Radiative and mechanical feedback into the molecular gas in the Large Magellanic Cloud. I. N159W

We present Herschel SPIRE Fourier Transform Spectrometer (FTS) observations of N159W, an active star-forming region in the Large Magellanic Cloud (LMC). In our observations, a number of far-infrared cooling lines including CO(4-3) to CO(12-11), [CI] 609 and 370 micron, and [NII] 205 micron are clearly detected. With an aim of investigating the physical conditions and excitation processes of molecular gas, we first construct CO spectral line energy distributions (SLEDs) on 10 pc scales by combining the FTS CO transitions with ground-based low-J CO data and analyze the observed CO SLEDs using non-LTE radiative transfer models. We find that the CO-traced molecular gas in N159W is warm (kinetic temperature of 153-754 K) and moderately dense (H2 number density of (1.1-4.5)e3 cm-3). To assess the impact of the energetic processes in the interstellar medium on the physical conditions of the CO-emitting gas, we then compare the observed CO line intensities with the models of photodissociation regions (PDRs) and shocks. We first constrain the properties of PDRs by modelling Herschel observations of [OI] 145, [CII] 158, and [CI] 370 micron fine-structure lines and find that the constrained PDR components emit very weak CO emission. X-rays and cosmic-rays are also found to provide a negligible contribution to the CO emission, essentially ruling out ionizing sources (ultraviolet photons, X-rays, and cosmic-rays) as the dominant heating source for CO in N159W. On the other hand, mechanical heating by low-velocity C-type shocks with ~10 km/s appears sufficient enough to reproduce the observed warm CO.Comment: accepted for publication in A&

PHANGS CO kinematics: disk orientations and rotation curves at 150 pc resolution

We present kinematic orientations and high resolution (150 pc) rotation curves for 67 main sequence star-forming galaxies surveyed in CO (2-1) emission by PHANGS-ALMA. Our measurements are based on the application of a new fitting method tailored to CO velocity fields. Our approach identifies an optimal global orientation as a way to reduce the impact of non-axisymmetric (bar and spiral) features and the uneven spatial sampling characteristic of CO emission in the inner regions of nearby galaxies. The method performs especially well when applied to the large number of independent lines-of-sight contained in the PHANGS CO velocity fields mapped at 1'' resolution. The high resolution rotation curves fitted to these data are sensitive probes of mass distribution in the inner regions of these galaxies. We use the inner slope as well as the amplitude of our fitted rotation curves to demonstrate that CO is a reliable global dynamical mass tracer. From the consistency between photometric orientations from the literature and kinematic orientations determined with our method, we infer that the shapes of stellar disks in the mass range of log($\rm M_{\star}(M_{\odot})$)=9.0-10.9 probed by our sample are very close to circular and have uniform thickness.Comment: 19 figures, 36 pages, accepted for publication in ApJ. Table of PHANGS rotation curves available from http://phangs.org/dat

Star Cluster Classification using Deep Transfer Learning with PHANGS-HST

Currently available star cluster catalogues from HST imaging of nearby galaxies heavily rely on visual inspection and classification of candidate clusters. The time-consuming nature of this process has limited the production of reliable catalogues and thus also post-observation analysis. To address this problem, deep transfer learning has recently been used to create neural network models which accurately classify star cluster morphologies at production scale for nearby spiral galaxies (D < 20 Mpc). Here, we use HST UV-optical imaging of over 20,000 sources in 23 galaxies from the Physics at High Angular Resolution in Nearby GalaxieS (PHANGS) survey to train and evaluate two new sets of models: i) distance-dependent models, based on cluster candidates binned by galaxy distance (9-12 Mpc, 14-18 Mpc, 18-24 Mpc), and ii) distance-independent models, based on the combined sample of candidates from all galaxies. We find that the overall accuracy of both sets of models is comparable to previous automated star cluster classification studies (~60-80 per cent) and show improvement by a factor of two in classifying asymmetric and multi-peaked clusters from PHANGS-HST. Somewhat surprisingly, while we observe a weak negative correlation between model accuracy and galactic distance, we find that training separate models for the three distance bins does not significantly improve classification accuracy. We also evaluate model accuracy as a function of cluster properties such as brightness, colour, and SED-fit age. Based on the success of these experiments, our models will provide classifications for the full set of PHANGS-HST candidate clusters (N ~ 200,000) for public release.Comment: 16 pages, 10 figure

Wide-field CO isotopologue emission and the CO-to-H2_2 factor across the nearby spiral galaxy M101

