An FUV-optical Approach for Studying Hierarchical Star Formation in Nearby Galaxies with UVIT

Abstract Young star-forming clumps (SFCs) emit strongly in the ultraviolet (UV), making UV imaging ideal for detecting them. The Ultraviolet Imaging Telescope (UVIT) on board AstroSat, with 1 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover> <mml:mrow> <mml:mo>.</mml:mo> </mml:mrow> <mml:mrow> <mml:mtext>″</mml:mtext> </mml:mrow> </mml:mover> </mml:math> 5 resolution, has enabled the characterization of recently formed (up to 300 Myr) SFCs on tens of parsec scales in nearby galaxies. The spatial distribution of SFCs with different ages can provide insights into the hierarchy of star formation. This study presents a seminovel approach to characterize SFCs in two nearby spiral galaxies, NGC 5457 and NGC 1313, by combining UVIT far-UV (FUV) data with g -band data from the Dark Energy Camera Legacy Survey. We tested and optimized our method on NGC 5457 and showed that after proper background subtraction, the FUV − g color of SFCs can serve as an equally reliable age indicator as the widely used FUV − near-UV (NUV) color. Next, we parameterized the star formation hierarchy in NGC 5457 using the two-point correlation function and found good agreement between the hierarchical parameters derived using FUV − NUV and FUV − g based ages. Using our FUV − g based SFC ages, we also constrained the global hierarchical parameter of NGC 1313 for the first time. The development of our FUV − g based method is motivated by the fact that the NUV channel of the UVIT is not operational, and there is a wealth of archival UVIT FUV-only observations of nearby galaxies. This work demonstrates the potential of our method in constraining the SFC’ ages and investigating hierarchical star formation in nearby galaxies using FUV and optical observations.