THREE LINERs UNDER THE CHANDRA X-RAY MICROSCOPE

摘要

We use observations of three galaxies hosting low-ionization nuclear emission-line regions (LINERs; NGC 404, NGC 4736, and NGC 4579) with the Chandra X-Ray Observatory to study their power sources. We find very diverse properties within this small group: NGC 404 has an X-ray faint nucleus with a soft, thermal spectrum; NGC 4736 harbors a plethora of discrete X-ray sources in and around its nucleus; and NGC 4579 has a dominant nuclear point source embedded in a very extended, diffuse nebulosity. From their multiwavelength properties we conclude the following about the power sources in these LINERs: the nucleus of NGC 404 is the site of a weak, compact starburst, whose X-ray emission is due to gas heated by stellar winds and supernovae; NGC 4736 appears to be in a recent or aging star-burst phase, where the X-ray emission is dominated by a dense cluster of X-ray binaries; and NGC 4579 is powered by accretion onto a supermassive black hole. We detect 39 discrete sources in NGC 4736 and 21 in NGC 4579, most with L_X > 10~(37) ergs s~(-1). One source in the disk of NGC 4579 appears to be an ultraluminous X-ray binary with L_X(2-10 keV) = 9 x 10~(39) ergs s~(-1), but it could also be a background quasar. The most luminous discrete sources have simple power-law spectra, which along with their luminosities suggest that these are X-ray binaries accreting near or above the Eddington rate for a neutron star. By comparing the luminosity functions of discrete X-ray sources in these and other galaxies, we find a potential connection between the age of the stellar population and the slope of the cumulative X-ray luminosity function: galaxies with primarily old stellar populations have steeper X-ray luminosity functions than starburst galaxies. We suggest that this difference results from the contribution of high-mass X-ray binaries from the young stellar population to the upper end of the luminosity function.