2009AJ....137.3263G

A few nearby interacting galaxies are known that host elevated numbers of ultraluminous X-ray sources. Here we report the results of a multiwavelength study of the X-ray source population in the field of the interacting pair of galaxies NGC 5774/5775. A total of 49 discrete sources are detected, including 12 ultraluminous X-ray source candidates with luminosities above 10³⁹ erg/s in the 0.5-8.0 keV X-ray band. X-ray source positions are mapped onto optical and radio images to search for potential counterparts. Twelve sources in the field have optical counterparts. Optical colors are used to differentiate these sources, which are mostly located outside the optical extent of the interacting galaxies, as potential globular clusters (2), one compact blue dwarf galaxy, and quasars (5). We obtained optical spectra of two of the latter, which confirm that they are background quasars. We expect three background sources in the field of these two galaxies. These results are used to determine the true X-ray population of these two interacting galaxies, which are connected with two bridges. Two high-mass X-ray binaries are detected on these two bridges, suggesting their formation through the interaction-induced star formation episode. NGC 5774 is an extremely low star forming galaxy with five X-ray sources plus three ultraluminous X-ray source candidates. Observed X-ray population of this galaxy does not scale with the star formation rate (SFR) alone but it may scale jointly with the mass of the galaxy and the SFR. Twenty-four X-ray sources (excluding the active galactic nucleus, AGN) are detected in NGC 5775. and its X-ray luminosity function is consistent with that of other interacting galaxies, suggesting that these galaxies have comparable numbers of luminous sources. No X-ray point source was detected at the center of this galaxy to a limiting luminosity of 3x10³⁷ erg/s. Wind/outflow is detected from the central region of NGC 5775. Subsolar diffuse gas with temperature ∼0.31±0.04 keV is present in this galaxy, which suggest that NGC 5775 is in the beginning of the evolutionary process. Twelve ultraluminous X-ray source candidates are detected within the D₂₅ isophotes of NGC 5774/5775. Several of them are highly variable X-ray sources that fall below the detection levels in one of two X-ray observations spaced 15 months apart. Two ultraluminous X-ray sources are located in the halo of NGC 5775 and one of them is hosted in a globular star cluster. Four of the remaining 10 candidates have powerlaw X-ray spectra with photon indices around 1.8 and are extremely luminous with no optical counterparts. One of these four objects is the brightest (∼10⁴¹ erg/s) with a possible 6.2 hr period and it varied by more than a factor of 500. Two of the rest six ultraluminous X-ray source candidates are having steep-powerlaw X-ray spectra and are embedded in diffuse Hα emission, which are probably ionized nebulae. These nebulae could be due to energetic supernova explosions or to continuous inflation by jets. Rest four ultraluminous X-ray source candidates are flat-powerlaw X-ray sources hosted in either young star clusters or bright star forming complexes. Two of them are radio sources. Finally, we find that the number of ultraluminous X-ray source candidates in interacting/merging galaxies are correlated with the far-infrared, K-band, and UV luminosities of their host galaxies, suggesting that the formation and evolution of ultraluminous X-ray sources depend not only on the SFR but also on the mass of their host galaxies.