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

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 ($>92\mathrm{\%}$ of sources), while 74$\mathrm{\%}$ 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 $L_{\nu ,\mathrm{F}2100\mathrm{W}}>{10}^{19}{\mathrm{W}\mathrm{H}\mathrm{z}}^{-1}$. Young embedded source candidates selected at 21 $\mu$m are found below this threshold and have $M_{\star }<{10}^3{M}_{\odot }$.