Looking for obscured young star clusters in NCG 1313
Abstract
Using recently acquired HST NIR observations (J, Paβ 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 Hα/Paβ extinctions; 2. compact HII regions that appear as point-like sources in the Paβ emission map. The NUV--optical--NIR photometry of the candidate clusters is used to derive their ages, masses and extinctions via a least-2 SED broad and narrow--band fitting process. The 100 clusters in the final samples have masses in the range 10(M/M)=2.5-3.5 and moderate extinctions, E(B-V)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, E(B-V)<0.25 mag, already at very young ages (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 Hα and Paβ) 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, 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.