A robust two-parameter description of the stellar profile of elliptical galaxies

Abstract

The stellar density profile a galaxy is typically summarised with two numbers: total stellar mass and half-light radius. The total mass of a galaxy, however, is not a well-defined quantity, due to the finite depth of photometric observations and the arbitrariness of the distinction between galaxy and diffuse intra-group light. This limits our ability to make accurate comparisons between models and observations. I wish to provide a more robust two-parameter description of the stellar density distribution of elliptical galaxies, in terms of quantities that can be measured unambiguously. I propose to use the stellar mass enclosed within 10 kpc in projection, $M_{\ast ,10}$, and the mass-weighted stellar density slope within the same aperture, ${\Gamma }_{\ast ,10}$, for this purpose. I measured the distribution in $M_{\ast ,10}$ and ${\Gamma }_{\ast ,10}$ of a sample of elliptical galaxies from the Sloan Digital Sky Survey and the Galaxy And Mass Assembly survey, using photometry from the Hyper Suprime-Cam survey. The pair of values of $(M_{\ast ,10},{\Gamma }_{\ast ,10})$ can be used to predict the stellar density profile in the inner 10 kpc of a galaxy with better than 20% accuracy. Similarly, $M_{\ast ,10}$ and ${\Gamma }_{\ast ,10}$ can be combined to obtain a proxy for stellar velocity dispersion at least as good as the stellar mass fundamental plane. As a first application, I then compared the observed $M_{\ast ,10}-{\Gamma }_{\ast ,10}$ relation of elliptical galaxies with that of similarly selected galaxies in the EAGLE Reference simulation. Observed and simulated galaxies match at $M_{\ast ,10}=10^{11}{M}_{\odot }$, but the EAGLE $M_{\ast ,10}-{\Gamma }_{\ast ,10}$ relation is shallower and has a larger intrinsic scatter compared to observations. This new parameterisation of the stellar density profile of massive elliptical galaxies provides a more robust way of comparing results from different photometric surveys and from hydrodynamical simulations.

arXiv comment13 pages, 13 figures, submitted to Astronomy & Astrophysics

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• The dark matter halo masses of elliptical galaxies as a function of observationally robust quantities
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