PHANGS-JWST First Results: Massive Young Star Clusters and New Insights from JWST Observations of NGC 1365

TL;DR

This study leverages JWST's infrared capabilities combined with HST data to uncover and analyze the most massive young star clusters in NGC 1365, revealing new insights into their obscuration, formation, and the galaxy's dynamics.

Contribution

It presents the first JWST observations of NGC 1365, discovering 16 new massive young star clusters and providing new understanding of their obscuration timescales and formation triggers.

Findings

NGC 1365 hosts the richest population of massive young clusters within 30 Mpc.

Approximately 13% of clusters are so embedded they are invisible in optical I band.

Star cluster formation is influenced by gas and dust stream interactions in the galaxy's bar.

Abstract

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 paper we combine new infrared images taken with JWST with our optical HST images of the star-bursting 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 $\sim$ 30 star clusters that are more massive than 10$^6$ Msolar 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 ($\sim$13$\%$) are so deeply embedded that they cannot be seen in the I band (AV $\gt$ 10 mag), and that 11 of 30 ($\sim$37$\%$) 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 obscured in the visible for $\sim$ 1.3 $\pm$ 0.7 Myr, and are either completely or partially obscured for $\sim$ 3.7 $\pm$ 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.