Dark matter–baryon scaling relations from Einasto halo fits to SPARC galaxy rotation curves^⋆

¹ Université de Strasbourg, CNRS, UMR 7550, Observatoire astronomique de Strasbourg, Strasbourg, France
² Department of Physics, Institute for Advanced Studies in Basic Sciences, 11365-9161 Zanjan, Iran
e-mail: amir.ghari@iasbs.ac.ir
³ Departement of Physics and Astronomy, University of Victoria, Victoria, BC V8P 1A1, Canada
⁴ Helmholtz-Institut für Strahlen- und Kernphysik (HISKP), Universität Bonn, Bonn, Germany

Received: 15 November 2018
Accepted: 26 January 2019

Abstract

Dark matter–baryon scaling relations in galaxies are important in order to constrain galaxy formation models. Here, we provide a modern quantitative assessment of these relations by modelling the rotation curves of galaxies from the Spitzer Photometry and Accurate Rotation Curves (SPARC) database with the Einasto dark halo model. We focus in particular on the comparison between the original SPARC parameters, with constant mass-to-light ratios for bulges and discs, and the parameters for which galaxies follow the tightest radial acceleration relation. We show that fits are improved in the second case, and that the pure halo scaling relations also become tighter. We report that the density at the radius where the slope is −2 is strongly anti-correlated to this radius and to the Einasto index. The latter is close to unity for a large number of galaxies, indicative of large cores. In terms of dark matter–baryon scalings, we focus on relations between the core properties and the extent of the baryonic component, which are relevant to the cusp-core transformation process. We report a positive correlation between the core size of halos with small Einasto index and the stellar disc scale-length, and between the averaged dark matter density within 2 kpc and the baryon-induced rotational velocity at that radius. This finding is related to the consequence of the radial acceleration relation on the diversity of rotation curve shapes, quantified by the rotational velocity at 2 kpc. While a tight radial acceleration relation slightly decreases the observed diversity compared to the original SPARC parameters, the diversity of baryon-induced accelerations at 2 kpc is sufficient to induce a large diversity, incompatible with current hydrodynamical simulations of galaxy formation, while maintaining a tight radial acceleration relation.