Dr Thomas Williams

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.

Abstract:

We compare measurements of star formation efficiency to cloud-scale gas properties across the PHANGS– ALMA sample. Dividing 67 galaxies into 1.5 kpc scale regions, we calculate the molecular gas depletion time τdepmol=Σmol/ΣSFR and the star formation efficiency per freefall time ϵffmol=τff/τdepmol for each region. Then we test how τdepmol and ϵffmol vary as functions of the regional mass-weighted mean molecular gas properties on cloud scales (60–150 pc): gas surface density, 〈Σmolcloud〉 , velocity dispersion, 〈σmolcloud〉 , virial parameter, 〈αvircloud〉 , and gravitational freefall time, 〈τffcloud〉 . 〈τffcloud〉 and τdepmol correlate positively, consistent with the expectation that gas density plays a key role in setting the rate of star formation. Our fiducial measurements suggest τdepmol∝〈τffcloud〉0.5 and ϵffmol≈0.34% , though the exact numbers depend on the adopted fitting methods. We also observe anticorrelations between τdepmol and 〈Σmolcloud〉 and between τdepmol and 〈σmolcloud〉 . All three correlations may reflect the same underlying link between density and star formation efficiency combined with systematic variations in the degree to which self-gravity binds molecular gas in galaxies. We highlight the τdepmol – 〈σmolcloud〉 relation because of the lower degree of correlation between the axes. Contrary to theoretical expectations, we observe an anticorrelation between τdepmol and 〈αvircloud〉 and no significant correlation between ϵffmol and 〈αvircloud〉 . Our results depend sensitively on the adopted CO-to-H2 conversion factor, with corrections for excitation and emissivity effects in inner galaxies playing an important role. We emphasize that our simple methodology and clean selection allow for easy comparison to numerical simulations and highlight this as a logical next direction.