arXiv subjects

Jérôme Pety

Publications and source records attributed to Jérôme Pety.

At least 55 records · Page 3Linked to original sources

arXiv2022

A CO isotopologue Line Atlas within the Whirlpool galaxy Survey (CLAWS)

We present the CO isotopologue Line Atlas within the Whirpool galaxy Survey (CLAWS) based on an IRAM 30-m large programme which provides a benchmark study of numerous, faint CO isotopologues in the mm-wavelength regime across the full disc of M51 (NGC 5194). The survey's core goal is to use the low-J CO isotopologue lines to constrain CO excitation and chemistry, and therefrom the local physical conditions of the gas. In this survey paper, we describe the CLAWS observing and data reduction strategies. We map the J=1-0 and 2-1 transitions of the CO isotopologues $^{12}$CO,$^{13}$CO, C$^{18}$O and C$^{17}$O, as well as several supplementary lines within the 1 mm and 3 mm window (CN(1-0), CS(2-1), CH$_3$OH(2-1), N$_2$H$^+$(1-0), HC$_3$N(10-9)) at ~1 kpc resolution. A total observation time of 149 h offers unprecedented sensitivity. We use these data to explore several CO isotopologue line ratios in detail, study their radial (and azimuthal) trends and investigate whether changes in line ratios stem from changes in ISM properties such as gas temperatures, densities or chemical abundances. For example, we find negative radial trends for the $^{13}$CO}/$^{12}$CO, C$^{18}$O/$^{12}$CO and C$^{18}$O/$^{13}$CO line ratios in their J=1-0 transitions. We also find variations with local environment, such as higher $^{12}$CO(2-1)/(1-0) or $^{13}$CO/$^{12}$CO(1-0) line ratios in interarm regions compared to spiral arm regions. We propose that these aforementioned variations of CO line ratios are most likely due to a variation of the optical depth, while abundance variations due to selective nucleosynthesis on a galaxy-wide scale could also play a role. We also study the CO spectral line energy distribution (SLED) using archival JCMT $^{12}$CO(3-2) data and find a variation of the SLED shape with local environmental parameters further underlying changes in optical depth, gas temperatures or densities.

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arXiv2022

A 2-3 mm high-resolution molecular line survey towards the centre of the nearby spiral galaxy NGC 6946

The complex physical, kinematic, and chemical properties of galaxy centres make them interesting environments to examine with molecular line emission. We present new $2-4$" (${\sim}75{-}150$ pc at $7.7$ Mpc) observations at 2 and 3 mm covering the central $50$" (${\sim}1.9$ kpc) of the nearby double-barred spiral galaxy NGC 6946 obtained with the IRAM Plateau de Bure Interferometer. We detect spectral lines from ten molecules: CO, HCN, HCO$^+$, HNC, CS, HC$_3$N, N$_2$H$^+$, C$_2$H, CH$_3$OH, and H$_2$CO. We complemented these with published 1mm CO observations and 33 GHz continuum observations to explore the star formation rate surface density ${Σ_{\mathrm{SFR}}}$ on 150 pc scales. In this paper, we analyse regions associated with the inner bar of NGC 6946 $-$ the nuclear region (NUC), the northern (NBE), and southern inner bar end (SBE) and we focus on short-spacing corrected bulk (CO) and dense gas tracers (HCN, HCO$^+$, and HNC). We find that HCO$^+$ correlates best with ${Σ_{\mathrm{SFR}}}$, but the dense gas fraction ($f_{\mathrm{dense}}$) and star formation efficiency of the dense gas (${\mathrm{SFE_{dense}}}$) fits show different behaviours than expected from large-scale disc observations.The SBE has a higher ${Σ_{\mathrm{SFR}}}$, $f_{\mathrm{dense}}$, and shocked gas fraction than the NBE. We examine line ratio diagnostics and find a higher CO(2-1)/CO(1-0) ratio towards NBE than for the NUC. Moreover, comparison with existing extragalactic datasets suggests that using the HCN/HNC ratio to probe kinetic temperatures is not suitable on kiloparsec and sub-kiloparsec scales in extragalactic regions. Lastly, our study shows that the HCO$^+$/HCN ratio might not be a unique indicator to diagnose AGN activity in galaxies.

