AChandraX‐Ray Survey of Ultraluminous Infrared Galaxies

We present results from Chandra observations of 14 ultraluminous infraredgalaxies (ULIRGs; log(L_IR/L_Sun) >= 12) with redshifts between 0.04 and 0.16.The goals of the observations were to investigate any correlation betweeninfrared color or luminosity and the properties of the X-ray emission and toattempt to determine whether these objects are powered by starbursts or activegalactic nuclei (AGNs). The sample contains approximately the same number ofhigh and low luminosity objects and ``warm'' and ``cool'' ULIRGs. All 14galaxies were detected by Chandra. Our analysis shows that the X-ray emissionof the two Seyfert 1 galaxies in our sample are dominated by AGN. The remaining12 sources are too faint for conventional spectral fitting to be applicable.Hardness ratios were used to estimate the spectral properties of these faintsources. The photon indices for our sample plus the Chandra-observed samplefrom Ptak et al.(2003) peak in the range of 1.0-1.5, consistent withexpectations for X-ray binaries in a starburst, an absorbed AGN, or hotbremsstrahlung from a starburst or AGN. The values of photon index for theobjects in our sample classified as Seyferts (type 1 or 2) are larger than 2,while those classified as HII regions or LINERs tend to be less than 2. Thehard X-ray to far-infrared ratios for the 12 weak sources are similar to thoseof starbursts, but we cannot rule out the possibility of absorbed, possiblyCompton-thick, AGNs in some of these objects. Two of these faint sources werefound to have X-ray counterparts to their double optical and infrared nuclei.Comment: 40 pages, 5 tables, 14 figures, accepted by Ap