Dr Thomas Williams

Abstract:

We provide new planetary nebula luminosity function (pnlf) distances to 19 nearby spiral galaxies that were observed with VLT/MUSE by the PHANGS collaboration. Emission line ratios are used to separate planetary nebulae (pne) from other bright [O, III] emitting sources like compact supernovae remnants (snrs) or H ii regions. While many studies have used narrowband imaging for this purpose, the detailed spectral line information provided by integral field unit (ifu) spectroscopy grants a more robust way of categorizing different [O, III] emitters. We investigate the effects of snr contamination on the pnlf and find that we would fail to classify all objects correctly, when limited to the same data narrowband imaging provides. However, the few misclassified objects usually do not fall on the bright end of the luminosity function, and only in three cases does the distance change by more than 1σ. We find generally good agreement with literature values from other methods. Using metallicity constraints that have also been derived from the same ifu data, we revisit the pnlf zero-point calibration. Over a range of 8.34 < 12 + log (O/H) < 8.59, our sample is consistent with a constant zero-point and yields a value of M∗ = -4.542+0.103-0.059, mag, within 1σ of other literature values. MUSE pushes the limits of pnlf studies and makes galaxies beyond 20 Mpc accessible for this kind of analysis. This approach to the pnlf shows great promise for leveraging existing archival ifu data on nearby galaxies.

Abstract:

We use high-resolution maps of the molecular interstellar medium (ISM) in the centres of eighty-six nearby galaxies from the millimetre-Wave Interferometric Survey of Dark Object Masses (WISDOM) and Physics at High Angular Resolution in Nearby GalaxieS (PHANGS) surveys to investigate the physical mechanisms setting the morphology of the ISM at molecular cloud scales. We show that early-type galaxies tend to have smooth, regular molecular gas morphologies, while the ISM in spiral galaxy bulges is much more asymmetric and clumpy when observed at the same spatial scales. We quantify these differences using non-parametric morphology measures (Asymmetry, Smoothness and Gini), and compare these measurements with those extracted from idealised galaxy simulations. We show that the morphology of the molecular ISM changes systematically as a function of various large scale galaxy parameters, including galaxy morphological type, stellar mass, stellar velocity dispersion, effective stellar mass surface density, molecular gas surface density, star formation efficiency and the presence of a bar. We perform a statistical analysis to determine which of these correlated parameters best predicts the morphology of the ISM. We find the effective stellar mass surface (or volume) density to be the strongest predictor of the morphology of the molecular gas, while star formation and bars maybe be important secondary drivers. We find that gas self-gravity is not the dominant process shaping the morphology of the molecular gas in galaxy centres. Instead effects caused by the depth of the potential well such as shear, suppression of stellar spiral density waves and/or inflow affect the ability of the gas to fragment.

Abstract:

We measure the low-J CO line ratios R 21 CO (2-1)/CO (1-0), R 32 CO (3-2)/CO (2-1), and R 31 CO (3-2)/CO (1-0) using whole-disk CO maps of nearby galaxies. We draw CO (2-1) from PHANGS-ALMA, HERACLES, and follow-up IRAM surveys; CO (1-0) from COMING and the Nobeyama CO Atlas of Nearby Spiral Galaxies; and CO (3-2) from the James Clerk Maxwell Telescope Nearby Galaxy Legacy Survey and Atacama Pathfinder Experiment Large APEX Sub-Millimetre Array mapping. All together, this yields 76, 47, and 29 maps of R 21, R 32, and R 31 at 20″ ∼1.3 kpc resolution, covering 43, 34, and 20 galaxies. Disk galaxies with high stellar mass, log(M∗/Mo˙)=10.25-11, and star formation rate (SFR) = 1-5 M o˙ yr-1, dominate the sample. We find galaxy-integrated mean values and a 16%-84% range of R 21 = 0.65 (0.50-0.83), R 32 = 0.50 (0.23-0.59), and R 31 = 0.31 (0.20-0.42). We identify weak trends relating galaxy-integrated line ratios to properties expected to correlate with excitation, including SFR/M ∗ and SFR/L CO. Within galaxies, we measure central enhancements with respect to the galaxy-averaged value of ∼ 0.18-0.14+0.09 dex for R 21, 0.27-0.15+0.13 dex for R 31, and 0.08-0.09+0.11 dex for R 32. All three line ratios anticorrelate with galactocentric radius and positively correlate with the local SFR surface density and specific SFR, and we provide approximate fits to these relations. The observed ratios can be reasonably reproduced by models with low temperature, moderate opacity, and moderate densities, in good agreement with expectations for the cold interstellar medium. Because the line ratios are expected to anticorrelate with the CO (1-0)-to-H2 conversion factor, αCO1-0, these results have general implications for the interpretation of CO emission from galaxies.

