Journal Article

On the nature of the H <span class="smallCaps">ii</span> regions in the extended ultraviolet disc of NGC 4625

Q. E. Goddard, F. Bresolin, R. C. Kennicutt, E. V. Ryan-Weber and F. F. Rosales-Ortega

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 412, issue 2, pages 1246-1258
Published in print April 2011 | ISSN: 0035-8711
Published online March 2011 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2010.17990.x
On the nature of the H ii regions in the extended ultraviolet disc of NGC 4625

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Using deep Subaru/Faint Object Camera And Spectrograph (FOCAS) spectra of 34 H ii 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 H ii region abundances, including R23, [N ii]/[O ii], [N ii]/Hα as well as the [O iii] λ4363 auroral line, which we detected in three of the H ii 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 H ii regions appear to deviate from the ionization sequence defined in the [N ii]/Hα versus [O iii]/Hβ diagnostic diagram by bright extragalactic H ii regions. Using theoretical models we conclude that the most likely explanations for these deviations are either related to the time evolution of the H ii regions or stochastic variations in the ionizing stellar populations of these low-mass H ii 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.

Keywords: galaxies: abundances; galaxies: individual: NGC 4625; galaxies: ISM; galaxies: structure

Journal Article.  9849 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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