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arXiv2022

The PHANGS-MUSE survey -- Probing the chemo-dynamical evolution of disc galaxies

We present the PHANGS-MUSE survey, a programme using the MUSE IFS at the ESO VLT to map 19 massive $(9.4 < \log(M_{*}/M_\odot) < 11.0)$ nearby (D < 20 Mpc) star-forming disc galaxies. The survey consists of 168 MUSE pointings (1'x1' each), a total of nearly 15 Million spectra, covering ~1.5 Million independent spectra. PHANGS-MUSE provides the first IFS view of star formation across different local environments (including galaxy centres, bars, spiral arms) in external galaxies at a median resolution of 50~pc, better than the mean inter-cloud distance in the ionised interstellar medium. This `cloud-scale' resolution allows detailed demographics and characterisations of HII regions and other ionised nebulae. PHANGS-MUSE further delivers a unique view on the associated gas and stellar kinematics, and provides constraints on the star formation history. The PHANGS-MUSE survey is complemented by dedicated ALMA CO(2-1) and multi-band HST observations, therefore allowing us to probe the key stages of the star formation process from molecular clouds to HII regions and star clusters. This paper describes the scientific motivation, sample selection, observational strategy, data reduction and analysis process of the PHANGS-MUSE survey. We present our bespoke automated data-reduction framework, which is built on the reduction recipes provided by ESO, but additionally allows for mosaicking and homogenisation of the point spread function. We further present a detailed quality assessment and a brief illustration of the potential scientific applications of the large set of PHANGS-MUSE data products generated by our data analysis framework. The data cubes and analysis data products described in this paper represent the basis for the first PHANGS-MUSE public data release and are available in the ESO archive and via the Canadian Astronomy Data Centre.

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arXiv2021

Low-J CO Line Ratios From Single Dish CO Mapping Surveys and PHANGS-ALMA

We measure the low-J CO line ratio R21=CO(2-1)/CO(1-0), R32=CO(3-2)/CO(2-1), and R31 = 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 JCMT NGLS and APEX LASMA mapping. Altogether this yields 76, 47, and 29 maps of R21, R32, and R31 at 20" \sim 1.3 kpc resolution, covering 43, 34, and 20 galaxies. Disk galaxies with high stellar mass, log10 M_* [Msun]=10.25-11 and star formation rate, SFR=1-5 Msun/yr, dominate the sample. We find galaxy-integrated mean values and 16%-84% range of R21 = 0.65 (0.50-0.83), R32=0.50 (0.23-0.59), and R31=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 \sim 0.18^{+0.09}_{-0.14} dex for R21, 0.27^{+0.13}_{-0.15} dex for R31, and 0.08^{+0.11}_{-0.09} dex for R32. All three line ratios anti-correlate with galactocentric radius and positively correlate with the local star formation rate surface density and specific star formation rate, 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 ISM. Because the line ratios are expected to anti-correlate with the CO(1-0)-to-H_2 conversion factor, alphaCO^(1-0), these results have general implications for the interpretation of CO emission from galaxies.

