Comparing the Inner and Outer Star Forming Complexes in the Nearby Spiral Galaxies NGC 628, NGC 5457 and NGC 6946 using UVIT Observations

TL;DR

This study uses UVIT far-UV observations to compare star-forming complexes inside and outside the optical radius in three nearby galaxies, revealing differences in size, metallicity, and stability, and suggesting dark matter influences star formation.

Contribution

It provides a detailed comparison of inner and outer disk star-forming complexes in three galaxies using UVIT data, highlighting the role of dark matter in outer disk star formation.

Findings

Outer disk SFCs are ten times smaller than inner disk SFCs.

Outer SFCs are metal-poor compared to inner SFCs.

Outer disks in isolated galaxies are stable yet still form stars, implying dark matter influence.

Abstract

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.