Dr Thomas Williams

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.

Abstract:

Polycyclic aromatic hydrocarbon (PAH) emission is widely used to trace the distribution of molecular gas in the interstellar medium, exhibiting a tight correlation with CO(2-1) emission across nearby galaxies. Using PHANGS-JWST and PHANGS-Atacama Large Millimeter/submillimeter Array (ALMA) data, we identify localized regions where this correlation fails, with CO flux exceeding that predicted from 7.7 μm PAH emission by more than an order of magnitude. These outlier regions are found in 20 out of 70 galaxies and are located in galaxy centers and bars, without signs of massive star formation. We explore two scenarios to explain the elevated CO-to-PAH ratios, which can either be due to suppressed PAH emission or enhanced CO emissivity. We examine PAH emission in other bands (3.3 and 11.3 μm) and the dust-continuum-dominated bands (10 and 21 μm), finding consistently high CO-to-PAH (or CO-to-dust continuum) emission ratios, suggesting that 7.7 μm PAH emission is not particularly suppressed. In some outlier regions, PAH sizes and spectral energy distribution of the radiation differ slightly from nearby control regions with normal CO-to-PAH ratios, though without a consistent trend. We find that the outlier regions show higher CO velocity dispersions (ΔvCO). This increase in ΔvCO lowers CO optical depth and raises its emissivity for a given gas mass. Our results favor a scenario where shear along the bar lanes and shocks at the bar ends elevate CO emissivity, leading to the breakdown of the CO-PAH correlation. Future JWST spectroscopy and deep ALMA observations of CO isotopologues will provide critical tests of this scenario.

Abstract:

We investigate the relationship between star formation and X-ray emission from the hot interstellar medium (ISM) on ∼kiloparsec scales in NGC 4254 (M99) by combining spatially resolved star formation histories (SFHs) and Bayesian X-ray spectral fitting. We measure subgalactic star formation rates (SFR) by modeling spectrophotometric UV-IR data with flexible SFHs, and we produce point-source-subtracted maps of the diffuse X-ray emission using Chandra data. We extract and fit the spectra of five regions selected by their SFR density ΣSFR, deriving hot gas luminosities and plasma temperatures. We examine the subgalactic kT–ΣSFR and LXgas−ΣSFR scaling relations in NGC 4254 and compare to predictions from simple models of the feedback into the ISM from core collapse supernovae (CCSNe). The hot gas emission from NGC 4254 is consistent with thermalization of ≈40%–50% of the energy from CCSNe in the ISM, and mass-loading of the CCSNe ejecta, which decreases as ΣSFR−1/3 . Our optimized model implies a temperature and X-ray production efficiency that scale as kT=(0.72−0.18+0.26keV)ΣSFR0.34±0.10 and η=(0.03−0.01+0.02)ΣSFR0.34±0.18 , respectively, for ΣSFR = 0.01–0.13 M⊙ yr−1 kpc−2. We also compare the properties of the hot ISM to other ISM phases using data from the PHANGS program. The diffuse X-ray emission of a given region is on average 200 times fainter than the Hα emission, and we see evidence that the hot ISM is overpressurized compared to the large-scale dynamical equilibrium pressure of the galaxy, consistent with expansion of the hot ISM into the ambient medium.

Abstract:

Abstract Studying samples of young star-forming regions allows us to statistically examine the evolution of their natal gas and dust along with the associated timescales in the volatile early stages of their lives. With the PHANGS survey data, we analyze the diversity of spectral energy distributions (SEDs) for the rich sample of massive star-forming regions found in NGC 1365. By combining unique detections across a variety of datasets from HST, JWST, and ALMA images, we produce a catalog of 85 star-forming regions located in the central starbursting region of NGC 1365. Prior to analysis, we observe clear saturation effects in our four JWST/MIRI images, and implement a saturation-correction method which allows us to recover data for 23 of 32 saturation-affected regions in these images. We then perform photometry in 13 HST & JWST images which are convolved to match the resolution of MIRI/F2100W (~64pc), allowing us to probe star clusters as well as their immediate surroundings. Upon deriving their properties from SED-fitting using CIGALE, we observe that regions selected with progressively redder wavebands are younger and generally more reddened. We also identify three SED features correlated with age: 1) sources with a positive near-infrared slope ((F300W+F360M)/(2×F200W)) are by median half the age of those with negative near-infrared slopes; turnover occurs around 6 Myr, 2) the relative strength of dust emission (F2100W/F200W) and 3) PAH emission (F335M/F300M) both show that larger such ratios correlate with younger ages. Considering our working resolution, these features are robust to the inclusion of nearby emission surrounding star clusters.

Abstract:

The effect of supernovae (SNe) on star formation in the interstellar medium (ISM) depends sensitively on where SNe explode with respect to ISM clouds. Observationally, SN ISM environments characterized by spatially resolved gas maps can empirically guide the placement of SNe in subgrid models, but unfortunately such measurements remain scarce, as SNe are rare and often distant. Here we demonstrate a new approach—mapping the ISM around evolved massive stars that are soon to explode. These provide a substantially larger sample of “explosion sites” (than just historical SNe) in nearby galaxies that have high-resolution atomic and molecular ISM maps from the Jansky Very Large Array and Atacama Large Millimeter/submillimeter Array. We demonstrate this technique in the well-resolved Local Group spiral M33 by analyzing the 50 pc scale projected ISM densities around red supergiants (RSGs; 8–30 M⊙ stars) Wolf–Rayet stars (W-Rs; >30M⊙ stars), and supernova remnants. We find a mass-dependent correlation between stars and gas clouds, with at least 45% of W-Rs and up to 77% of RSGs having no detectable H2 at their pixel locations. In the sample with H2 detections, we find that more-massive younger progenitors are coincident with denser gas. We show that the density distributions for stars >15 M⊙ are statistically distinct from random alignment of stars and gas in M33. Our work provides the first observationally derived estimate of the fraction of the SN-producing stellar population correlated with ISM density peaks. We demonstrate how this can be compared with galaxy simulations, and advocate similar comparisons to the community for constraining subgrid models.