2013ApJ...768L..22S

We studied the optical spectrum of HLX-1 during its latest outburst, using the FORS2 spectrograph on the Very Large Telescope. We detect an Hα emission line centered at λ = 6718.9±0.9 Å and find that its projected radial velocity with respect to the nucleus of ESO 243-49 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 Hα emission line is resolved with FWHM ~400 km/s, suggesting a nebular rather than disk origin for the emission. Its luminosity (L_Hα~ a few 10³⁷ erg/s, equivalent width ~70 Å) is also consistent with emission from a nebula photoionized by HLX-1.