Looking for Obscured Young Star Clusters in NGC 1313

Abstract

Using recently acquired Hubble Space Telescope NIR observations (J, Paβ, and H bands) of the nearby galaxy NGC 1313, 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 or (2) compact H ii 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-χ 2 spectral energy distribution broad- and narrowband fitting process. The 100 clusters in the final samples have masses in the range 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 are used to estimate the typical timescales for clearing the natal gas cloud, resulting in between 3 and 5 Myr, ∼1 Myr older than what was estimated from NUV–optical-based cluster studies. This difference hints at a bias for optical-only-based studies, which James Webb Space Telescope will address in the coming years.

The dependence of the hierarchical distribution of star clusters on galactic environment

“…Due to both its physical and morphological properties, including the presence of a bar and an irregular appearance, NGC 1313 has been compared to the Large Magellanic Cloud (de Vaucouleurs 1963). Hannon et al (2019) and Messa et al (2021) have recently analysed the properties of NGC 1313's star clusters and H II regions, using H α narrow-band and near-infrared Paβ observations. LEGUS observed NGC 1313 with two distinct pointings, and we use a mosaic prepared from the two pointings for the analysis in this study.…”

Momentum feedback from marginally-resolved HII regions in isolated disc galaxies

“…By contrast, if the young stars remain embedded for long periods of time, then the lower branch remains close to the galactic average value. This is seen in the simulations of Fujimoto et al (2019), who find a duration of 23 ± 1 Myr, nearly an order of magnitude longer than observed (Whitmore et al 2014;Hollyhead et al 2015;Grasha et al 2018Grasha et al , 2019Hannon et al 2019;Kruijssen et al 2019;Chevance et al 2020b;Kim et al 2020a;Messa et al 2021). In our simulations, this time-scale ranges from 4.4 Myr (HII region momentum runs) up to > 5 Myr (runs without HII region momentum; representing a lower limit, because the duration of co-existence cannot exceed the adopted duration of the young stellar phase, which is 5 Myr).…”

The Panchromatic Hubble Andromeda Treasury: Triangulum Extended Region (PHATTER). III. The Mass Function of Young Star Clusters in M33

“…The lower threshold is adopted because during the first 10 Myr of a clusters life, clusters are embedded, making optical observations difficult, and our sample incomplete. In addition, young embedded groupings of stars (< 10 Myr), are still forming through hierarchical merging of sub-clumps, making it unclear whether the resulting structure will be a long-lived, gravitationally bound cluster (Allison et al 2010;Gieles et al 2012;Messa et al 2021). The upper threshold is where CMD fitting becomes less accurate due to the cluster's main sequence turnoff dropping below the 50% detection limit for the stellar photometry.…”