A comparison of the UV and HI properties of the extended UV (XUV) disk galaxies NGC 2541, NGC 5832 and ESO406-042

Abstract

We present a UV study of 3 extended UV (XUV) galaxies that we have observed with the UVIT and the GMRT. XUV galaxies show filamentary or diffuse star formation well beyond their optical disks, in regions where the disk surface density lies below the threshold for star formation. GALEX observations found that surprisingly 30% of all the nearby spiral galaxies have XUV disks. The XUV galaxies can be broadly classified as Type 1 and Type 2 XUV disks. The Type 1 XUV disks have star formation that is linked to that in their main disk, and the UV emission appears as extended, filamentary spiral arms. The UV luminosity is associated with compact star forming regions along the extended spiral arms. The star formation is probably driven by slow gas accretion from nearby galaxies or the intergalactic medium (IGM). But the Type 2 XUV disks have star formation associated with an outer low luminosity stellar disk that is often truncated near the optical radius of the galaxy. The nature of the stellar disks in Type 2 XUV disks are similar to that of the diffuse stellar disks of low surface brightness galaxies. The star formation in Type 2 XUV disks is thought to be due to rapid gas accretion or gas infall from nearby high velocity clouds (HVCs), interacting galaxies or the IGM. In this paper, we investigate the star formation properties of the XUV regions of two Type 2 galaxies and one mixed XUV type galaxy and compare them with the neutral hydrogen (HI) emisison in their disks. We present preliminary results of our UVIT (FUV and NUV) observations of NGC 2541, NGC 5832 and ESO406-042, as well as GMRT observations of their HI emission. We describe the UV emission morphology, estimate the star formation rates and compare it with the HI distribution in these Type 2 and mixed XUV galaxies.

KMTNet Nearby Galaxy Survey: Overview and Survey Description

“…This indicates that SF actively occurs in the outer disk and may serve as indirect evidence for inside-out disk formation (e.g., Padave et al 2021). While the physical origin of this feature is still under debate (e.g., Gil de Paz et al 2005;Dong et al 2008;Emami et al 2019;Das et al 2021), the flux ratio of Hα to UV in the XUV disks appears to be lower than that in typical star-forming regions (e.g., Zaritsky & Christlein 2007;Goddard et al 2010). A similar discrepancy between UV and Hα has also been reported in dwarf galaxies (e.g., Lee et al 2009).…”

A UVIT Look at Star Formation in Nearby Interacting/Merging Galaxies

“…The observations were conducted mainly in the far-UV (FUV) band (1300-1800 A) with the Ultraviolet Imaging Telescope (UVIT), on board the AstroSat Satellite. The details of the filters and instrument are briefly described in (Das et al 2021). The advantage of using the UVIT is that it has a wide field of view of approximately 28 and a spatial resolution ∼ 1.5 , which is good enough to resolve the large star forming clusters UVIT observations of star formation 115 in nearby galaxies.…”

The PHANGS-AstroSat Atlas of Nearby Star-forming Galaxies

“…Different scenarios are proposed for triggering the star formation process in XUV disks, such as interactions, perturbations, and gas accretion (Bush et al 2008(Bush et al , 2010Das et al 2021). In addition, spiral density waves could propagate from the inner to outer disk and result in gravitationally unstable regions that eventually form stars (Lemonias et al 2011).…”