Kinematics of the intermediate mass black hole candidate HLX-1

Abstract

We studied the optical spectrum of HLX-1 during its latest outburst, using the FORS2 spectrograph on the Very Large Telescope. We detect an Halpha emission line centered at lambda = (6718.9 +/- 0.9) Ang and find that its projected radial velocity with respect to the nucleus of ESO243-49 is (424 +/- 27) km/s, while the maximum rotational velocity of the stars in that galaxy is ~209 km/s. 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 Halpha emission line is resolved with full width at half maximum ~400 km/s, suggesting a nebular rather than disk origin for the emission. Its luminosity (L_{Halpha} ~ a few 10^{37} erg/s, equivalent width ~70 Ang) is also consistent with emission from a nebula photo-ionized by HLX-1.