Abstract
We present a study of X-ray luminosity functions (XLFs) of ultraluminous X-ray source (ULX) candidates (Lx >10^39 erg/s) and their host galaxy properties. In this work, we present the largest Chandra X-ray observations to date and new methods to compare different ULX host galaxy properties and reconfirm the results from previous studies. Our ULX samples are a collection of 5 different catalogs based on Chandra observations with a total of 339 ULXs within 100 Mpc, and we called it ‘All ULXs’ sample. For comparison, we also build a ‘Complete ULXs’ sample consisting candidates above 4 sigma detection significance, within D25 isophotal ellipse and the whole galaxy is fully covered by Chandra observations. We select the maximum flux of a source estimated by the Portable, Interactive Multi-Mission Simulator (PIMMS) to build XLFs as the parent sample or ‘All ULXs’ sample. In addition to the XLFs of ULXs, we also report X-ray spectral fitting results with power law and disk blackbody models. To characterize the host galaxy properties, we estimate galaxy-wide star formation rate (SFR) based on IRAS observations, while stellar-mass is estimated from the B magnitude (B_T^0) corrected for galactic and internal extinction, and for redshift in the Third Reference Catalog of Bright Galaxies (RC3). We calculate the galactocentric distance of ULXs to the center of the host galaxy by considering inclination angle based on the major-to-minor ratio in the RC3, and find that it is similar to high-mass X-ray binaries (HMXBs) in our Galaxy. We build XLFs in both ‘All ULXs’ and ‘Complete ULXs’ samples and fit them with both power law and exponential cut-off power law models according to morphology, SFR, stellar-mass and galactocentric distance. This is the first time to be done by using XLFs for studying ULX’s host galaxy properties. We find that an obvious cut-off feature in XLFs and suggest that it could be a global feature in terms of host galaxy properties. We select ULXs to construct the corresponding XLFs. For the first time, we also give a number density of ULXs according to different host galaxy properties. The most denser environment are ULXs in irregular/peculiar and high SFR group and we find that the XLFs are significantly difference for high and low SFR. It suggests that SFR is the most important factor than others to affect both number density and luminosity of ULXs. We also apply KS-test to test whether two groups/sub-samples are from the same parent distribution. The results suggest that there is a significant difference between high and low SFR groups in both ‘All ULXs’ and ‘Complete ULXs’ samples. The average HR1 and HR2 colors show no significant difference between groups. We also compare host galaxy properties to each other according to different X-ray spectral shape. These results suggest that the spectral shape may not be affected by different host galaxy properties. However, the luminosity can be affected by host galaxy properties. We also briefly compared the host galaxies of hyperluminous X-ray sources (HLXs) and ULXs. Two of the HLX candidates have IRAS observations. We find that they are located in high SFR galaxy or active galaxy. It would be generally consistent with our results that SFR is crucial factor to determine the luminosity of ULX.