Dr Thomas Williams

Abstract:

High-resolution JWST images of nearby spiral galaxies reveal polycyclic aromatic hydrocarbon (PAH) structures that potentially trace molecular clouds, even CO-dark regions. For this paper, we identified ISM cloud structures in PHANGS-JWST 7.7μm PAH emission maps for 66 galaxies, smoothed to a common physical resolution of 30 pc and at native resolution. We extracted 108,466 cloud structures in the 30 pc sample and 146,040 clouds in the native resolution sample. We then calculated their molecular properties following a linear conversion from PAH to CO. Given the tendency for clouds in galaxy centers to overlap in velocity space, we opted to flag these clouds and omit them from the analysis in this work. The remaining clouds correspond to giant molecular clouds, such as those detected in CO(2-1) emission by ALMA, or lower surface density clouds that either fall below the ALMA detection limits of existing maps or genuinely have no molecular counterpart. We specifically used the homogenized sample for our analysis. Upon cross-matching the PAH clouds to the ALMA CO clouds at a homogenized resolution of 90 pc in 27 galaxies, we find that 41 $%$ of the PAH clouds are associated with a CO counterpart. We also show that the converted molecular cloud properties of the PAH clouds do not differ much when compared in different galactic environments. However, outside the central environment, the highest molecular mass surface density clouds are preferentially found in spiral arms. We further apply a lognormal fit to the mass spectra to an unprecedented extragalactic completeness limit of 2 , 10^ 3 , and find that spiral arms contain the most massive clouds compared to other galactic environments. Our findings support the idea that spiral arm gravitational potentials foster the formation of high surface density clouds, and that lower surface density clouds form in the interarm regions. The cloud values show a decline of a factor of ∼ 1.5-2 toward the outer 2-3 R_e. However, the trend largely varies in individual galaxies, with flat, decreasing, and even no trend as a function of R_̊m gal. Factors such as large-scale processes, galaxy types, and morphologies might influence the observed trends. We note that combining homogenized molecular properties of individual galaxies leads to the loss of information about the physical processes that are driving deviations in trends of those properties across different galactic environments. We published two catalogs at the CDS, one at the common resolution of 30 pc and another at the native resolution. We expect them to have broad utility for future studies of PAH clouds, molecular clouds, and star formation.

Abstract:

We present radial profiles of surface brightness in UV and IR bands, estimate stellar mass surface density (Σ⋆) and star formation rate surface density (ΣSFR), and predict the CO-to-H2 conversion factor (αCO) for over 5000 local galaxies with stellar mass M⋆ ≥ 109.3 M⊙. We build these profiles and measure galaxy half-light radii using GALEX and WISE images from the z0MGS program, with special care given to highly inclined galaxies. From the UV and IR surface brightness profiles, we estimate Σ⋆ and ΣSFR and use them to predict αCO with state-of-the-art empirical prescriptions. We validate our (kpc-scale) αCO predictions against observational estimates, finding the best agreement when accounting for CO-dark gas as well as CO emissivity and excitation effects. The CO-dark correction plays a primary role in lower-mass galaxies, whereas CO emissivity and excitation effects become more important in higher-mass and more actively star-forming galaxies, respectively. We compare our estimated αCO to observed galaxy-integrated SFR to CO luminosity ratio as a function of M⋆. A large compilation of literature data suggests that star-forming galaxies with M⋆ = 109.5–11 M⊙ show strong anticorrelations of SFR/ L′CO(1–0)∝M⋆−0.29 and SFR/ L′CO(2–1)∝M⋆−0.40 . The estimated αCO trends, when combined with a constant molecular gas depletion time tdep, can only explain ≈1/3 of these SFR/ L′CO trends. This suggests that tdep being systematically shorter in lower-mass star-forming galaxies is the main cause of the observed SFR/ L′CO variations. We publish all data products from this work, including galaxy sizes, UV and IR surface brightness profiles, Σ⋆, ΣSFR, and αCO estimates.

