Shedding light on the nature of ESO 243-49 HLX-1

TL;DR

This study investigates the nature of the intermediate-mass black hole candidate HLX-1 near galaxy ESO 243-49, proposing it results from a minor merger and analyzing observational data with simulations to understand its properties.

Contribution

It introduces a merger hypothesis for HLX-1's origin and combines photometric analysis with N-body/SPH simulations to match observed surface brightness profiles.

Findings

Synthetic brightness profiles agree with observations at late merger stages.

The FUV emission is more extended and fuzzy than models predict.

The merger scenario explains the properties of HLX-1's counterpart.

Abstract

The point-like X-ray source HLX-1, close to the S0 galaxy ESO 243-49, is one of the strongest intermediate-mass black hole candidates. We discuss the hypothesis that ESO 243-49 is undergoing a minor merger with a gas-rich disc galaxy. We propose that the counterpart of HLX-1 coincides with the nucleus of the secondary galaxy. We re-analyze the available photometric HST data, and we compare them with the results of N-body/smoothed particle hydrodynamics simulations. 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 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.