Ionized Gas Kinematics around an Ultra-luminous X-ray Source in NGC 5252 : Additional Evidence for an Off-nuclear AGN

TL;DR

This study uses optical observations to confirm that an ultra-luminous X-ray source in NGC 5252 is likely a low-mass galaxy nucleus, showing ordered gas motions and ionization consistent with an off-nuclear active galactic nucleus.

Contribution

The paper provides new integral-field spectroscopic evidence supporting the ULX as a low-mass galaxy nucleus, not a background object, with detailed kinematic and metallicity analysis.

Findings

Ionized gas near the ULX shows ordered rotation.

The ULX is likely a low-mass galaxy nucleus.

Gas metallicity around the ULX is low, similar to other low-mass AGNs.

Abstract

The Seyfert 2 galaxy NGC 5252 contains a recently identified ultra-luminous X-ray (ULX) source that has been suggested to be a possible candidate off-nuclear low-mass active galactic nucleus. We present follow-up optical integral-field unit observations obtained using GMOS on the Gemini-North telescope. In addition to confirming that the ionized gas in the vicinity of the ULX is kinematically associated with NGC 5252, the new observations reveal ordered motions consistent with rotation around the ULX. The close coincidence of the excitation source of the line-emitting gas with the position of the ULX further suggests that ULX itself is directly responsible for the ionization of the gas. The spatially resolved measurements of [N II] $\lambda$ 6584/H$\alpha$ surrounding the ULX indicate a low gas-phase metallicity, consistent with those of other known low-mass active galaxies but not that of its more massive host galaxy. These findings strengthen the proposition that the ULX is not a background source, but rather that it is the nucleus of a small, low-mass galaxy accreted by NGC 5252.