Dr Thomas Williams

Abstract:

CO(1–0) and CO(2–1) are commonly used as bulk molecular gas tracers. The CO line ratios (especially CO(2–1)/CO(1–0)–R21) vary within and among galaxies, yet previous studies on R21 and alike often rely on measurements constructed by combining data from facilities with substantial relative calibration uncertainties that have the same order as physical line ratio variations. Hence, robustly determining systematic R21 variations is challenging. Here, we compare CO(1–0) and CO(2–1) mapping data from ALMA for 14 nearby galaxies, at a common physical resolution of 1.7 kpc. Our data set includes new ALMA (7 m+TP) CO(1–0) maps of 12 galaxies. We investigate R21 variation to understand its dependence on global galaxy properties, kiloparsec-scale environmental factors, and its correlation with star formation rate (SFR) surface density and metallicity. We find that the galaxy-to-galaxy scatter is 0.05 dex. This is lower than previous studies, which reported over 0.1 dex variation, likely reflecting significant flux calibration uncertainties in single-dish surveys. Within individual galaxies, R21 has a typical mean value of ∼0.64 and 0.1 dex variation, with an increase to ∼0.75 toward galactic centers. We find strong correlations between R21 and various galactic parameters, particularly SFR surface density, which shows a power-law slope of 0.10–0.11 depending on the adopted binning/fitting methods. Our findings suggest that, for studies covering main-sequence galaxy samples, assuming a fixed R21 = 0.64 does not significantly bias kiloparsec-scale molecular gas mass estimates from CO(2–1). Instead, systematic uncertainties from flux calibration and the CO-to-H2 conversion factor account for more systematic scatter of CO-derived molecular gas properties.

Abstract:

We present the Local Group L-Band Survey, a Karl G. Jansky Very Large Array (VLA) survey producing the highest-quality 21 cm and 1–2 GHz radio continuum images to date, for the six VLA-accessible, star-forming, Local Group galaxies. Leveraging the VLA’s spectral multiplexing power, we simultaneously survey the 21 cm line at high 0.4 km s−1 velocity resolution, the 1–2 GHz polarized continuum, and four OH lines. For the massive spiral M31, the dwarf spiral M33, and the dwarf irregular galaxies NGC 6822, IC 10, IC 1613, and the Wolf–Lundmark–Melotte Galaxy, we use all four VLA configurations and the Green Bank Telescope to reach angular resolutions of <5″ (10–20 pc) for the 21 cm line with <1020 cm−2 column density sensitivity, and even sharper views (<2″; 5–10 pc) of the continuum. Targeting these nearby galaxies (D ≲ 1 Mpc) reveals a sharp, resolved view of the atomic gas, including 21 cm absorption, and continuum emission from supernova remnants and H ii regions. These data sets can be used to test theories of the abundance and formation of cold clouds, the driving and dissipation of interstellar turbulence, and the impact of feedback from massive stars and supernovae. Here, we describe the survey design and execution, scientific motivation, data processing, and quality assurance. We provide a first look at and publicly release the wide-field 21 cm H i data products for M31, M33, and four dwarf irregular targets in the survey, which represent some of the highest-physical-resolution 21 cm observations of any external galaxies beyond the LMC and SMC.

Abstract:

We present a molecular gas dynamical supermassive black hole (SMBH) mass measurement in the nearby barred lenticular galaxy NGC 1574, using Atacama Large Millimeter/sub-millimeter Array observations of the ¹²CO(2-1) emission line with synthesised beam full-widths at half-maximum of 0.″078×0.″070 (≈7.5×6.7 pc²). The observations are the first to spatially resolve the SMBH's sphere of influence (SoI), resulting in an unambiguous detection of the Keplerian velocity increase due to the SMBH towards the centre of the gas disc. We also detect a previously known large-scale kinematic twist of the CO velocity map, due to a position angle (PA) warp and possible mild non-circular motions, and we resolve a PA warp within the central 0.″2×0.″2 of the galaxy, larger than that inferred from previous intermediate-resolution data. By forward modelling the data cube, we infer a SMBH mass of (6.2±1.2)×107 M⊙ (1σ confidence interval), slightly smaller than but statistically consistent with the SMBH mass derived from the previous intermediate-resolution data that did not resolve the SoI, and slightly outside the 1σ scatter of the SMBH mass–stellar velocity dispersion relation. Our measurement thus emphasises the importance of observations that spatially resolve the SMBH SoI for accurate SMBH mass measurements and gas dynamical modelling.

Abstract:

The inner regions of the Milky Way are known to contain an enigmatic population of prominent molecular clouds characterized by extremely broad lines. The physical origin of these ‘extended velocity features’ (EVFs) is still debated, although a connection with the ‘dust lanes’ of the Galactic bar has been hypothesized. In this paper, we search for analogous features in the dust lanes of nearby barred galaxies using the PHANGS–ALMA CO(2-1) survey. We aim to confirm existence of EVFs in other galaxies and to take advantage of the external perspective to gain insight into their origin. We study a sample of 29 barred galaxies and find that 34 per cent contain one or more EVFs, while the remaining lack obvious signs of EVFs. Upon analysing the physical properties of the EVFs, we find they possess large virial parameters, ranging from few hundreds to several thousand, indicating that they are strongly out-of-equilibrium. The most likely explanation for their origin is extreme cloud–cloud collisions with relative velocities in excess of 100 km s−1 in highly non-circular flow driven by the bar. This interpretation is consistent with previous high-resolution observations in Milky Way. Further corroboration of this interpretation comes from the inspection of high-sensitivity infrared observations from the PHANGS–JWST Treasury Survey that reveals streams of gas that appear to be hitting the dust lanes at locations where EVFs are found. We argue that EVFs are the clearest examples of cloud–cloud collisions available in literature and represent a unique opportunity to study cloud collisions and their impact on star formation.

Abstract:

We present 0.6–3.2 pc resolution mid-infrared (MIR) JWST images at 7.7 μm (F770W) and 21 μm (F2100W) covering the main star-forming regions of two of the closest star-forming low-metallicity dwarf galaxies, NGC 6822 and Wolf–Lundmark–Melotte (WLM). The images of NGC 6822 reveal filaments, edge-brightened bubbles, diffuse emission, and a plethora of point sources. By contrast, most of the MIR emission in WLM is pointlike, with a small amount of extended emission. Compared to solar-metallicity galaxies, the ratio of 7.7 μm intensity ( IνF770W ), tracing polycyclic aromatic hydrocarbons (PAHs), to 21 μm intensity ( IνF2100W ), tracing small, warm dust grain emission, is suppressed in these low-metallicity dwarfs. Using Atacama Large Millimeter/submillimeter Array CO(2–1) observations, we find that detected CO intensity versus IνF770W at ≈2 pc resolution in dwarfs follows a similar relationship to that at solar metallicity and lower resolution, while the CO versus IνF2100W relationship in dwarfs lies significantly below that derived from solar-metallicity galaxies at lower resolution, suggesting more pronounced destruction of CO molecules at low metallicity. Finally, adding in Local Group L-Band Survey 21 cm H i observations from the Very Large Array, we find that IνF2100W and IνF770W versus total gas ratios are suppressed in NGC 6822 and WLM compared to solar-metallicity galaxies. In agreement with dust models, the level of suppression appears to be at least partly accounted for by the reduced galaxy-averaged dust-to-gas and PAH-to-dust mass ratios in the dwarfs. Remaining differences are likely due to spatial variations in dust model parameters, which should be an exciting direction for future work in local dwarf galaxies.