The Size of the Cooling Region of Hot Gas in Two Elliptical Galaxies

Some early-type galaxies show O VI emission, a tracer of gas at 10 K, and a predicted product of gas cooling from the X-ray emitting temperatures. We studied the spatial extent and velocity structure of this cooling gas by obtaining spectra of the O VI doublet in NGC 4636 and NGC 5846 with the Far Ultraviolet Spectroscopic Explorer . For NGC 4636, the central LWRS pointing shows that the O VI lines are double-peaked and symmetrical about the systemic velocity of the galaxy, with a separation of 210 km s. A LWRS observation 30 from the center failed to show additional O VI emission. For NGC 5846, three spectra were obtained with 4 × 20 apertures (MDRS) at the center and 4 to the east and west of the center. The O VI line flux seen in the previous LWRS is contained in the sum of the smaller apertures, with most of the flux in a single non-central MDRS aperture, suggesting a size for the emission ≤ 0.5 kpc; the emission consists of a blue and red peak. For both galaxies, the O VI velocity structure is similar to that of the optical [NII] emission and is consistent with rotation. The compactness and velocity structure of the O VI emission rules out cooling flow models with broadly distributed mass drop-out but is consistent with cooling flow models where the cooling occurs primarily in the central region. The 10 K gas may be the end state of the O VI emitting gas. Subject headings: galaxies: individual (NGC 4636, NGC 5846) — galaxies: ISM — cooling flows — X-rays: galaxies present address: Kavli Institute for Astrophysics and Space Science, MIT, Cambridge, MA 02139