KINEMATICS OF THE INTERMEDIATE-MASS BLACK HOLE CANDIDATE HLX-1

We studied the optical spectrum of HLX-1 during its latest outburst, using the FORS2 spectrograph on the Very Large Telescope. We detect an H alpha emission line centered at [lambda] = 6718.9 + or - 0.9 [Angstrom] and find that its projected radial velocity with respect to the nucleus of ESO 243-49 is 424 + or - 27 km s super(-1), while the maximum rotational velocity of the stars in that galaxy is [approximate]209 km s super(-1). This suggests that HLX-1 and its surrounding stars were not formed in situ, but came either from a disrupted dwarf galaxy or from a nuclear recoil. We also find that the H alpha emission line is resolved with FWHM [approximate]400 km s super(-1), suggesting a nebular rather than disk origin for the emission. Its luminosity (L sub(H alpha ) [approximate] a few 10 super(37) erg s super(-1), equivalent width [approximate]70 [Angstrom]) is also consistent with emission from a nebula photoionized by HLX-1.