2016MNRAS.460.1382S

We report a tight linear relation between the H i circular velocity measured at 6 R_e and the stellar velocity dispersion measured within 1 R_e for a sample of 16 early-type galaxies with stellar mass between 10^10 and 10^11 M_☉. The key difference from previous studies is that we only use spatially resolved v_circ(H i) 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_circ(H i)= 1.33 σ_e with an observed scatter of just 12 per cent. 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_circ(H i)-σ_e relation is shallower than those based on v_circ measurements obtained from stellar kinematics and modelling at smaller radius, implying that v_circ declines with radius - as in bulge-dominated spirals. Indeed, the value of v_circ(H i) is typically 25 per cent lower than the maximum v_circ derived at ∼0.2 R_e from dynamical models. Under the assumption of power-law total density profiles ρ ∝ r^-γ, our data imply an average logarithmic slope <γ> =2.18±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.