Linear relation between HI circular velocity and stellar velocity dispersion in early-type galaxies, and slope of the density profiles

Abstract

We report a tight linear relation between the HI circular velocity measured at 6 $R_{\mathrm{e}}$ and the stellar velocity dispersion measured within 1 $R_{\mathrm{e}}$ for a sample of 16 early-type galaxies with stellar mass between $10^{10}$ and $10^{11}$ ${\mathrm{M}}_{\odot }$. The key difference from previous studies is that we only use spatially resolved $v_{\mathrm{circ}}$(HI) measurements obtained at large radius for a sizeable sample of objects. We can therefore link a kinematical tracer of the gravitational potential in the dark-matter dominated outer regions of galaxies with one in the inner regions, where baryons control the distribution of mass. We find that $v_{\mathrm{circ}}$(HI) = 1.33 $\ddot{I}{ƒ}_{\mathrm{e}}$ with an observed scatter of just 12 percent. This indicates a strong coupling between luminous and dark matter from the inner- to the outer regions of early-type galaxies, analogous to the situation in spirals and dwarf irregulars. The $v_{\mathrm{circ}}$(HI)-$\ddot{I}{ƒ}_{\mathrm{e}}$ relation is shallower than those based on $v_{\mathrm{circ}}$ measurements obtained from stellar kinematics and modelling at smaller radius, implying that \vcirc\ declines with radius -- as in bulge-dominated spirals. Indeed, the value of $v_{\mathrm{circ}}$(HI) is typically 25 percent lower than the maximum $v_{\mathrm{circ}}$ derived at $\sim 0.2R_{\mathrm{e}}$ from dynamical models. Under the assumption of power-law total density profiles $\ddot{I}\propto r^{-{\hat{I}}^3}$, our data imply an average logarithmic slope $⟨{\hat{I}}^3⟩=2.18\pm 0.03$ across the sample, with a scatter of 0.11 around this value. The average slope and scatter agree with recent results obtained from stellar kinematics alone for a different sample of early-type galaxies.

MNRAS, accepted