HI study of the environment around ESO 243-49, the host galaxy of an intermediate mass black hole

TL;DR

This study investigates the environment of the intermediate mass black hole candidate HLX-1 in galaxy ESO 243-49 through radio, optical, and X-ray observations, finding evidence of environmental effects and a possible merger event.

Contribution

It provides new radio HI observations of ESO 243-49 and its surroundings, constrains the gas content, and suggests environmental depletion and merger activity affecting the galaxy and its black hole candidate.

Findings

No HI emission detected in ESO 243-49, upper limit of a few 10^8 M_sun.

Detected ~5 x 10^8 M_sun of HI in nearby galaxy AM 0108-462, less than typical for similar galaxies.

Evidence suggests cluster environment depletes HI gas and indicates a merger event in AM 0108-462.

Abstract

The lenticular galaxy ESO 243-49 hosts the ultraluminous X-ray source HLX-1, the best candidate intermediate mass black hole (IMBH) currently known. The environments of IMBHs remain unknown, however the proposed candidates include the nuclei of dwarf galaxies or globular clusters. Evidence at optical wavelengths points at HLX-1 being the remnant of an accreted dwarf galaxy. Here we report the Australia Telescope Compact Array radio observations of HI emission in and around ESO 243-49 searching for signatures of a recent merger event. No HI line emission is detected in ESO 243-49 with a 5$\sigma$ upper limit on the HI gas mass of a few $10^8 M_{\odot}$. A likely reason for this non-detection is the cluster environment depleting ESO 243-49's HI gas reservoir. The upper limit is consistent with an interpretation of HLX-1 as a dwarf satellite of ESO 243-49, however more sensitive observations are required for a detection. We detect ~$5 \times 10^8 M_{\odot}$ of HI gas in the peculiar spiral galaxy AM 0108-462, located at a projected distance of ~170 kpc from ESO 243-49. This amount of HI gas is ~10 times less than in spiral galaxies with similar optical and near-infrared properties in the field, strengthening the conclusion that the cluster environment indeed depletes the HI gas reservoir of these two galaxies. Here we also report observations of AM 0108-462 in several optical and near-infrared bands using the Magellan 6.5 m telescopes, and archival X-ray and ultraviolet observations with XMM-Newton and Swift. These data combined with the HI line data suggest it is likely that AM 0108-462 is experiencing a merger event.