratio of dynamical- to stellar population-based estimates of the mass of a galaxy ($M_*^{rm JAM}/M_*$) correlates with $sigma_e$, if $M_*$ is estimated using the same IMF for all galaxies and the stellar M/L ratio within each galaxy is constant. This correlation may indicate that, in fact, the IMF is more dwarf-rich for galaxies with large $sigma$. We use this correlation to estimate a dynamical or IMF-corrected stellar mass, $M_*^{rm alpha_{JAM}}$, from $M_{*}$ and $sigma_e$ for a sample of $6 times 10^5$ SDSS galaxies for which spatially resolved kinematics is not available. We also compute the `virial mass estimate $k(n,R),R_e,sigma_R^2/G$, where $n$ is the Sersic index, in the SDSS and ATLAS$^{rm 3D}$ samples. We show that an $n$-dependent correction must be applied to the $k(n,R)$ values provided by Prugniel & Simien (1997). Our analysis also shows that the shape of the velocity dispersion profile in the ATLAS$^{rm 3D}$ sample varies weakly with $n$: $(sigma_R/sigma_e) = (R/R_e)^{-gamma(n)}$. The resulting stellar mass functions, based on $M_*^{rm alpha_{JAM}}$ and the recalibrated virial mass, are in good agreement. If the $M_*^{rm alpha_{JAM}}/M_* - sigma_e$ correlation is indeed due to the IMF, and stellar M/L gradients can be ignored, then our $phi(M_*^{rm alpha_{JAM}})$ is an estimate of the stellar mass function in which $sigma_e$-dependent variations in the IMF across the population have been accounted for. Using a Fundamental Plane based observational proxy for $sigma_e$ produces comparable results. By demonstrating that cheaper proxies are sufficiently accurate, our analysis should enable a more reliable census of the mass in stars for large galaxy samples, at a fraction of the cost. Our results are provided in tabular form.
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