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

Abstract

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 (H I). 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 H I 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) H I maps to those of far-and near-ultraviolet images obtained with GALEX, to explore how close the morphology and scales of H I and UV in these discs correlate. We find that XUV discs do not stand out by their effective radii in UV or H I. 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 H I 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 H I disc is a photo-dissociation product of the XUV regions and both H I and UV features originate in cold flows fueling the main galaxy.We define extended H I 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 H I disc.Previously, we identified the morphology space occupied by ongoing major mergers. Known XUV discs rarely reside in the merger-dominated part of H I 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 H I structures.

High Angular Momentum Halo Gas: A Feedback and Code-independent Prediction of LCDM

“…Additionally, Figure 6 demonstrates that the codirectional mass fraction of cold gas in the halo stayed consistently high over cosmic time (at least until z=1). We speculate that it may be possible that this high angular momentum accretion helps to explain observations of extended XUV disks (e.g., Thilker et al 2005Thilker et al , 2007Lemonias et al 2011;Holwerda et al 2012), local extended H I disks (e.g., García-Ruiz et al 2002;Oosterloo et al 2007;Christlein & Zaritsky 2008;Sancisi et al 2008;Walter et al 2008;Wang et al 2013;Huang et al 2014;Courtois et al 2015), and corotating cold halo gas around local Milky Way analogs (e.g., Diamond-Stanic et al 2016).…”

Far-ultraviolet morphology of star-forming filaments in cool core brightest cluster galaxies

“…Galaxies of different Hubble type appropriately stratify within the optical CAS volume, which has been expanded to include other wavelength regimes over the years (including for extragalactic FUV imaging; see e.g. Holwerda, Pirzkal & Heiner 2012).…”

Magnetic fields near the peripheries of galactic discs

“…Many papers on the topic (e.g. Moss et al 1998) imply that a significant magnetic field may exist beyond 10 kpc, because the stellar discs of galaxies (and, indeed, their gaseous discs) can be traced far beyond the usually adopted optical radius (usually taken as the radius R 25 within the isophote of 25th B-magnitude per square arcsec, see Gentile et al 2007;Hunter et al 2011;Holwerda et al 2012).…”