Multiwavelength study of the X-Ray sources in NGC 5018

Restricted Access. An open-access version is available at arXiv.org (one of the alternative locations)The E3 giant elliptical galaxy NGC 5018 was observed with the Chandra X-Ray Observatory's Advanced CCD Imaging Spectrometer for 30 ks on 2001 April 14. Results of the analysis of these X-ray data as well as of complementary optical, infrared, and radio data are reported. Seven X-ray point sources, including the nucleus, were detected. If they are intrinsic to NGC 5018, then all six nonnuclear sources have luminosities exceeding 1039 ergs s-1 in the 0.5-8.0 keV energy band, placing them in the class of ultraluminous X-ray sources. Comparison of X-ray source positions to archival Hubble Space Telescope Wide Field Planetary Camera 2 (HST WFPC2) images reveals that four of the six nonnuclear sources are spatially coincident with bright, MV<~-8.6 mag objects. These four objects have optical magnitudes and (V-I) colors consistent with globular clusters in NGC 5018, but they also have X-ray-to-optical flux ratios consistent with background active galactic nuclei (AGNs). Strong, unpolarized radio emission has been detected from one of the optically bright counterparts. Another optically bright counterpart was observed to vary by ~1 mag in optical observations taken on 1997 July 28 and 1999 February 4. Extended X-ray emission is detected within an ~15" radius of the galaxy center at a luminosity of ~1040 ergs s-1 in the X-ray band. Its thermal X-ray spectrum (kT~0.4 keV) and its spatial coincidence with strong Hα emission are consistent with a hot gas origin. The nucleus itself may be a weak X-ray source, LX<~3.5×1039 ergs s-1, which displays a radio spectrum typical of AGNs