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

TL;DR

This study supports the idea that the ultraluminous X-ray source HLX-1 is the nucleus of a satellite galaxy merging with ESO 243-49, using N-body simulations and photometric data to constrain the scenario.

Contribution

The paper presents detailed N-body/smoothed particle hydrodynamics simulations combined with photometric analysis to support the minor merger origin of HLX-1, including synthetic surface brightness profiles matching observations.

Findings

Synthetic profiles agree with observations at late merger stages.

FUV emission indicates a younger stellar component.

Star formation history supports the minor merger scenario.

Abstract

The point-like X-ray source HLX-1, close to the S0 galaxy ESO 243-49, is the brightest known ultraluminous X-ray source and one the strongest intermediate-mass black hole candidates. We argue that the counterpart of HLX-1 may be the nucleus of a satellite galaxy, undergoing minor merger with the S0 galaxy. We investigate this scenario by running a set of N-body/smoothed particle hydrodynamics simulations of the minor merger between a S0 galaxy and a gas-rich bulgy satellite galaxy, and by comparing the results with the available photometric Hubble Space Telescope (HST) data of ESO 243-49 and of the HLX-1 counterpart. In particular, we derive synthetic surface brightness profiles for the simulated counterpart of HLX-1 in six HST filters, ranging from far ultraviolet (FUV) to infrared wavelengths. Such synthetic profiles include a contribution from the stellar population associated with the simulated disrupted satellite and a contribution from an irradiated disc model. These are in reasonable agreement with the observed surface brightness profiles of the HLX-1 counterpart, provided that the merger is at sufficiently late stage (>~2.5 Gyr since the first pericentre passage). The main difference between models and observations is in the FUV band, where the HST image shows a fuzzy and extended emission. We show that the spectral energy distribution of the bulge of ESO 243-49 cannot be explained with 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.