SDSS-IV MaNGA: the inner density slopes of nearby galaxies

We derive the mass-weighted total density slopes within the effective (half-light) radius, γ′, for more than 2000 nearby galaxies from the SDSS-IV (Sloan Digital Sky Survey IV) MaNGA survey using Jeans-anisotropic-models applied to integral field unit observations. Our galaxies span a wide range of the stellar mass (109 M⊙ < M* < 1012 M⊙) and the velocity dispersion (30 km s−1 < σv < 300 km s−1). We find that for galaxies with velocity dispersion σv > 100 km s−1, the density slope has a mean value 〈γ′〉 = 2.24 and a dispersion σγ = 0.22, almost independent of velocity dispersion, consistent with previous lensing and stellar dynamical analysis. We also quantitatively confirm with high accuracy a turnover in the γ′–σv relation is present at σ ∼ 100 km s−1, below which the density slope decreases rapidly with σv, consistent with the results reported by previous analysis of ${\rm ATLAS^{\rm 3D}}$ survey. Our analysis shows that a large fraction of dwarf galaxies (below M* = 1010 M⊙) have total density slopes shallower than 1, which implies that they may reside in cold dark matter haloes with shallow density slopes. We compare our results with that of galaxies in hydrodynamical simulations of EAGLE, Illustris, and IllustrisTNG projects, and find all simulations predict shallower density slopes for massive galaxies with high σv. Finally, we explore the dependence of γ′ on the positions of galaxies in haloes, namely centrals versus satellites, and find that for the same velocity dispersion, the amplitude of γ′ is higher for satellite galaxies by about 0.1.