Hidden in Plain Sight: UVIT and MUSE Discovery of a Large, Diffuse Star-Forming Galaxy

Abstract: We report the discovery of a nearby large, diffuse galaxy that shows star formation, using Ultra Violet Imaging Telescope (UVIT) far-UV observations, archival optical data from Multi-Unit Spectroscopic Explorer (MUSE) and Dark Energy Camera Legacy Survey (DECaLS), and InfraRed Survey Facility (IRSF) near-infrared observations. The galaxy was not detected earlier due to its superposition with the background galaxy, NGC 6902A. They were together mistakenly classified as an interacting system. NGC 6902A is at a redshift of 0.05554, but MUSE observations indicate that the interacting tail is a separate star-forming, foreground galaxy at a redshift of 0.00980. We refer to the new galaxy as UVIT J202258.73-441623.8 (UVIT J2022). The near-infrared observations show that UVIT J2022 has a stellar mass of 8.7$\times$10$^{8}$M$_{\odot}$. Its inner disk (R$<$4 kpc) shows UV and H$\alpha$ emission from ongoing massive star formation. The rest of the disk is extremely low luminosity, has a low stellar surface density, and extends out to a radius of R$\sim$9 kpc. The velocity and metallicity distribution maps and the star formation history indicate that UVIT J2022 has undergone three bursts of star formation. The latest episode is ongoing, which is supported by the presence of widespread H$\alpha$ and UV emission in its inner disk. The galaxy also shows patchy spiral arms in far-UV, and there is a metallicity enhancement along a bar-like feature. UVIT J2022 is thus a unique example of triggered star formation in a diffuse galaxy, resulting in the growth of its inner stellar disk. Our study raises the intriguing possibility that (i) there could be similar diffuse galaxies that have been mistakenly interpreted as interacting galaxies due to their superposition, and (ii) UV or H$\alpha$ could be a way to detect such diffuse galaxies in our local universe.