Physically motivated fit to mass surface density profiles observed in galaxies

TL;DR

This study investigates whether polytropic models can accurately describe the stellar mass surface density profiles in galaxies, finding they are comparable to Sersic profiles and revealing correlations with galaxy mass.

Contribution

The paper develops a fitting code for polytropic profiles and systematically applies it to a large galaxy sample, demonstrating their effectiveness across different galaxy types and masses.

Findings

Polytropes and Sersic profiles are indistinguishable within observational errors.

Propol fits outperform Sersic profiles for low-mass galaxies (log M* < 9).

Polytropic index correlates with galaxy mass, clustering around 5.

Abstract

Polytropes have gained renewed interest because they account for several seemingly-disconnected observational properties of galaxies. Here we study if polytropes are also able to explain the stellar mass distribution within galaxies. We develop a code to fit surface density profiles using polytropes projected in the plane of the sky (propols). Sersic profiles are known to be good proxies for the global shapes of galaxies and we find that, ignoring central cores, propols and Sersic profiles are indistinguishable within observational errors (within 5 % over 5 orders of magnitude in surface density). The range of physically meaningful polytropes yields Sersic indexes between 0.4 and 6. The code has been systematically applied to ~750 galaxies with carefully measured mass density profiles and including all morphological types and stellar masses (7 < log (Mstar/Msun) < 12). The propol fits are systematically better than Sersic profiles when log(Mstar/Msun) < 9 and systematically worst when log(Mstar/Msun) > 10. Although with large scatter, the observed polytropic indexes increase with increasing mass and tend to cluster around m=5. For the most massive galaxies, propols are very good at reproducing their central parts, but they do not handle well cores and outskirts altogether. Polytropes are self-gravitating systems in thermal meta-equilibrium as defined by the Tsallis entropy. Thus, the above results are compatible with the principle of maximum Tsallis entropy dictating the internal structure in dwarf galaxies and in the central region of massive galaxies.