Authors: N.A.Webb, A.Guerou, B.Ciambur, A.Detoeuf, M.Coriat, O.Godet, D.Barret, F.Combes, T.Contini, A.W.Graham, T.J.Maccarone, M.Mrkalj, M.Servillat, I.Schroetter, and K.Wiersema.
Abstract:
Aims: ESO 243-49 HLX-1 (otherwise known as HLX-1) is an intermediate
mass black hole (IMBH) candidate located 8′′ from the centre of the
edge-on S0 galaxy ESO 243-49. How the black hole came to be associated
with this galaxy and the nature of the environment in which it resides
are still unclear. Using multi-wavelength observations we aim to
constrain the nature of the medium surrounding HLX-1, search for
evidence of past mergers with ESO 243-49 and constrain parameters of
the galaxy, including the mass of the expected central supermassive
black hole, essential for future modelling of the interaction of the
IMBH and ESO 243-49.
Methods. We reduce and analyse proprietary
integral field unit observations of the galaxy ESO 243-49 taken with
the MUSE instrument on the VLT. Using complementary multi-wavelength
data, including X-Shooter, HST, Swift, Chandra and ATCA data, we
further investigate the vicinity of HLX-1. We additionally estimate
the mass of the central supermassive black hole in ESO 243-49 using
the (black hole mass)–(host spheroid Sérsic index) scaling relation,
the central velocity dispersion and the fundamental plane of black
hole activity and examine the nature of the host galaxy.
Results: No
evidence for a recent minor-merger that could result in the presence
of the IMBH is discerned, but the data suggest that minor mergers may
have occurred in the history of ESO 243-49. The MUSE data reveal a
rapidly rotating disc in the centre of the galaxy, around the
supermassive black hole. The mass of the supermassive black hole at
the centre of ESO 243-49 is estimated to be
0.5–23×107 MSun. Studying the
spectra of HLX-1, that were taken in the low/hard state, we determine
Hα flux variability by at least a factor 6, compared to optical
spectra taken during the high/soft state. This Hα flux variability
over one year indicates that the line originates close to the
intermediate mass black hole, excluding the possibility that the line
emanates from a surrounding nebula or a star cluster. The large
variability associated with the X-ray states of HLX-1 confirms that
the optical line is associated with the object and therefore validates
the distance to HLX-1.