PHANGS-JWST first results: massive young star clusters and new insights from JWST Observations of NGC 1365

A primary new capability of JWST is the ability to penetrate the dust in star-forming galaxies to identify and study
the properties of young star clusters that remain embedded in dust and gas. In this Letter we combine new infrared
images taken with JWST with our optical Hubble Space Telescope (HST) images of the starbursting barred
(Seyfert2) spiral galaxy NGC 1365. We find that this galaxy has the richest population of massive young clusters
of any known galaxy within 30 Mpc, with ∼30 star clusters that are more massive than 106 Me and younger than
10 Myr. Sixteen of these clusters are newly discovered from our JWST observations. An examination of the optical
images reveals that 4 of 30 (∼13%) are so deeply embedded that they cannot be seen in the Hubble I band (AV  10
mag), and that 11 of 30 (∼37%) are missing in the HST B band, so age and mass estimates from optical
measurements alone are challenging. These numbers suggest that massive clusters in NGC 1365 remain completely
obscured in the visible for ∼1.3 ± 0.7 Myr and are either completely or partially obscured for ∼3.7 ± 1.1 Myr. We
also use the JWST observations to gain new insights into the triggering of star cluster formation by the collision of
gas and dust streamers with gas and dust in the bar. The JWST images reveal previously unknown structures (e.g.,
bridges and overshoot regions from stars that form in the bar) that help us better understand the orbital dynamics of
barred galaxies and associated star-forming rings. Finally, we note that the excellent spatial resolution of the
NIRCAM F200W filter provides a better way to separate barely resolved compact clusters from individual stars
based on their sizes.