Stellar properties of the host galaxy of an ultraluminous X-ray source in NGC 5252

An ultraluminous X-ray source (ULX) in NGC 5252 has been known as a strong candidate for an off-nuclear intermediate-mass black hole (IMBH). We present near-infrared imaging data of the ULX obtained with the William Herschel Telescope. Using this data we estimate a stellar mass associated with the ULX of ≈10^{7.9 ± 0.1} M_⊙, suggesting that it could be (the remnant of) a dwarf galaxy that is in the process of merging with NGC 5252. Based on a correlation between the mass of the central black hole (BH) and host galaxy, the ULX is powered by a 10⁵ M_⊙ BH. Alternatively, if the BH mass is ≈10⁶ M_⊙ or larger, the host galaxy of the ULX must have been heavily stripped during the merger. The ULX K_s-band luminosity is two orders of magnitude smaller than that expected from an ordinary active galactic nucleus with the observed [O III] luminosity, which also suggests the ULX lacks a dusty torus. We discuss how these findings provide suggestive evidence that the ULX is hosting an IMBH.