Abstract:

The relative distribution of molecular gas and star formation in galaxies gives insight into the physical processes and timescales of the cycle between gas and stars. In this work, we track the relative spatial configuration of CO and Hα emission at high resolution in each of our galaxy targets and use these measurements to quantify the distributions of regions in different evolutionary stages of star formation: from molecular gas without star formation traced by Hα to star-forming gas, and to H ii regions. The large sample, drawn from the Physics at High Angular resolution in Nearby GalaxieS ALMA and narrowband Hα (PHANGS-ALMA and PHANGS-Hα) surveys, spans a wide range of stellar masses and morphological types, allowing us to investigate the dependencies of the gas-star formation cycle on global galaxy properties. At a resolution of 150 pc, the incidence of regions in different stages shows a dependence on stellar mass and Hubble type of galaxies over the radial range probed. Massive and/or earlier-type galaxies in our sample exhibit a significant reservoir of molecular gas without star formation traced by Hα, while lower-mass galaxies harbor substantial H ii regions that may have dispersed their birth clouds or formed from low-mass, more isolated clouds. Galactic structures add a further layer of complexity to the relative distribution of CO and Hα emission. Trends between galaxy properties and distributions of gas traced by CO and Hα are visible only when the observed spatial scale is ≪500 pc, reflecting the critical resolution requirement to distinguish stages of the star formation process.

Abstract:

The PHANGS program is building the first data set to enable the multiphase, multiscale study of star formation across the nearby spiral galaxy population. This effort is enabled by large survey programs with the Atacama Large Millimeter/submillimeter Array (ALMA), MUSE on the Very Large Telescope, and the Hubble Space Telescope (HST), with which we have obtained CO(2-1) imaging, optical spectroscopic mapping, and high-resolution UV-optical imaging, respectively. Here, we present PHANGS-HST, which has obtained NUV-U-B-V-I imaging of the disks of 38 spiral galaxies at distances of 4-23 Mpc, and parallel V- and I-band imaging of their halos, to provide a census of tens of thousands of compact star clusters and multiscale stellar associations. The combination of HST, ALMA, and VLT/MUSE observations will yield an unprecedented joint catalog of the observed and physical properties of ∼100,000 star clusters, associations, H ii regions, and molecular clouds. With these basic units of star formation, PHANGS will systematically chart the evolutionary cycling between gas and stars across a diversity of galactic environments found in nearby galaxies. We discuss the design of the PHANGS-HST survey and provide an overview of the HST data processing pipeline and first results. We highlight new methods for selecting star cluster candidates, morphological classification of candidates with convolutional neural networks, and identification of stellar associations over a range of physical scales with a watershed algorithm. We describe the cross-observatory imaging, catalogs, and software products to be released. The PHANGS high-level science products will seed a broad range of investigations, in particular, the study of embedded stellar populations and dust with the James Webb Space Telescope, for which a PHANGS Cycle 1 Treasury program to obtain eight-band 2-21 μm imaging has been approved.