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arXiv2021

NOEMA High Fidelity Imaging of the Molecular Gas in and around M82

We present a 154 pointing IRAM NOEMA mosaic of the CO(1-0) line emission in and around the nearby starburst galaxy M82. The observations, complemented by zero--spacing observations, reach a spatial resolution of $\sim$30 pc ($\sim 1.9^{\prime\prime}$) at 5.0 km s$^{-1}$ spectral resolution, sufficient to resolve the molecular gas in the central starburst disk, the outflow, as well as the tidal streamers. The resulting moment and peak brightness maps show a striking amount of structure. Using a clump decomposition algorithm, we analyse the physical properties (e.g., radii $R$, line widths $σ$, and masses $M$) of $\sim2000$ molecular clouds. To first order, the clouds' properties are very similar, irrespective of their environment. This also holds for the size-line width relations of the clouds. The distribution of clouds in the $σ^2/R$ vs. column density $Σ$ space suggests that external pressure does not play a significant role in setting their physical parameters in the outflow and the streamers. We find that the clouds in the streamers stay approximately constant in size ($R \sim 50$ pc) and mass ($M \sim 10^5$ M$_\odot$) and do not vary with their projected distance from M82's center. The clouds in the outflow, on the other hand, appear to decrease in size and mass with distance towards the Southern outflow. The reduction in the molecular gas luminosity could be indicative of cloud evaporation of embedded clouds in the hot outflow.

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arXiv2021

PHANGS-ALMA: Arcsecond CO(2-1) Imaging of Nearby Star-Forming Galaxies

We present PHANGS-ALMA, the first survey to map CO J=2-1 line emission at ~1" ~ 100pc spatial resolution from a representative sample of 90 nearby (d<~20 Mpc) galaxies that lie on or near the z=0 "main sequence" of star-forming galaxies. CO line emission traces the bulk distribution of molecular gas, which is the cold, star-forming phase of the interstellar medium. At the resolution achieved by PHANGS-ALMA, each beam reaches the size of a typical individual giant molecular cloud (GMC), so that these data can be used to measure the demographics, life-cycle, and physical state of molecular clouds across the population of galaxies where the majority of stars form at z=0. This paper describes the scientific motivation and background for the survey, sample selection, global properties of the targets, ALMA observations, and characteristics of the delivered ALMA data and derived data products. As the ALMA sample serves as the parent sample for parallel surveys with VLT/MUSE, HST, AstroSat, VLA, and other facilities, we include a detailed discussion of the sample selection. We detail the estimation of galaxy mass, size, star formation rate, CO luminosity, and other properties, compare estimates using different systems and provide best-estimate integrated measurements for each target. We also report the design and execution of the ALMA observations, which combine a Cycle~5 Large Program, a series of smaller programs, and archival observations. Finally, we present the first 1" resolution atlas of CO emission from nearby galaxies and describe the properties and contents of the first PHANGS-ALMA public data release.

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arXiv2021

On the duration of the embedded phase of star formation

Feedback from massive stars plays a key role in molecular cloud evolution. After the onset of star formation, the young stellar population is exposed by photoionization, winds, supernovae, and radiation pressure from massive stars. Recent observations of nearby galaxies have provided the evolutionary timeline between molecular clouds and exposed young stars, but the duration of the embedded phase of massive star formation is still ill-constrained. We measure how long massive stellar populations remain embedded within their natal cloud, by applying a statistical method to six nearby galaxies at 20-100 pc resolution, using CO, Spitzer 24$\rm\,μm$, and H$α$ emission as tracers of molecular clouds, embedded star formation, and exposed star formation, respectively. We find that the embedded phase (with CO and 24$\rm\,μm$ emission) lasts for $2{-}7$ Myr and constitutes $17{-}47\%$ of the cloud lifetime. During approximately the first half of this phase, the region is invisible in H$α$, making it heavily obscured. For the second half of this phase, the region also emits in H$α$ and is partially exposed. Once the cloud has been dispersed by feedback, 24$\rm\,μm$ emission no longer traces ongoing star formation, but remains detectable for another $2{-}9$ Myr through the emission from ambient CO-dark gas, tracing star formation that recently ended. The short duration of massive star formation suggests that pre-supernova feedback (photoionization and winds) is important in disrupting molecular clouds. The measured timescales do not show significant correlations with environmental properties (e.g. metallicity). Future JWST observations will enable these measurements routinely across the nearby galaxy population.

