Kinematics of the intermediate mass black hole candidate HLX-1

TL;DR

This study analyzes the optical spectrum of HLX-1 during an outburst, revealing emission features and velocities that suggest an extragalactic origin and nebular emission, providing insights into its nature and environment.

Contribution

First detailed optical spectral analysis of HLX-1 during outburst, indicating its possible origin from a disrupted dwarf galaxy or recoil, and characterizing its nebular emission.

Findings

Halpha emission line detected at 6718.9 Å

Radial velocity of HLX-1 is ~424 km/s

Halpha line width suggests nebular origin

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.