2015MNRAS.447.1951M

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 5σ upper limit on the Hi gas mass of a few 10⁸M_☉. 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 ∼ 5x 10^8M_☉ 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.