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