Quantified HI morphology - VI. The morphology of extended discs in UV and HI

Extended ultraviolet (XUV) discs have been found in a substantial fraction of late-type - S0, spiral and irregular - galaxies. Similarly, most late-type spirals have an extended gas disc, observable in the 21-cm radio line (HI). The morphology of galaxies can be quantified well using a series of scale-invariant parameters; concentration-asymmetry-smoothness (CAS), Gini, M-20, and G(M) parameters. In this series of papers, we apply these to HI column density maps to identify mergers and interactions, lopsidedness and now XUV discs. In this paper, we compare the quantified morphology and effective radius (R-50) of the Westerbork observations of neutral Hydrogen in Irregular and SPiral galaxies Project (WHISP) HI maps to those of far-and near-ultraviolet images obtained with GALEX, to explore how close the morphology and scales of HI and UV in these discs correlate. We find that XUV discs do not stand out by their effective radii in UV or HI. However, the concentration index in far-ultraviolet (FUV) appears to select some XUV discs. And known XUV discs can be identified via a criterion using asymmetry and M-20; 80 per cent of XUV discs are included but with 55 per cent contamination. This translates into 61 candidate XUV disc out of our 266 galaxies, 23 per cent consistent with previous findings. Otherwise, the UV and HI morphology parameters do not appear closely related. Our motivation is to identify XUV discs and their origin. We consider three scenarios; tidal features from major mergers, the typical extended HI disc is a photo-dissociation product of the XUV regions and both HI and UV features originate in cold flows fueling the main galaxy. We define extended HI and UV discs based on their concentration (C-HI > 5 and C-FUV > 4 respectively), but that these two subsamples never overlap in the WHISP sample. This appears to discount a simple photo-dissociation origin of the outer HI disc. Previously, we identified the morphology space occupied by ongoing major mergers. Known XUV discs rarely reside in the merger-dominated part of HI morphology space but those that do are type 1. The exceptions, XUV discs in ongoing mergers, include the previously identified UGC 4862 and UGC 7081, 7651, and 7853. This suggests cold flows as the origin for the XUV complexes and their surrounding HI structures.