Tracing young star-forming clumps in the nearby flocculent spiral galaxy NGC 7793 with UVIT imaging

Abstract

Star formation in galaxies is a hierarchical process with a wide range of scales, from smaller clusters to larger stellar complexes. Here, we present an ultra-violet imaging study of the nearby flocculent spiral galaxy NGC 7793, observed using the Ultra-Violet Imaging Telescope (UVIT). We find that the disk scale length estimated in Far-UV (2.64 ± 0.16 kpc) is larger than that in Near-UV (2.21 ± 0.21 Kpc) and optical (1.08 Kpc), which supports the inside-out growth scenario of the galaxy disk. The star-forming UV disk is also found to be contained within the extent of H i gas, with a column density greater than 1021 cm−2. With the spatial resolution of UVIT (1 pixel ~ 6.8 pc), we identified 2046 young star-forming clumps in the galaxy with radii between ~12 and 70 pc, which matches well with the size of giant molecular clouds (GMCs) detected in the galaxy. Around 61% of the regions identified in our study are younger than 20 Myr, which points to a recent enhancement in star formation across the galaxy. We also note that the youngest star-forming regions, with an age of <10 Myr, distinctly trace the flocculent arms of the galaxy. The estimated mass of the clumps covers a range between 103 and 106M⊙. We notice a gradient in the mass distribution of identified clumps along the spiral arms. We also study the nuclear star cluster of the galaxy, finding that the stellar populations in the cluster outskirts are younger than the inner part.