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arXiv2021

Applying the Tremaine-Weinberg Method to Nearby Galaxies: Stellar Mass-Based Pattern Speeds, and Comparisons with ISM Kinematics

We apply the Tremaine-Weinberg method to 19 nearby galaxies using stellar mass surface densities and velocities derived from the PHANGS-MUSE survey, to calculate (primarily bar) pattern speeds ($Ω_{\rm P}$). After quality checks, we find that around half (10) of these stellar mass-based measurements are reliable. For those galaxies, we find good agreement between our results and previously published pattern speeds, and use rotation curves to calculate major resonance locations (co-rotation radii and Lindblad resonances). We also compare these stellar-mass derived pattern speeds with H$α$ (from MUSE) and CO($J=2{-}1$) emission from the PHANGS-ALMA survey. We find that in the case of these clumpy ISM tracers, this method erroneously gives a signal that is simply the angular frequency at a representative radius set by the distribution of these clumps ($Ω_{\rm clump}$), and that this $Ω_{\rm clump}$ is significantly different to $Ω_{\rm P}$ ($\sim$20% in the case of H$α$, and $\sim$50% in the case of CO). Thus, we conclude that it is inadvisable to use "pattern speeds" derived from ISM kinematics. Finally, we compare our derived pattern speeds and co-rotation radii, along with bar properties, to the global parameters of these galaxies. Consistent with previous studies, we find that galaxies with a later Hubble type have a larger ratio of co-rotation radius to bar length, more molecular-gas rich galaxies have higher $Ω_{\rm P}$, and more bulge-dominated galaxies have lower $Ω_{\rm P}$. Unlike earlier works, however, there are no clear trends between the bar strength and $Ω_{\rm P}$, nor between the total stellar mass surface density and the pattern speed.

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arXiv2021

Resolving the Dust-to-Metals Ratio and CO-to-H$_2$ Conversion Factor in the Nearby Universe

We investigate the relationship between the dust-to-metals ratio (D/M) and the local interstellar medium environment at ~2 kpc resolution in five nearby galaxies: IC342, M31, M33, M101, and NGC628. A modified blackbody model with a broken power-law emissivity is used to model the dust emission from 100 to 500 um observed by Herschel. We utilize the metallicity gradient derived from auroral line measurements in HII regions whenever possible. Both archival and new CO rotational line and HI 21 cm maps are adopted to calculate gas surface density, including new wide field CO and HI maps for IC342 from IRAM and the VLA, respectively. We experiment with several prescriptions of CO-to-H$_2$ conversion factor, and compare the resulting D/M-metallicity and D/M-density correlations, both of which are expected to be non-negative from depletion studies. The D/M is sensitive to the choice of the conversion factor. The conversion factor prescriptions based on metallicity only yield too much molecular gas in the center of IC342 to obtain the expected correlations. Among the prescriptions tested, the one that yields the expected correlations depends on both metallicity and surface density. The 1-$σ$ range of the derived D/M spans 0.40-0.58. Compared to chemical evolution models, our measurements suggest that the dust growth time scale is much shorter than the dust destruction time scale. The measured D/M is consistent with D/M in galaxy-integrated studies derived from infrared dust emission. Meanwhile, the measured D/M is systematically higher than the D/M derived from absorption, which likely indicates a systematic offset between the two methods.

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arXiv2021

Giant Molecular Cloud Catalogues for PHANGS-ALMA: Methods and Initial Results

We present improved methods for segmenting CO emission from galaxies into individual molecular clouds, providing an update to the CPROPS algorithms presented by Rosolowsky & Leroy (2006; arXiv:astro-ph/0601706 ). The new code enables both homogenization of the noise and spatial resolution among data, which allows for rigorous comparative analysis. The code also models the completeness of the data via false source injection and includes an updated segmentation approach to better deal with blended emission. These improved algorithms are implemented in a publicly available python package, PYCPROPS. We apply these methods to ten of the nearest galaxies in the PHANGS-ALMA survey, cataloguing CO emission at a common 90 pc resolution and a matched noise level. We measure the properties of 4986 individual clouds identified in these targets. We investigate the scaling relations among cloud properties and the cloud mass distributions in each galaxy. The physical properties of clouds vary among galaxies, both as a function of galactocentric radius and as a function of dynamical environment. Overall, the clouds in our target galaxies are well-described by approximate energy equipartition, although clouds in stellar bars and galaxy centres show elevated line widths and virial parameters. The mass distribution of clouds in spiral arms has a typical mass scale that is 2.5x larger than interarm clouds and spiral arms clouds show slightly lower median virial parameters compared to interarm clouds (1.2 versus 1.4).

