Multiwavelength Study of the Bright X-Ray Source Population in the Interacting Galaxies NGC 5774/NGC 5775

Abstract

A few nearby interacting galaxies are known that host elevated number 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 10 39 ergs s −1 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 3 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 starformation episode.NGC 5774 is an extremely low starforming 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 starformation rate alone but it may scale jointly with the mass of the galaxy and the starformation rate.

CHANDRAOBSERVATIONS OF THE NUCLEAR STAR CLUSTER AND ULTRALUMINOUS X-RAY SOURCES IN NGC 2139

“…The potential counterparts show diverse photometric properties, consistent with results from earlier studies of luminous Xray sources in galaxies (e.g., Ptak et al 2006;Ghosh et al 2009). Photometry listed in Table 2 is given in AB magnitudes measured in apertures with radius 0.…”

“…By these criteria none of the NGC 2139 sources show evidence of variability, with the exception of Sources 8 and 16 for the soft (0.5-1.2 keV) bandpass only. Surveys of X-ray point sources of comparable luminosity in other galaxies show similarly modest evidence of variability over the timescales we sample (e.g., Zezas et al 2006;Grimm et al 2007;Ghosh et al 2009). In the absence of variability the possibility exists that a source listed in Table 1 could be a compact group of multiple lower luminosity binaries that remains spatially unresolved in the Chandra image.…”

A search for radio counterparts to Chandra ULX candidates

“…There is only one clear association of a FIRST radio emission peak for the location of ULX 4. This source is coincident with the source reported (numbered 47) in Ghosh et al (2009). Detailed estimations for the peak and the FIRST location of the source are given in Table 1 .…”

Five fake ultra-luminous X-ray sources

“…Here, we continue our previous efforts (Arp et al 2004;Ghosh et al 2009;Gutiérrez 2006;Gutiérrez & López-Corredoira 2005, 2007López-Corredoira & Gutiérrez 2006) to identify and characterize optical counterparts of ULXs. We put special emphasis on the identification of foreground and background contaminants to establish a correct census and statistics of real ULXs.…”