X-ray emission and internal kinematics in early-type galaxies. I. Observations

Abstract

Long slit spectroscopic data for 7 early-type galaxies with X-ray emission have been analyzed to derive velocity dispersion and radial velocity profiles. Major axis rotation curves out to $R\sim R_e$ are presented. Adding these new data to those available in the literature, we have built a sample of early-type galaxies with detected X-ray emission and known kinematics (central velocity dispersion $σ_0$ and maximum rotational velocity $v_r$). Using this sample we investigate from an observational point of view the role of rotation and flattening on the X-ray emission, particularly with regard to the X-ray underluminosity of flat systems. The trend between the X-ray to optical ratio $L_X/L_B$, a measure of the hot gas content of the galaxies, and $v_r/σ_0$ is L-shaped, with the X-ray brightest objects confined at $v_r/σ_0 <0.4$, both for Es and S0s. Neither for low or intermediate, nor for high $L_X/L_B$, there is any clear correlation between X-ray emission and rotational properties. The trend between $L_X/L_B$ and the ellipticity $ε$ is also L-shaped: there are no high $L_X/L_B$ objects with high $ε$. The existence of a relation between $v_r/σ_0$ and $ε$ makes it difficult to assess whether rotation or flattening is at the basis of the trends found. Our observational findings are finally discussed in connection with the effects that rotation and flattening are predicted to have in the cooling flow and in the wind/outflow/inflow scenarios.

10 pages, plain tex, plus 7 .ps figures and tex-macro mn.tex -- accepted by MNRAS (main journal)