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 \pm 0.9 Å and find that its projected radial velocity with respect to the nucleus of ESO 243-49 is 424 \pm 27 km s ^ { -1 } , while the maximum rotational velocity of the stars in that galaxy is \approx 209 km s ^ { -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 full width at half maximum \approx 400 km s ^ { -1 } , suggesting a nebular rather than disk origin for the emission . Its luminosity ( L _ { { H } \alpha } \approx a few 10 ^ { 37 } erg s ^ { -1 } , equivalent width \approx 70 Å ) is also consistent with emission from a nebula photo-ionized by HLX-1 .