On the nature of the H II regions in the extended ultraviolet disc of NGC 4625

Abstract

Using deep Subaru/Faint Object Camera And Spectrograph (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]/Hα as well as the [Oiii]λ4363 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 ionization sequence defined in the [Nii]/Hα versus [Oiii]/Hβ 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 ionizing 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. © 2010 The Authors Monthly Notices of the Royal Astronomical Society © 2010 RAS.