On the Nature of the HII Regions in the Extended Ultraviolet Disc of NGC 4625

TL;DR

This study investigates the chemical abundance gradient and ionization properties of HII regions in the extended ultraviolet disc of NGC 4625, revealing a flattened gradient and potential abundance discontinuity at the disc transition.

Contribution

It provides the first detailed measurement of abundance gradients out to 2.5 times the optical radius in NGC 4625 using deep spectroscopic data and multiple calibration methods.

Findings

Abundance gradient flattens at the disc transition.

Potential abundance discontinuity near the inner-outer disc boundary.

Deviations in ionization diagnostics suggest effects of stellar evolution or stochasticity.

Abstract

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.