Carbon monoxide (CO) emission is the most widely used tracer of the bulk molecular gas in the interstellar medium (ISM) in extragalactic studies. The CO-to-H$_2$ conversion factor, $\alpha_{\rm CO}$, links the observed CO emission to the total molecular gas mass. However, no single prescription perfectly describes the variation of $\alpha_{\rm CO}$ across all environments across galaxies as a function of metallicity, molecular gas opacity, line excitation, and other factors. Using resolved spectral line observations of CO and its isotopologues, we can constrain the molecular gas conditions and link them to a variation in the conversion factor. We present new IRAM 30-m 1mm and 3mm line observations of $^{12}$CO, $^{13}$CO, and C$^{18}$O} across the nearby galaxy M101. Based on the CO isotopologue line ratios, we find that selective nucleosynthesis and opacity changes are the main drivers of the variation in the line emission across the galaxy. Furthermore, we estimated $\alpha_{\rm CO(1-0)}$ using different approaches, including (i) the dust mass surface density derived from far-IR emission as an independent tracer of the total gas surface density and (ii) LTE-based measurements using the optically thin $^{13}$CO(1-0) intensity. We find an average value of $\alpha_{\rm CO}=4.4{\pm}0.9\rm\,M_\odot\,pc^{-2}(K\,km\,s^{-1})^{-1}$ across the galaxy, with a decrease by a factor of 10 toward the 2 kpc central region. In contrast, we find LTE-based values are lower by a factor of 2-3 across the disk relative to the dust-based result. Accounting for $\alpha_{\rm CO}$ variations, we found significantly reduced molecular gas depletion time by a factor 10 in the galaxy's center. In conclusion, our result suggests implications for commonly derived scaling relations, such as an underestimation of the slope of the Kennicutt Schmidt law, if $\alpha_{\rm CO}$ variations are not accounted for.Comment: Accepted for publication in A&A, 25 pages, 15 figure

The lifecycle of molecular clouds in nearby star-forming disc galaxies

It remains a major challenge to derive a theory of cloud-scale (⁠≲100 pc) star formation and feedback, describing how galaxies convert gas into stars as a function of the galactic environment. Progress has been hampered by a lack of robust empirical constraints on the giant molecular cloud (GMC) lifecycle. We address this problem by systematically applying a new statistical method for measuring the evolutionary timeline of the GMC lifecycle, star formation, and feedback to a sample of nine nearby disc galaxies, observed as part of the PHANGS-ALMA survey. We measure the spatially resolved (∼100 pc) CO-to-H α flux ratio and find a universal de-correlation between molecular gas and young stars on GMC scales, allowing us to quantify the underlying evolutionary timeline. GMC lifetimes are short, typically 10−30Myr⁠, and exhibit environmental variation, between and within galaxies. At kpc-scale molecular gas surface densities ΣH2≥8M⊙pc−2⁠, the GMC lifetime correlates with time-scales for galactic dynamical processes, whereas at ΣH2≤8M⊙pc−2 GMCs decouple from galactic dynamics and live for an internal dynamical time-scale. After a long inert phase without massive star formation traced by H α (75-90 per cent of the cloud lifetime), GMCs disperse within just 1−5Myr once massive stars emerge. The dispersal is most likely due to early stellar feedback, causing GMCs to achieve integrated star formation efficiencies of 4-10 per cent. These results show that galactic star formation is governed by cloud-scale, environmentally dependent, dynamical processes driving rapid evolutionary cycling. GMCs and H II regions are the fundamental units undergoing these lifecycles, with mean separations of 100−300pc in star-forming discs. Future work should characterize the multiscale physics and mass flows driving these lifecycles.MC and JMDK gratefully acknowledge funding from the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) through an Emmy Noether Research Group (grant number KR4801/1-1) and the DFG Sachbeihilfe (grant number KR4801/2-1). JMDK, APSH, SMRJ, and DTH gratefully acknowledge funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme via the ERC Starting Grant MUSTANG (grant agreement number 714907). MC, JMDK, SMRJ, and DTH acknowledge support from the Australia-Germany Joint Research Cooperation Scheme (UA-DAAD, grant number 57387355). APSH, SMRJ, and DTH are fellows of the International Max Planck Research School for Astronomy and Cosmic Physics at the University of Heidelberg (IMPRS-HD). BG gratefully acknowledges the support of the Australian Research Council as the recipient of a Future Fellowship (FT140101202). CNC, AH, and JP acknowledge funding from the Programme National ‘Physique et Chimie du Milieu Interstellaire’ (PCMI) of the Centre national de la recherche scientifique/Institut national des sciences de l’Univers (CNRS/INSU) with the Institut de Chimie/Institut de Physique (INC/INP), co-funded by the Commissariat a l’ ` energie ´ atomique et aux energies alternatives (CEA) and the Centre ´ national d’etudes spatiales (CNES). AH acknowledges support ´ by the Programme National Cosmology et Galaxies (PNCG) of CNRS/INSU with the INP and the Institut national de physique nucleaire et de physique des particules (IN2P3), co-funded by ´ CEA and CNES. PL, ES, CF, DL, and TS acknowledge funding from the ERC under the European Union’s Horizon 2020 research and innovation programme (grant agreement No. 694343). The work of AKL, JS, and DU is partially supported by the National Science Foundation (NSF) under Grants No. 1615105, 1615109, and 1653300. AKL also acknowledges partial support from the National Aeronautics and Space Administration (NASA) Astrophysics Data Analysis Program (ADAP) grants NNX16AF48G and NNX17AF39G. ER acknowledges the support of the Natural Sciences and Engineering Research Council of Canada (NSERC), funding reference number RGPIN-2017-03987. FB acknowledges funding from the ERC under the European Union’s Horizon 2020 research and innovation programme (grant agreement No. 726384). GB is supported by the Fondo de Fomento al Desarrollo Cient´ıfico y Tecnologico of the Comisi ´ on Nacional de ´ Investigacion Cient ´ ´ıfica y Tecnologica (CONICYT/FONDECYT), ´ Programa de Iniciacion, Folio 11150220. SCOG acknowledges ´ support from the DFG via SFB 881 ‘The Milky Way System’ (subprojects B1, B2, and B8) and also via Germany’s Excellence Strategy EXC-2181/1–390900948 (the Heidelberg STRUCTURES Excellence Cluster). KK gratefully acknowledges funding from the DFG in the form of an Emmy Noether Research Group (grant number KR4598/2-1, PI Kreckel). AU acknowledges support from the Spanish funding grants AYA2016-79006-P (MINECO/FEDER) and PGC2018-094671-B-I00 (MCIU/AEI/FEDER)

