Dr Thomas Williams

Abstract:

The PHANGS project is assembling a comprehensive, multiwavelength data set of nearby (∼5–20 Mpc), massive star-forming galaxies to enable multiphase, multiscale investigations into the processes that drive star formation and galaxy evolution. To date, large survey programs have provided molecular gas (CO) cubes with the Atacama Large Millimeter/submillimeter Array, optical integral field unit (IFU) spectroscopy with the Very Large Telescope/Multi-Unit Spectroscopic Explorer (MUSE), high-resolution near-ultraviolet–optical imaging in five broadband filters with Hubble Space Telescope (HST), and infrared imaging in NIRCAM+MIRI filters with JWST. Here we present PHANGS-HST-Hα, which has obtained high-resolution (∼2–10 pc), narrowband imaging in the F658N or F657N filters with the HST/WFC3 camera of the warm ionized gas in the first 19 nearby galaxies observed in common by all four of the PHANGS large programs. We summarize our data reduction process, with a detailed discussion of the production of flux-calibrated, Milky Way extinction-corrected, continuum-subtracted Hα maps. PHANGS-MUSE IFU spectroscopy data are used to background-subtract the HST-Hα maps and to determine the [N ii] correction factors for each galaxy. We describe our public data products (the data released as part of this work include the reduced drizzled narrowband images and the flux-calibrated, continuum-subtracted Hα maps for each galaxy; these images are available for download via MAST at https://archive.stsci.edu/hlsp/phangs.html, as well as at the Canadian Astronomy Data Centre as part of the PHANGS archive at https://www.canfar.net/storage/vault/list/phangs/RELEASES) and highlight a few key science cases enabled by the PHANGS-HST-Hα observations.

Abstract:

We present a comparison of observed polycyclic aromatic hydrocarbon (PAH) feature ratios in 19 nearby galaxies with a grid of theoretical expectations for near- and mid-infrared dust emission. The PAH feature ratios are drawn from Cycle 1 JWST observations and are measured for 7224 stellar clusters and 29,176 stellar associations for which we have robust ages and mass estimates from Hubble Space Telescope five-band photometry. Though there are galaxy-to-galaxy variations, the observed PAH feature ratios largely agree with the theoretical models, particularly those that are skewed toward more ionized and larger PAH size distributions. For each galaxy we also extract PAH feature ratios for 200 pc wide circular regions in the diffuse interstellar medium, which serve as a noncluster/association control sample. Compared to what we find for stellar clusters and associations, the 3.3 μm/7.7 μm and 3.3 μm/11.3 μm ratios from the diffuse interstellar medium are ∼0.10–0.15 dex smaller. When the observed PAH feature ratios are compared to the radiation field hardness as probed by the [O iii]/Hβ ratio, we find anticorrelations for nearly all galaxies in the sample. These results together suggest that the PAH feature ratios are driven by the shape and intensity of the radiation field and that the smallest PAHs—observed via JWST F335M imaging—are increasingly “processed” or destroyed in regions with the most intense and hard radiation fields.

Abstract:

We present a measurement of the supermassive black hole (SMBH) mass of the nearby lenticular galaxy NGC 383, based on Atacama Large Millimeter/sub-millimeter Array (ALMA) observations of the ¹²CO(2-1) emission line with an angular resolution of 0.″050×0.″024 (≈16×8 pc²). These observations spatially resolve the nuclear molecular gas disc down to ≈41,300 Schwarzschild radii and the SMBH sphere of influence by a factor of ≈24 radially, better than any other SMBH mass measurement using molecular gas to date. The high resolution enables us to probe material with a maximum circular velocity of ≈1040 km/s^-1, even higher than those of the highest-resolution SMBH mass measurements using megamasers. We detect a clear Keplerian increase (from the outside in) of the line-of-sight rotation velocities, a slight offset between the gas disc kinematic (i.e. the position of the SMBH) and morphological (i.e. the centre of the molecular gas emission) centres, an asymmetry of the innermost rotation velocity peaks and evidence for a mild position angle warp and/or non-circular motions within the central ≈0.″3 arcsec. By forward modelling the mass distribution and ALMA data cube, we infer a SMBH mass of (3.58±0.19)×10⁹ M⊙ (1σ confidence interval), more precise (5%) but consistent within ≈1.4σ with the previous measurement using lower-resolution molecular gas data. Our measurement emphasises the importance of high spatial resolution observations for precise SMBH mass determinations.

Abstract:

The structure and chemistry of the dusty interstellar medium (ISM) are shaped by complex processes that depend on the local radiation field, gas composition, and dust grain properties. Of particular importance are polycyclic aromatic hydrocarbons (PAHs), which emit strong vibrational bands in the mid-infrared, and play a key role in the ISM energy balance. We recently identified global correlations between PAH band and optical line ratios across three nearby galaxies, suggesting a connection between PAH heating and gas ionization throughout the ISM. In this work, we perform a census of the PAH heating–gas ionization connection using ∼700,000 independent pixels that probe scales of 40–150 pc in 19 nearby star-forming galaxies from the PHANGS survey. We find a universal relation between log PAH(11.3 μm/7.7 μm) and log ([S ii]/Hα) with a slope of ∼0.2 and a scatter of ∼0.025 dex. The only exception is a group of anomalous pixels that show unusually high (11.3 μm/7.7 μm) PAH ratios in regions with old stellar populations and high starlight-to-dust emission ratios. Their mid-infrared spectra resemble those of elliptical galaxies. Active galactic nucleus hosts show modestly steeper slopes, with a ∼​​​​​​10% increase in PAH(11.3 μm/7.7 μm) in the diffuse gas on kiloparsec scales. This universal relation implies an emerging simplicity in the complex ISM, with a sequence that is driven by a single varying property: the spectral shape of the interstellar radiation field. This suggests that other properties, such as gas-phase abundances, gas ionization parameter, and grain charge distribution, are relatively uniform in all but specific cases.