PHANGS-JWST First Results: The 21 \(\mu\)m Compact Source Population

Abstract

We use PHANGS-JWST data to identify and classify 1271 compact 21 \(\mu\)m sources in four nearby galaxies using MIRI F2100W data. We identify sources using a dendrogram-based algorithm, and we measure the background-subtracted flux densities for JWST bands from 2 \(\mu\)m to 21 \(\mu\)m. Using the SED in JWST as well as HST bands, plus ALMA and MUSE/VLT observations, we classify the sources by eye. Then we use this classification to define regions in color-color space, and so establish a quantitative framework for classifying sources. We identify 1085 sources as belonging to the ISM of the target galaxies with the remainder being dusty stars or background galaxies. These 21 \(\mu\)m sources are strongly spatially associated with HII regions (\(>92\%\) of sources), while 74\(\%\) of sources are coincident with a stellar association defined in the HST data. Using SED fitting, we find that the stellar masses of the 21 \(\mu\)m sources span a range of 10\(^{2}\) to 10\(^{4}~M_\odot\) with mass-weighted ages down to 2 Myr. There is a tight correlation between attenuation-corrected H\(\alpha\) and 21 \(\mu\)m luminosity for \(L_{\nu,\mathrm{F2100W}}>10^{19}~\mathrm{W~Hz}^{-1}\). Young embedded source candidates selected at 21 \(\mu\)m are found below this threshold and have \(M_\star < 10^{3}~M_\odot\).