A Comparison of the UV and HI Properties of the Extended UV (XUV) Disk
  Galaxies NGC 2541, NGC 5832 and ESO406-042
  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) emission 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.