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arXiv2020

Pre-supernova feedback mechanisms drive the destruction of molecular clouds in nearby star-forming disc galaxies

It is a major open question which physical processes stop the accretion of gas onto giant molecular clouds (GMCs) and limit the efficiency at which gas is converted into stars within these GMCs. While feedback from supernova explosions has been the popular feedback mechanism included in simulations of galaxy formation and evolution, `early' feedback mechanisms such as stellar winds, photoionisation and radiation pressure are expected to play an important role in dispersing the gas after the onset of star formation. These feedback processes typically take place on small scales ($\sim 10-100$ pc) and their effects have therefore been difficult to constrain in environments other than the Milky Way. We apply a novel statistical method to $\sim 1$" resolution maps of CO and Ha emission across a sample of nine nearby disc galaxies, in order to measure the time over which GMCs are dispersed by feedback from young, high-mass stars, as a function of the galactic environment. We find that GMCs are typically dispersed within $\sim$ 3 Myr after the emergence of unembedded high-mass stars, showing no significant trend with galactocentric radius. Comparison with analytical predictions demonstrates that, independently of the environment, early feedback mechanisms (particularly photoionisation and stellar winds) play a crucial role in dispersing GMCs and limiting their star formation efficiency in nearby galaxies. Finally, we show that the efficiency at which the energy injected by these early feedback mechanisms couples with the parent GMC is relatively low (a few tens of per cent), such that the vast majority of momentum and energy emitted by the young stellar populations escapes the parent GMC.

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arXiv2020

Molecular Gas Properties on Cloud Scales Across the Local Star-forming Galaxy Population

Using the PHANGS-ALMA CO (2-1) survey, we characterize molecular gas properties on ${\sim}$100 pc scales across 102,778 independent sightlines in 70 nearby galaxies. This yields the best synthetic view of molecular gas properties on cloud scales across the local star-forming galaxy population obtained to date. Consistent with previous studies, we observe a wide range of molecular gas surface densities (3.4 dex), velocity dispersions (1.7 dex), and turbulent pressures (6.5 dex) across the galaxies in our sample. Under simplifying assumptions about sub-resolution gas structure, the inferred virial parameters suggest that the kinetic energy of the molecular gas typically exceeds its self-gravitational binding energy at ${\sim}$100 pc scales by a modest factor (1.3 on average). We find that the cloud-scale surface density, velocity dispersion, and turbulent pressure (1) increase towards the inner parts of galaxies, (2) are exceptionally high in the centers of barred galaxies (where the gas also appears less gravitationally bound), and (3) are moderately higher in spiral arms than in inter-arm regions. The galaxy-wide averages of these gas properties also correlate with the integrated stellar mass, star formation rate, and offset from the star-forming main sequence of the host galaxies. These correlations persist even when we exclude regions with extraordinary gas properties in galaxy centers, which contribute significantly to the inter-galaxy variations. Our results provide key empirical constraints on the physical link between molecular cloud populations and their galactic environment.

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arXiv2020

Quantitative inference of the $H_2$ column densities from 3 mm molecular emission: A case study towards Orion B

