On the Nature of the HII Regions in the Extended Ultraviolet Disc of NGC
  4625
  Using deep Subaru/FOCAS spectra of 34 HII regions in both the inner and outer
parts of the extended ultraviolet (XUV) disc galaxy NGC 4625 we have measured
an abundance gradient out to almost 2.5 times the optical isophotal radius. We
applied several strong line abundance calibrations to determine the HII region
abundances, including R23, [NII]/[OII], [NII]/Ha as well as the [OIII]4363
auroral line, which we detected in three of the HII regions. We find that at
the transition between the inner and outer disc the abundance gradient becomes
flatter. In addition, there appears to be an abundance discontinuity in
proximity of this transition. Several of our target HII regions appear to
deviate from the ionisation sequence defined in the [NII]/Ha vs. [OIII]/Hb
diagnostic diagram by bright extragalactic HII regions. Using theoretical
models we conclude that the most likely explanations for these deviations are
either related to the time evolution of the HII regions, or stochastic
variations in the ionising stellar populations of these low mass HII regions,
although we are unable to distinguish between these two effects. Such effects
can also impact on the reliability of the strong line abundance determinations.
