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Register a new userStellar mass functions and implications for a variable IMF
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Added by
Mariangela Bernardi
Publication date
2017
fields
Physics
and research's language is
English
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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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Within a galaxy the stellar mass-to-light ratio $Upsilon_*$ is not constant. Spatially resolved kinematics of nearby early-type galaxies suggest that allowing for a variable initial mass function (IMF) returns significantly larger $Upsilon_*$ gradients than if the IMF is held fixed. If $Upsilon_*$ is greater in the central regions, then ignoring the IMF-driven gradient can overestimate $M_*^{rm dyn}$ by as much as a factor of two for the most massive galaxies, though stellar population estimates $M_*^{rm SP}$ are also affected. Large $Upsilon_*$-gradients have four main consequences: First, $M_*^{rm dyn}$ cannot be estimated independently of stellar population synthesis models. Second, if there is a lower limit to $Upsilon_*$ and gradients are unknown, then requiring $M_*^{rm dyn}=M_*^{rm SP}$ constrains them. Third, if gradients are stronger in more massive galaxies, then $M_*^{rm dyn}$ and $M_*^{rm SP}$ can be brought into agreement, not by shifting $M_*^{rm SP}$ upwards by invoking constant bottom-heavy IMFs, as advocated by a number of recent studies, but by revising $M_*^{rm dyn}$ estimates in the literature downwards. Fourth, accounting for $Upsilon_*$ gradients changes the high-mass slope of the stellar mass function $phi(M_*^{rm dyn})$, and reduces the associated stellar mass density. These conclusions potentially impact estimates of the need for feedback and adiabatic contraction, so our results highlight the importance of measuring $Upsilon_*$ gradients in larger samples.
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