Comparing the Inner and Outer Star Forming Complexes in the Nearby
  Spiral Galaxies NGC 628, NGC 5457 and NGC 6946 using UVIT Observations
  We present a far-UV (FUV) study of the star-forming complexes (SFCs) in three
nearby galaxies using the Ultraviolet Imaging Telescope (UVIT). The galaxies
are close to face-on and show significant outer disk star formation. Two of
them are isolated (NGC 628, NGC 6946), and one is interacting with distant
companions (NGC 5457). We compared the properties of the SFCs inside and
outside the optical radius (R$_{25}$). We estimated the sizes, star formation
rates (SFRs), metallicities, and the Toomre Q parameter of the SFCs. We find
that the outer disk SFCs are at least ten times smaller in area than those in
the inner disk. The SFR per unit area ($\Sigma_{SFR}$) in both regions have
similar mean values, but the outer SFCs have a much smaller range of
$\Sigma_{SFR}$. They are also metal-poor compared to the inner disk SFCs. The
FUV emission is well correlated with the neutral hydrogen gas (\HI)
distribution and is detected within and near several \HI~holes. Our estimation
of the Q parameter in the outer disks of the two isolated galaxies suggests
that their outer disks are stable (Q$>$1). However, their FUV images indicate
that there is ongoing star formation in these regions. This suggests that there
may be some non-luminous mass or dark matter in their outer disks, which
increases the disk surface density and supports the formation of local
gravitational instabilities. In the interacting galaxy, NGC 5457, the baryonic
surface density is sufficient (Q$<$1) to trigger local disk instabilities in
the outer disk.
