A 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) onboard AstroSat, with 1.5arcsec 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 semi-novel approach to characterize SFCs in two nearby spiral galaxies, NGC 5457 and NGC 1313, by combining UVIT FUV data with g-band data from the Dark Energy Camera Legacy Survey (DECaLS). 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-NUV color. Next, we parametrized the star formation hierarchy in NGC 5457 using the two-point correlation function (TPCF) and found good agreement between the hierarchy parameters derived using FUV-NUV and FUV-g based ages. Using our FUV-g based SFC ages, we also constrained the global hierarchy 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.

Figures (10)

• Figure 1: Left: False-color composite image of NGC 5457, where blue, yellow, and red represent FUV, NUV, and g-band emissions, respectively. Right: False-color composite image of NGC 1313, where blue and red represent FUV and g-band emissions, respectively. In both panels, the golden box indicates the HST FoV ($3\arcmin \times 3\arcmin$ per pointing), adopted from menon2021dependence. NGC 5457 is covered by four pointings ($36~\mathrm{arcmin}^2$), while NGC 1313 is covered by two pointings ($18~\mathrm{arcmin}^2$).
• Figure 2: A schematic illustrating our local background estimation method - as applied on the UVIT FUV image of NGC 5457. The galaxy is divided into five concentric zones (red rings). Small (orange) circular apertures of increasing radius (radii = 15, 20, 30, 40, 60) were placed in each zone for the local background estimation.
• Figure 3: Left to right : The radial flux profile of NGC 5457 in FUV, NUV and g-band. Solid lines represent the normalized total flux per pixel, and dotted lines show the normalized local background flux per pixel. The normalization is performed with respect to the total flux measured at the innermost radial bin.
• Figure 4: Left: Attenuation + local background corrected CMD (FUV$-$NUV colors vs FUV magnitude) for NGC 5457. Right: Attenuation + local background corrected CMD (FUV$-$g colors vs FUV magnitude) for NGC 5457. In both the CMDs, the blue/green points represent the local background corrected colors and magnitudes, whereas the grey points represent the sky background corrected colors and magnitudes. The four dashed tracks with diamonds represent the synthetic CMDs of star clusters of varying stellar masses. The numbers written in black on top of the tracks represent the ages of the simulated star clusters in SB99. Only the SFCs with magnitude errors cut smaller than 0.10 have been used in this analysis.
• Figure 5: Top left : SFC age map of NGC 5457 derived using the local background corrected FUV$-$NUV color. Top right : SFC age map of NGC 5457 derived using the local background corrected FUV$-$g color. Bottom left : Spatial age difference map for NGC 5457 where the colorbar represents the difference between the local background corrected FUV$-$g color based ages and local background corrected FUV$-$NUV color based ages., Bottom right : A comparison between FUV$-$NUV and FUV$-$g color-based ages reveals excellent agreeement, with a Pearson correlation coefficient value of 0.82.