Chandra observations of low-mass X-ray binaries and diffuse gas in the early-type galaxies NGC 4365 and NGC 4382 (M85)


We used the Chandra X-Ray Observatory ACIS-S3 to image the X-ray-faint elliptical galaxy NGC 4365 and lenticular galaxy NGC 4382. The observations resolve much of the X-ray emission into 99 and 58 sources, respectively, most of which are low-mass X-ray binaries (LMXBs) associated with each of the galaxies. Within one effective radius of NGC 4365, about 45% of the counts are resolved into sources, 30% are attributed to unresolved LMXBs, and 25% are attributed to diffuse gas. Within two effective radii of NGC 4382, about 22% of the counts are resolved into sources, 33% are attributed to unresolved LMXBs, and 45% are attributed to diffuse gas. We identify 18 out of the 37 X-ray sources in a central field in NGC 4365 with globular clusters. The luminosity functions of the resolved sources for both galaxies are best fitted with cutoff power laws whose cutoff luminosity is approximate to (0.9-3.1) x 10(39) ergs s(-1). These luminosities are much larger than those previously measured for similar galaxies; we do not find evidence for a break in the luminosity function at the Eddington luminosity of a 1.4 M-. neutron star. The spatial distributions of the resolved sources for both galaxies are broader than the distribution of optical stars. In both galaxies, a hard powerlaw model fits the summed spectrum of all of the sources. The unresolved emission is best fitted by the sum of a soft MEKAL model, representing emission from diffuse gas, and a hard power law, presumed to be from unresolved LMXBs. There is some evidence that the temperature of the diffuse gas increases with increasing radius. A standard beta model fits the radial distribution of the diffuse gas in both galaxies. In the elliptical galaxy NGC 4365, the best-fit core radius is very small, while the SO galaxy NGC 4382 has a larger core radius. This may indicate that the gas in NGC 4382 is rotating significantly.

binaries : close, galaxies : elliptical and lenticular, cD, galaxies : ISM, X-rays : binaries, X-rays : galaxies, X-rays : ISM, GLOBULAR-CLUSTER SYSTEMS, ELLIPTIC GALAXIES, SPECTRAL PROPERTIES, FAINT, POPULATION, DISTANCES, EMISSION, COLORS, FIELD, ASCA, Astronomy & Astrophysics
Sivakoff, G., Sarazin, C., Irwin, J. (2003): Chandra Observations of Low-Mass X-Ray Binaries and Diffuse Gas in the Early-Type Galaxies NGC 4365 and 4382 (M85). The Astrophysical Journal, 599(1). DOI: 10.1086/379115