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.

We present ∼ 0.1" resolution (∼ 10 pc) ALMA observations of a molecular cloud identified in the merging Antennae galaxies with the potential to form a globular cluster, nicknamed the "Firecracker." Since star formation has not yet begun at an appreciable level in this region, this cloud provides an example of what the birth environment of a globular cluster may have looked like before stars form and disrupt the natal physical conditions. Using emission from 12 CO(2-1), 12 CO(3-2), 13 CO(2-1), HCN(4-3), and HCO + (4-3) molecular lines, we are able to resolve the cloud's structure and find that it has a characteristic radius of 22 pc and a mass of 1-9×10 6 M . We also put constraints on the abundance ratios of 12 CO/ 13 CO and H 2 / 12 CO. Based on the calculation of the mass, we determine that the commonly used CO-to-H 2 conversion factor in this region varies spatially, with average values in the range X CO = (0.12 − 1.1) × 10 20 cm −2 (K km s −1 ) −1 . We demonstrate that if the cloud is bound (as is circumstantially suggested by its bright, compact morphology), an external pressure in excess of P/k > 10 8 K cm −3 is required. This would be consistent with theoretical expectations that globular cluster formation requires high pressure environments, much higher than typical values found in the Milky Way. The position-velocity diagram of the cloud and its surrounding material suggests that this high pressure may be produced by ram pressure from the collision of filaments. The radial profile of the column density can be fit with both a Gaussian and a Bonnor-Ebert profile. If the Bonnor-Ebert fit is taken to be indicative of the cloud's physical structure, it would imply the cloud is gravitationally stable and pressure-confined. The relative line strengths of HCN and HCO + in this region also suggest that these molecular lines can be used as a tracer for the evolutionary stage of a cluster.

Henize 2-10 (He 2-10) is a nearby (D = 9 Mpc) starbursting blue compact dwarf galaxy that boasts a high star formation rate and a low-luminosity active galactic nucleus. He 2-10 is also one of the first galaxies in which embedded super star clusters (SSCs) were discovered. SSCs are massive, compact star clusters that will impact their host galaxies dramatically when their massive stars evolve. Here, we discuss radio, submillimeter, and infrared observations of He 2-10 from 1.87 μm to 6 cm in high angular resolution (∼0.3″), which allows us to disentangle individual clusters from aggregate complexes as identified at lower resolution. These results indicate the importance of spatial resolution to characterize SSCs, as low resolution studies of SSCs average over aggregate complexes that may host SSCs at different stages of evolution. We explore the thermal, nonthermal, and dust emission associated with the clusters along with dense molecular tracers to construct a holistic review of the natal SSCs that have yet to dramatically disrupt their parent molecular clouds. We assess the production rate of ionizing photons, extinction, total mass, and the star formation efficiency (SFE) associated with the clusters. Notably, we find that the SFE for the some of the natal clusters is high (>70%), which suggests that these clusters could remain bound even after the gas is dispersed from the system from stellar feedback mechanisms. If they remain bound, these SSCs could survive to become objects indistinguishable from globular clusters.Unified Astronomy Thesaurus concepts: Interstellar medium (847); Star clusters (1567); H ɪɪ regions (694); Star formation (1569); Blue compact dwarf galaxies (165); Galaxies (573)

The Molecular Ridge in the LMC extends several kiloparsecs south from 30 Doradus, and it contains ∼30% of the molecular gas in the entire galaxy. However, the southern end of the Molecular Ridge is quiescent—it contains almost no massive star formation, which is a dramatic decrease from the very active massive-star-forming regions 30 Doradus, N159, and N160. We present new Atacama Large Millimeter/submillimeter Array and Atacama Pathfinder Experiment observations of the Molecular Ridge at a resolution as high as ∼16″ (∼3.9 pc) with molecular lines 12CO(1-0), 13CO(1-0), 12CO(2-1), 13CO(2-1), and CS(2-1). We analyze these emission lines with our new multiline non-LTE fitting tool to produce maps of T kin, n H 2 , and N CO across the region based on models from RADEX. Using simulated data for a range of parameter space for each of these variables, we evaluate how well our fitting method can recover these physical parameters for the given set of molecular lines. We then compare the results of this fitting with LTE and X CO methods of obtaining mass estimates and how line ratios correspond with physical conditions. We find that this fitting tool allows us to more directly probe the physical conditions of the gas and estimate values of T kin, n H 2 , and N CO that are less subject to the effects of optical depth and line-of-sight projection than previous methods. The fitted n H 2 values show a strong correlation with the presence of young stellar objects (YSOs), and with the total and average mass of the associated YSOs. Typical star formation diagnostics, such as mean density, dense gas fraction, and virial parameter do not show a strong correlation with YSO properties.

The properties of young massive clusters (YMCs) are key to understanding the star formation mechanism in starburst systems, especially mergers. We present Atacama Large Millimeter/submillimeter Array high-resolution (∼10 pc) continuum (100 and 345 GHz) data of YMCs in the overlap region of the Antennae galaxy. We identify six sources in the overlap region, including two sources that lie in the same giant molecular cloud (GMC). These YMCs correspond well with radio sources in lower-resolution continuum (100 and 220 GHz) images at GMC scales (∼60 pc). We find most of these YMCs are bound clusters through virial analysis. We estimate their ages to be ∼1 Myr and that they are either embedded or just beginning to emerge from their parent cloud. We also compare each radio source with a Paβ source, and find they have consistent total ionizing photon numbers, which indicates they are tracing the same physical source. By comparing the free–free emission at ∼10 pc scale and ∼60 pc scale, we find that ∼50% of the free–free emission in GMCs actually comes from these YMCs. This indicates that roughly half of the stars in massive GMCs are formed in bound clusters. We further explore the mass correlation between YMCs and GMCs in the Antennae and find it generally agrees with the predictions of the star cluster simulations. The most massive YMC has a stellar mass that is 1%–5% of its host GMC mass.