Quantified H i Morphology VII: The Morphology of Extended Disks in UV and H i

TL;DR

This study compares the morphology of extended UV and HI disks in galaxies, finding that certain morphological parameters can identify xuv disks and suggesting cold flows as their likely origin.

Contribution

It introduces a morphological criterion to identify xuv disks using UV and HI data, and explores their possible origins, emphasizing cold flows over merger or photo-dissociation scenarios.

Findings

80% of xuv disks are identified with a specific asymmetry and M20 criterion.

Extended HI and UV disks do not overlap, challenging simple photo-dissociation models.

Cold flows are suggested as the primary origin for xuv complexes and surrounding HI structures.

Abstract

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.