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Register a new userConnection between dynamically derived IMF normalisation and stellar populations
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Richard M. McDermid
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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.
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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.
In this letter we describe how we use stellar dynamics information to constrain the shape of the stellar IMF in a sample of 27 early-type galaxies from the CALIFA survey. We obtain dynamical and stellar mass-to-light ratios, $Upsilon_mathrm{dyn}$ and $Upsilon_{ast}$, over a homogenous aperture of 0.5~$R_{e}$. We use the constraint $Upsilon_mathrm{dyn} ge Upsilon_{ast}$ to test two IMF shapes within the framework of the extended MILES stellar population models. We rule out a single power law IMF shape for 75% of the galaxies in our sample. Conversely, we find that a double power law IMF shape with a varying high-mass end slope is compatible (within 1$sigma$) with 95% of the galaxies. We also show that dynamical and stellar IMF mismatch factors give consistent results for the systematic variation of the IMF in these galaxies.
The current knowledge on the stellar IMF is documented. It appears to become top-heavy when the star-formation rate density surpasses about 0.1Msun/(yr pc^3) on a pc scale and it may become increasingly bottom-heavy with increasing metallicity and in increasingly massive early-type galaxies. It declines quite steeply below about 0.07Msun with brown dwarfs (BDs) and very low mass stars having their own IMF. The most massive star of mass mmax formed in an embedded cluster with stellar mass Mecl correlates strongly with Mecl being a result of gravitation-driven but resource-limited growth and fragmentation induced starvation. There is no convincing evidence whatsoever that massive stars do form in isolation. Various methods of discretising a stellar population are introduced: optimal sampling leads to a mass distribution that perfectly represents the exact form of the desired IMF and the mmax-to-Mecl relation, while random sampling results in statistical variations of the shape of the IMF. The observed mmax-to-Mecl correlation and the small spread of IMF power-law indices together suggest that optimally sampling the IMF may be the more realistic description of star formation than random sampling from a universal IMF with a constant upper mass limit. Composite populations on galaxy scales, which are formed from many pc scale star formation events, need to be described by the integrated galactic IMF. This IGIMF varies systematically from top-light to top-heavy in dependence of galaxy type and star formation rate, with dramatic implications for theories of galaxy formation and evolution.
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.
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