A Chandra observation of the interacting pair of galaxies NGC 4485/NGC 4490

Abstract

(Abridged) We report the results of a 20 ks Chandra ACIS-S observation of the interacting galaxy pair NGC 4485/4490. A total of 29 discrete X-ray sources are found coincident with NGC 4490, but only one is found within NGC 4485. The sources range in observed X-ray luminosity from ~ 2 x 10^37 to ~ 4 x 10^39 erg/s. Extensive diffuse X-ray emission is detected coincident with the disk of NGC 4490, and in the tidal tail of NGC 4485, which appears to be thermal in nature and hence the signature of a hot ISM in both galaxies. However, the diffuse component accounts for only ~ 10% of the total X-ray luminosity of the system (2 x 10^40 erg/s, 0.5 - 8 keV), which arises predominantly in a handful of the brightest discrete sources. This diffuse emission fraction is unusually low for a galaxy pair that has many characteristics that would lead it to be classified as a starburst system, possibly as a consequence of the small gravitational potential well of the system. The discrete source population, on the other hand, is similar to that observed in other starburst systems, possessing a flat luminosity function slope of ~ -0.6 and a total of six ultraluminous X-ray sources (ULX). Five of the ULX are identified as probable black hole X-ray binary systems, and the sixth (which is coincident with a radio continuum source) is identified as an X-ray luminous supernova remnant. The ULX all lie in star-formation regions, providing further evidence of the link between the ULX phenomenon and active star formation. Importantly, this shows that even in star forming regions, the ULX population is dominated by accreting systems. We discuss the implications of this work for physical models of the nature of ULX, and in particular how it argues against the intermediate-mass black hole hypothesis.