Looking for obscured young star clusters in NCG 1313

Abstract

Using recently acquired HST NIR observations (J, Pa${\hat{I}}^2$ and H bands) of the nearby galaxy NGC1313, we investigate the timescales required by a young star cluster to emerge from its natal cloud. We search for extincted star clusters, potentially embedded in their natal cloud as either: 1. compact sources in regions with high H$\hat{I}\pm$/Pa${\hat{I}}^2$ extinctions; 2. compact HII regions that appear as point-like sources in the Pa${\hat{I}}^2$ emission map. The NUV--optical--NIR photometry of the candidate clusters is used to derive their ages, masses and extinctions via a least$-\ddot{I}{‡}^2$ SED broad and narrow--band fitting process. The 100 clusters in the final samples have masses in the range ${\log }_{10}(M/{\mathrm{M}}_{\odot })=2.5-3.5$ and moderate extinctions, $\mathrm{E}(\mathrm{B}-\mathrm{V})\lesssim 1.0$ mag. Focusing on the young clusters ($0-6$ Myr) we derive a weak correlation between extinction and age of the clusters. Almost half of the clusters have low extinctions, $\mathrm{E}(\mathrm{B}-\mathrm{V})<0.25$ mag, already at very young ages ($\le 3$ Myr), suggesting that dust is quickly removed from clusters. A stronger correlation is found between the morphology of the nebular emission (compact, partial or absent, both in H$\hat{I}\pm$ and Pa${\hat{I}}^2$) and cluster age. Relative fractions of clusters associated with a specific nebular morphology is used to estimate the typical timescales for clearing the natal gas cloud, resulting between 3 and 5 Myr, $\sim 1$ Myr older than what estimated from NUV--optical--based cluster studies. This difference hints to a bias for optically--only based studies, which JWST will address in the coming years.

29 pages (20 figures, 5 tables) + appendix (6 pages, 6 figures, 1 table). Submitted, feedback/comments are welcome