PHANGS: Constraining Star Formation Timescales Using the Spatial Correlations of Star Clusters and Giant Molecular Clouds

In the hierarchical view of star formation, giant molecular gas clouds (GMCs) undergo fragmentation to form small-scale structures made up of stars and star clusters. Here we study the connection between young star clusters and cold gas across a range of extragalactic environments by combining the high resolution (1") PHANGS-ALMA catalogue of GMCs with the star cluster catalogues from PHANGS-HST. The star clusters are spatially matched with the GMCs across a sample of 11 nearby star-forming galaxies with a range of galactic environments (centres, bars, spiral arms, etc.). We find that after 4-6 Myr the star clusters are no longer associated with any gas clouds. Additionally, we measure the autocorrelation of the star clusters and GMCs as well as their cross-correlation to quantify the fractal nature of hierarchical star formation. Young ($\leq$ 10 Myr) star clusters are more strongly autocorrelated on kpc and smaller spatial scales than the >10 Myr stellar populations, indicating that the hierarchical structure dissolves over time.Comment: 15 pages, 11 figures, 4 tables. Accepted to MNRAS Sept 6 202

The Physical Drivers and Observational Tracers of CO-to-H2 Conversion Factor Variations in Nearby Barred Galaxy Centers

The CO-to-H$_2$ conversion factor (\alpha_\rm{CO}) is central to measuring the amount and properties of molecular gas. It is known to vary with environmental conditions, and previous studies have revealed lower \alpha_\rm{CO} in the centers of some barred galaxies on kpc scales. To unveil the physical drivers of such variations, we obtained ALMA Band 3, 6, and 7 observations toward the inner 2 kpc of NGC 3627 and NGC 4321 tracing $^{12}$CO, $^{13}$CO, and C$^{18}$O lines on 100 pc scales. Our multi-line modeling and Bayesian likelihood analysis of these datasets reveal variations of molecular gas density, temperature, optical depth, and velocity dispersion, which are among the key drivers of \alpha_\rm{CO}. The central 300 pc nuclei in both galaxies show strong enhancement of temperature T_\rm{k}>100 K and density n_\rm{H_2}>10^3 cm$^{-3}$. Assuming a CO-to-H$_2$ abundance of $3\times10^{-4}$, we derive 4-15 times lower \alpha_\rm{CO} than the Galactic value across our maps, which agrees well with previous kpc-scale measurements. Combining the results with our previous work on NGC 3351, we find a strong correlation of \alpha_\rm{CO} with low-J $^{12}$CO optical depths (\tau_\rm{CO}), as well as an anti-correlation with T_\rm{k}. The \tau_\rm{CO} correlation explains most of the \alpha_\rm{CO} variation in the three galaxy centers, whereas changes in T_\rm{k} influence \alpha_\rm{CO} to second order. Overall, the observed line width and $^{12}$CO/$^{13}$CO 2-1 line ratio correlate with \tau_\rm{CO} variation in these centers, and thus they are useful observational indicators for \alpha_\rm{CO} variation. We also test current simulation-based \alpha_\rm{CO} prescriptions and find a systematic overprediction, which likely originates from the mismatch of gas conditions between our data and the simulations.Comment: Accepted for publication in ApJ; 30 pages of main text + 3 appendice