Deep FUV observations of NGC6946, NGC628 & NGC5457

The SFRs 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 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 nonluminous 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.