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

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 ∼ 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 vrot). This sample is used to 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 LX/LB, a measure of the hot gas content of the galaxies, and vrot/σc is L-shaped, with the X-ray brightest objects confined at vrot/σc<∼0.4, both for Es and S0s. Neither for low or intermediate, nor for high LX/LB, there is any clear correlation between X-ray emission and rotational properties. The trend between LX/LB and the ellipticity ǫ is also L-shaped, and resembles that between LX/LB and vrot/σc: there are no high LX/LB objects with high ǫ. The relationships between LX/LB, the anisotropy parameter (v/σ) ∗, and the isophotal shape parameter a4/a have also been investigated, but no significant trends have been found. The existence of a relation between vrot/σc and ǫ makes it difficult to assess on a purely observational ground whether rotation or flattening is at the basis of the L-shaped trends found, although the trend with vrot/σc is sharper than that with ǫ. Our observational findings are then 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 behavior.