Abstract:

The joint capabilities of the Hubble Space Telescope (HST) and JWST allow for an unparalleled look at the early lives of star clusters at near- and mid-infrared wavelengths. We present here a multiband analysis of embedded young stellar clusters in 11 nearby, star-forming galaxies, using the PHANGS-JWST and PHANGS-HST data sets. We use the Zooniverse citizen science platform to conduct an initial by-eye search for embedded clusters in near-UV/optical/near-infrared images that trace stellar continuum emission, the Paschenα and Hα recombination lines, and the 3.3 μm polycyclic aromatic hydrocarbon feature and its underlying continuum. With this approach, we identify 292 embedded cluster candidates for which we characterize their ages, masses, and levels of line-of-sight extinction by comparing the photometric data to predictions from stellar population models. The embedded cluster candidates have a median age of 4.5 Myr and an average line-of-sight extinction 〈AV〉 = 6.0 mag. We determine lower limits on source stellar masses, resulting in a median stellar mass of 103 M⊙. We use this sample of embedded cluster candidates to train multiple convolutional neural network models to carry out deep transfer learning-based searches for embedded clusters. With the aim of optimizing models for future catalog production, we compare results for four variations of training data using two neural networks. Confusion matrices for all eight model configurations, as well as inter-model identification trends, are presented. With refinement of the training sample, we determine that optimized models could serve as a pathway for future embedded cluster identification beyond our 11 galaxy sample.

Abstract:

Understanding how the dynamical state of the interstellar medium (ISM) changes across spatial scales can provide important insights into how the gas is organized and ultimately collapses to form stars. To this end, we present ALMA 12CO(2–1) observations at 7 pc (0 .″ 4) spatial resolution across a 1.4 kpc × 5.6 kpc ( 1.′3×1.′3 ) region located in the disk of the nearby (D = 3.5 Mpc), massive, star-forming galaxy NGC 253. We decompose this emission with a hierarchical, multiscale dendrogram algorithm to identify 2463 structures with deconvolved sizes ranging from ∼3 to 300 pc, complete to a limiting mass of 104 M⊙. By comparing the virial parameter of these structures against physical properties including size, mass, surface density, velocity dispersion, and hierarchical position, we carry out a comprehensive search for a preferred scale at which gravitationally bound structures emerge. Ultimately, we do not identify evidence of an emergent scale for bound objects in our data, nor do we find a significant correlation between the virial parameter and structure sizes. These findings suggest that simple observational estimates of gravitational binding cannot be used to define molecular clouds and emphasize the need for multiscale approaches to characterize the ISM.

Abstract:

Polycyclic aromatic hydrocarbons (PAHs) are widespread in the interstellar medium (ISM) of near solar metallicity galaxies, where they play a critical role in ISM heating, cooling, and reprocessing stellar radiation. The PAH fraction, the abundance of PAHs relative to total dust mass, is a key parameter in ISM physics. Using JWST and MUSE observations of 42 galaxies from the PHANGS survey, we analyzed the PAH fraction in over 17 000 H  II regions spanning a gas-phase oxygen abundance of 12 + log(O/H) = 8.0–8.8 ( Z ∼ 0.2–1.3 Z ⊙ ), and ∼400 isolated supernova remnants (SNRs). We find a significantly lower PAH fraction toward H  II regions compared to a reference sample of diffuse ISM areas at matched metallicity. At 12 + log(O/H) > 8.2, the PAH fraction toward H  II regions is strongly anti-correlated with the local ionization parameter, suggesting that PAH destruction is correlated with ionized gas and/or hydrogen-ionizing UV radiation. At lower metallicities, the PAH fraction declines steeply in H  II regions and in the diffuse ISM, likely reflecting less efficient PAH formation in metal-poor environments. Carefully isolating dust emission from the vicinity of optically identified supernova remnants, we see evidence of selective PAH destruction from measurements of lower PAH fractions, which is, however, indistinguishable at ∼50 pc scales. Overall, our results point to ionizing radiation as the dominant agent of PAH destruction within H  II regions; metallicity plays a key role in their global abundance in galaxies.