Dr Thomas Williams

Abstract:

Context. CO isotopologues are common tracers of the bulk molecular gas in extragalactic studies, providing insights into the physical and chemical conditions of the cold molecular gas, a reservoir for star formation. Aims. Since star formation occurs within molecular clouds, mapping CO isotopologues on the scale of clouds is important to understanding the processes driving star formation. However, achieving this mapping at such scales is challenging and time-intensive. The Surveying the Whirlpool Galaxy at Arcseconds with NOEMA (SWAN) survey addresses this by using the Institut de radioastronomie millimétrique (IRAM) NOrthern Extended Millimeter Array (NOEMA) to map the 13 CO(1−0) and C 18 O(1−0) isotopologues, alongside several dense gas tracers, in the nearby star-forming galaxy M51 at high sensitivity and spatial resolution (≈125 pc). Methods. We examine the 13 CO(1−0) to C 18 O(1−0) line emission ratio as a function of galactocentric radius and star formation rate surface density to infer how different chemical and physical processes affect this ratio at cloud scales across different galactic environments: nuclear bar, molecular ring, and northern and southern spiral arms. Results. In line with previous studies conducted at kiloparsec scales for nearby star-forming galaxies, we find a moderate positive correlation with galactocentric radius and a moderate negative correlation with star formation rate surface density across the field of view (FoV), with slight variations depending on the galactic environment. Conclusions. We propose that selective nucleosynthesis and changes in the opacity of the gas are the primary drivers of the observed variations in the ratio.

Abstract:

Tracing dense molecular gas, the fuel for star formation, is essential for understanding the evolution of molecular clouds and star-formation processes. We compared the emission of HCN (1–0), HNC (1–0), and HCO + (1–0) with the emission of N 2 H + (1–0) at cloud scales (125 pc) across the central 5 × 7 kpc of the Whirlpool galaxy, M51a, from “Surveying the Whirlpool galaxy at Arcseconds with NOEMA” (SWAN). We find that the integrated intensities of HCN, HNC, and HCO + are more steeply correlated with N 2 H + emission compared to the bulk molecular gas tracer CO, and we find variations in this relation across the center, molecular ring, northern, and southern disk of M51. Compared to HCN and HNC emission, the HCO + emission follows the N 2 H + emission more closely across the environments and physical conditions, such as the surface densities of molecular gas, stellar mass, star-formation rate, dynamical equilibrium pressure, and radius. Under the assumption that N 2 H + is a fair tracer of dense gas at these scales, this makes HCO + a more favorable dense gas tracer than HCN within the inner disk of M51. In all environments within our field of view, even when the central 2 kpc are removed, the ratio HCN/CO, which is commonly used to trace average cloud density, is only weakly dependent on molecular gas mass surface density. While ratios of other dense gas lines to CO show a steeper dependence on the surface density of molecular gas, this relation is still shallow in comparison to other nearby star-forming disk galaxies. One reason might be physical conditions in M51, which are different from other normal star-forming galaxies. Increased ionization rates, increased dynamical equilibrium pressure in the central few kiloparsecs, and the impact of the dwarf companion galaxy NGC 5195 are proposed mechanisms that might enhance HCN and HNC emission over HCO + and N 2 H + emission at larger-scale environments and cloud scales.

Abstract:

The large number of star clusters in nearby galaxies permits us to statistically test the predictions of stellar, dust, and gas models. Using Hubble Space Telescope (HST) broadband plus Hα imaging combined with JWST near-infrared imaging, we use a total of 10 filters spanning near-ultraviolet through near-infrared wavelengths to model key physical parameters, including age, mass, and reddening, of 6130 star clusters in 16 nearby spiral galaxies from the Physics at High Angular resolution in Nearby GalaxieS sample, focusing on their ages, masses, and reddenings. We find that HST/Hα and JWST/NIRCam 2–3.6 μm photometry significantly improves our ability to disentangle the age–reddening degeneracy between young, gas- and dust-rich clusters and older, dustless clusters. The near-infrared data provide strong constraints on hot continuum dust and small polycyclic aromatic hydrocarbon emission for populations where gas and dust are present. These hot dust constraints demonstrate that Bruzual & Charlot stellar population models do not align with the observed near-ultraviolet-optical-near-infrared spectral energy distributions of star clusters in the first 10 Myr. We note that for old and low-metallicity globular clusters, the inclusion of narrowband Hα and/or broadband near-infrared data does not improve the determination of age and reddening parameters, due to the lack of stars capable of heating dust in the near-infrared regime.

Abstract:

Spiral arms play a central role in disc galaxies, but their dynamical nature remains a long-standing open question. Azimuthal offsets between molecular gas and star formation are expected if gas crosses spiral arms, as predicted by quasi-stationary density wave theory. In this work, we measure offsets between CO and H α peaks in radial bins for 24 galaxies from the PHANGS survey that display a well-delineated spiral structure. The offsets exhibit substantial scatter, implying that star formation is not exclusively initiated at a coherent spiral shock. We define offsets such that positive values mean H α peaks lie ahead of CO peaks in the direction of galactic rotation. With this convention, 14 galaxies show mean positive CO-H α offsets, typically of a few hundred parsecs. In four of these 14 galaxies (17% of the total), offsets become smaller with increasing radius, as expected for a single quasi-stationary spiral density wave. Ten galaxies (42%) show positive mean offsets but no clear correlation with radius, which is compatible with multiple overlapping modes. In the remaining ten galaxies (42%), we find no significantly positive offsets, which could point to transient dynamical spirals or material arms, where gas and stars co-rotate with the spiral perturbation. Across the full sample, we find mostly positive offsets between CO peaks and the gravitational potential minimum, confirming that gas often crosses the spiral perturbation. For the four galaxies with clear positive offsets and a radial trend, we derived pattern speeds in good agreement with the literature. Overall, our results suggest that even well-delineated spirals in the local Universe can arise from a variety of underlying dynamical mechanisms.

Abstract:

We present high angular resolution (0.19 arcsec or pc) Atacama Large Millimeter/submillimeter Array observations of the CO(3–2) line emission of the galaxy NGC 4751. The data provide evidence for the presence of a central supermassive black hole (SMBH). Assuming a constant mass-to-light ratio (), we infer a SMBH mass M and a F160W filter stellar M/L, where the first uncertainties are statistical and the second systematic. Assuming a linearly spatially varying , we infer M and , where R is the galactocentric radius. We also present SMBH mass estimates using the Jeans Anisotropic Modelling (JAM) method and Very Large Telescope Spectrograph for INtegral Field Observations in the Near Infrared (SINFONI) stellar kinematics. Assuming a cylindrically aligned velocity ellipsoid (JAM), we infer M, and while assuming a spherically aligned velocity ellipsoid (JAM), we infer M. The SMBH mass assuming a constant is statistically consistent with that of JAM, whereas the mass assuming a linearly varying is consistent with both JAM and JAM (within the uncertainties). Our derived masses are larger than (and inconsistent with) one previous stellar dynamical measurement using the Schwarzschild orbit-superposition method and the same SINFONI kinematics.