Dr Thomas Williams

Abstract:

We use JWST NIRCam and MIRI imaging at 2, 4, 10, and 21 μm to study young, dusty compact sources in four nearby galaxies at distances of ∼1–5 Mpc (M33, NGC 300, NGC 7793, and NGC 5068). This work bridges well-characterized massive young stellar objects (MYSOs) in the Magellanic Clouds from the Spitzer Space Telescope SAGE survey to new studies of embedded clusters in more distant galaxies with JWST. Guided by the SAGE-LMC catalog, we define JWST color–magnitude selection criteria (F1000W versus F1000W − F2100W) and test them using resolution-degradation experiments. We identify 216, 32, 80, and 139 dusty young objects in the four galaxies, respectively. The selected population spans sources from systems dominated by a single MYSO to compact marginally resolved sources hosting multiple MYSOs. The color selection remains stable across 1–5 Mpc, and the 10 μm luminosity function retains a slope of α ∼ −2. However, blending and surface-brightness dilution remove fainter sources, leading to incompleteness of up to ∼50% at 5.2 Mpc and biasing the sample toward brighter objects (F1000W < 19 mag). The sample spans approximate stellar masses of ∼10–2 × 10⁵ M⊙. Spatial resolution affects the interpretation of mid-IR emission: clustering increases the fraction of emission attributed to compact sources in active regions, while blending into diffuse emission dominates in quiescent environments. Comparisons with polycyclic aromatic hydrocarbon (PAH)-selected young clusters in the PHANGS galaxy NGC 5068 show that our selection recovers ∼80% of the PAH-selected sources. We show that the practical limit for studying individual MYSOs with JWST is ∼3 Mpc. The resulting catalog provides a foundation for future resolved studies of star formation rates and early cluster evolution.

Abstract:

Context. The spatial distribution of giant molecular clouds (GMCs) at sub-kiloparsec scales encodes information about cloud formation and evolution. However, we still lack a general quantitative characterisation of molecular gas structure at this scale. Aims. We aim to provide a quantitative description of molecular gas structure at 150–1000 pc for a typical star-forming main sequence galaxy. We analyse how GMCs cluster together and how CO emission is spatially correlated with bright GMCs using a sample of 8984 GMCs from 40 galaxies observed by PHANGS-ALMA. Methods. We homogenized our data to a common spatial resolution of 150 pc and a mass sensitivity of 2.5 M ⊙ pc −2 to remove observational bias. We then calculated the nearest neighbour distances, neighbour number density, and two-point correlation functions (2PCFs) for the catalogued GMCs in each galaxy. When analysing the 2PCFs, we generated several control samples that reflect different null hypotheses on large spatial scales. We stacked integrated intensity CO emission profiles around the position of catalogued GMCs to probe the gas distribution on scales between the observational resolution and the typical GMC-GMC spacing. Results. Our measurements of cloud spacing and the number of neighbours show that GMC clustering follows the large-scale gas distribution. Once we accounted for this contribution, the peak excess clustering relative to the null hypothesis in the 2PCF dropped from 1 + ω ∼ 2.3 to 1.3, with the power-law slope flattened from −0.25 to 0. Stacks of CO intensity around local maxima show a strong clustering signal on scales smaller than the typical GMC-GMC separation. We show that this is largely the same signal captured by the ‘GMC size’ measured by CPROPS , with an additional ∼20% of the flux in an extended component beyond 500 pc. We find that our stacked profiles can be fit with a double Gaussian function plus a constant offset. The broad Gaussian component accounts for 70% of the over-density power above the constant background and is stronger around massive and gravitationally bound GMCs. Conclusions. Our measurements yield a general statistical description of the structure of CO emission from ≈150 pc to galactic scales that can serve as a benchmark for simulations of molecular cloud formation and destruction in galaxy disks. Our results indicate that galactic structure exerts a strong influence on the GMC distribution in galaxy disks and that the formation of massive gravitationally bound GMCs is related to strong local gas clustering.

Abstract:

We analyze H i 21 cm observations of the Local Group dwarf galaxy Wolf–Lundmark–Melotte (WLM) from the Local Group L-Band Survey to search for evidence of ram pressure stripping. While previous MeerKAT-16 observations of WLM showed evidence for off-galaxy atomic gas emission with a geometry suggestive of ram pressure stripping, our observations find no evidence for this stripped gas. We demonstrate that our observations would be sensitive to the claimed detections and suggest that an uncorrected observational flaw with the MeerKAT data led to the apparent off-galaxy emission. The lack of off-galaxy emission obviates the need for uncharacteristically high values of the density of the intergalactic medium in this region.

Abstract:

We present a multiwavelength catalog of mid-infrared-selected compact sources in 19 nearby galaxies, combining JWST NIRCam/MIRI, Hubble Space Telescope UV–optical broadband, Hα narrowband, and Atacama Large Millimeter/submillimeter Array CO observations. We detect 24,945 compact sources at 21 μm and 55,581 at 10 μm. Artificial star tests show 50% completeness limits of ∼5 μJy for the 10 μm catalog, and ∼24 μJy for the 21 μm catalog. We find that 21 μm compact sources contribute ∼20% of the total galaxy emission in that band, but only contribute 5% at 10 μm. We classify sources using stellar evolution and population synthesis models combined with empirical classifications derived from the literature. Our classifications include Hα-bright and dust-embedded optically faint clusters, red supergiants, oxygen-rich and carbon-rich asymptotic giant branch stars, and a range of rarer stellar types. In sampling a broad range of star-forming environments with a uniform, well-characterized selection, this catalog enables analyses of infrared-bright stellar populations. We find that Hα-faint sources account for only 10% of dusty (likely young) clusters, implying that the infrared-bright, optically faint phase of cluster evolution is short compared to the Hα-bright stage. The luminosity functions of 10 and 21 μm sources follow power-law distributions, with the 21 μm slope (−1.7 ± 0.1) similar to that of giant molecular cloud mass functions and ultraviolet bright star-forming complexes, while the 10 μm slope (−2.0 ± 0.1) is closer to that of young stellar clusters.