A minor merger scenario for the ultraluminous X-ray source ESO 243-49 HLX-1-II. Constraints from photometry

Abstract

The point-like X-ray source HLX-1, close to the S0 galaxy ESO 243-49, is the brightest knownultraluminous X-ray source and one the strongest intermediate-mass black hole candidates.Weargue that the counterpart of HLX-1 may be the nucleus of a satellite galaxy, undergoing minormerger with the S0 galaxy. We investigate this scenario by running a set of N-body/smoothedparticle hydrodynamics simulations of the minor merger between an S0 galaxy and a gas-richbulgy satellite galaxy, and by comparing the results with the available photometric HubbleSpace Telescope (HST) data of ESO 243-49 and of the HLX-1 counterpart. In particular, wederive synthetic surface brightness profiles for the simulated counterpart of HLX-1 in six HSTfilters, ranging from far-ultraviolet (FUV) to infrared wavelengths. Such synthetic profilesinclude a contribution from the stellar population associated with the simulated disruptedsatellite and a contribution from an irradiated disc model. These are in reasonable agreementwith the observed surface brightness profiles of the HLX-1 counterpart, provided that themerger is at sufficiently late stage (2.5 Gyr since the first pericentre passage). The maindifference between models and observations is in the FUV band, where the HST image showsa fuzzy and extended emission.We showthat the spectral energy distribution of the bulge of ESO 243-49 cannot be explainedwith a single old stellar population, but requires the existence of a younger stellar component.This is in good agreement with the star formation history derived from our N-body simulations,and is a further hint for the minor merger scenario.