Quantified H i Morphology VII: The Morphology of Extended Disks in UV
  and H i
  Extended UltraViolet (xuv) disks have been found in a substantial fraction of
late-type --S0, spiral and irregular-- galaxies. Similarly, most late-type
spirals have an extended gas disk, observable in the 21cm radio line (HI). The
morphology of galaxies can be quantified well using a series of scale-invariant
parameters; Concentration- Asymmetry-Smoothness (CAS), Gini, M20, and GM
parameters. In this paper, we compare the quantified morphology and effective
radius (R50) 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 disks correlate. We find that xuv
disks do not stand out by their effective radii in UV or HI. However, the
concentration index in FUV appears to select some xuv disks. And known xuv
disks can be identified via a criterion using Asymmetry and M20; 80% of xuv
disks are included but with 55% contamination. This translates into 61
candidate xuv disk out of our 266 galaxies, (23%) consistent with previous
findings. We consider three scenarios; tidal features from major mergers, the
typical extended Hi disk 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 disks based on their concentration (CHI > 5 and CFUV
> 4 respectively), but note that these two subsamples never overlap in the
WHISP sample. This appears to discount a simple photo-dissociation origin of
the outer HI disk. Previously, we identified the morphology space occupied by
ongoing major mergers. Known xuv disks rarely reside in the merger dominated
part of HI morphology space but those that do are Type 1. This suggests cold
flows as the origin for the xuv complexes and their surrounding HI structures.
