The dichotomy of dark matter fraction and total mass density slope of galaxies over five dex in mass

Abstract

We analyse the mass density distribution in the centres of galaxies across five orders of magnitude in mass range. Using high-quality spiral galaxy rotation curves and infrared photometry from SPARC, we conduct a systematic study of their central dark matter fraction ($f_{\rm DM}$) and their mass density slope ($\alpha$), within their effective radius. We show that lower-mass spiral galaxies are more dark matter dominated and have more shallow mass density slopes when compared with more massive galaxies, which have density profiles closer to isothermal. Low-mass ($M_{\star} \lesssim 10^{10}\, \rm M_{\odot}$) gas-rich spirals span a wide range of \fdm\ values, but systematically lower than in gas-poor systems of similar mass. With increasing galaxy mass, the values of \fdm\ decrease and the density profiles steepen. In the most massive late-type gas-poor galaxies, a possible flattening of these trends is observed. When comparing these results to massive ($M_{\star} \gtrsim 10^{10}M_{\odot}$) elliptical galaxies from SPIDER and to dwarf ellipticals from SMACKED, these trends result to be inverted. Hence, the values of both $f_{\rm DM}$ and $\alpha$, as a function of $M_{\star}$, exhibit a U-shape trend. At a fixed stellar mass, the mass density profiles in dwarf ellipticals are steeper than in spirals. These trends can be understood by stellar feedback from a more prolonged star formation period in spirals, causing a transformation of the initial steep density cusp to a more shallow profile via differential feedback efficiency by supernovae, and by galaxy mergers or AGN feedback in higher-mass galaxies.