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

Long-slit spectroscopic data for seven early-type galaxies with X-ray emission have been analysed to derive velocity dispersion and radial velocity profiles. Major axis rotation curves out to R ∼ Re 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 σc and maximum rotational velocity νrot). This sample is used to investigate from an observational point of view the role of rotation and flattening in the X-ray emission, particularly with regard to the X-ray underluminosity of flat systems. The trend between the X-ray to optical ratio LX/LB, a measure of the hot gas content of the galaxies, and νrot/σc is L-shaped, with the X-ray-brightest objects confined at νrot/σc ≲ 0.4, for both Es and S0s. Neither for low or intermediate LX/LB nor for high LX/LB is there any clear correlation between X-ray emission and rotational properties. The trend between LX/LB and the ellipticity ∈ is also L-shaped, and resembles that between LX/LB and νrot/σc: there are no high-Lx/LB objects with high ∈. The relationships between LX/LB, the anisotropy parameter (ν/σ)*, and the isophotal shape parameter a4/a have also been analysed, but no significant trends have been found. The existence of a relation between νrot/σc and ∈ makes it difficult to assess on purely observational grounds whether rotation or flattening is the basis of the L-shaped trends found, although the trend with νrot/σc is sharper than that with ∈. Our observational findings are discussed in connection with the effects that rotation and flattening are predicted to have in the steady-state cooling flow and in the evolutionary wind/outflow/inflow scenarios for the hot gas behaviour. © 1997 RAS.