Molecular hydrogen being unobservable in cold molecular clouds, the column density measurements of molecular gas currently rely either on dust emission observation in the far-IR or on star counting. (Sub-)millimeter observations of numerous trace molecules are effective from ground based telescopes, but the relationships between the emission of one molecular line and the H2 column density (NH2) is non-linear and sensitive to excitation conditions, optical depths, abundance variations due to the underlying physico-chemistry. We aim to use multi-molecule line emission to infer NH2 from radio observations. We propose a data-driven approach to determine NH2 from radio molecular line observations. We use supervised machine learning methods (Random Forests) on wide-field hyperspectral IRAM-30m observations of the Orion B molecular cloud to train a predictor of NH2, using a limited set of molecular lines as input, and the Herschel-based dust-derived NH2 as ground truth output. For conditions similar to the Orion B molecular cloud, we obtain predictions of NH2 within a typical factor of 1.2 from the Herschel-based estimates. An analysis of the contributions of the different lines to the predictions show that the most important lines are $^{13}$CO(1-0), $^{12}$CO(1-0), C$^{18}$O(1-0), and HCO$^+$(1-0). A detailed analysis distinguishing between diffuse, translucent, filamentary, and dense core conditions show that the importance of these four lines depends on the regime, and that it is recommended to add the N$_2$H$^+$(1-0) and CH$_3$OH(20-10) lines for the prediction of NH2 in dense core conditions. This article opens a promising avenue to directly infer important physical parameters from the molecular line emission in the millimeter domain. The next step will be to try to infer several parameters simultaneously (e.g., NH2 and far-UV illumination field) to further test the method. [Abridged]

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arXiv2020

Tracers of the ionization fraction in dense and translucent gas: I. Automated exploitation of massive astrochemical model grids

The ionization fraction plays a key role in the physics and chemistry of the neutral interstellar medium, from controlling the coupling of the gas to the magnetic field to allowing fast ion-neutral reactions that drive interstellar chemistry. Most estimations of the ionization fraction have relied on deuterated species such as DCO+, whose detection is limited to dense cores representing an extremely small fraction of the volume of the giant molecular clouds they are part of. As large field-of-view hyperspectral maps become available, new tracers may be found. We search for the best observable tracers of the ionization fraction based on a grid of astrochemical models. We build grids of models that sample randomly a large space of physical conditions (unobservable quantities such as gas density, temperature, etc.) and compute the corresponding observables (line intensities, column densities) and the ionization fraction. We estimate the predictive power of each potential tracer by training a Random Forest model to predict the ionization fraction from that tracer, based on these model grids. In both translucent medium and cold dense medium conditions, several observable tracers with very good predictive power for the ionization fraction are found. Several tracers in cold dense medium conditions are found to be better and more widely applicable than the traditional DCO+/HCO+ ratio. We also provide simpler analytical fits for estimating the ionization fraction from the best tracers, and for estimating the associated uncertainties. We discuss the limitations of the present study and select a few recommended tracers in both types of conditions. The method presented here is very general and can be applied to the measurement of any other quantity of interest (cosmic ray flux, elemental abundances, etc.) from any type of model (PDR models, time-dependent chemical models, etc.). (abridged)

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arXiv2020

Dynamical Equilibrium in the Molecular ISM in 28 Nearby Star-Forming Galaxies

We compare the observed turbulent pressure in molecular gas, $P_\mathrm{turb}$, to the required pressure for the interstellar gas to stay in equilibrium in the gravitational potential of a galaxy, $P_\mathrm{DE}$. To do this, we combine arcsecond resolution CO data from PHANGS-ALMA with multi-wavelength data that traces the atomic gas, stellar structure, and star formation rate (SFR) for 28 nearby star-forming galaxies. We find that $P_\mathrm{turb}$ correlates with, but almost always exceeds the estimated $P_\mathrm{DE}$ on kiloparsec scales. This indicates that the molecular gas is over-pressurized relative to the large-scale environment. We show that this over-pressurization can be explained by the clumpy nature of molecular gas; a revised estimate of $P_\mathrm{DE}$ on cloud scales, which accounts for molecular gas self-gravity, external gravity, and ambient pressure, agrees well with the observed $P_\mathrm{turb}$ in galaxy disks. We also find that molecular gas with cloud-scale ${P_\mathrm{turb}}\approx{P_\mathrm{DE}}\gtrsim{10^5\,k_\mathrm{B}\,\mathrm{K\,cm^{-3}}}$ in our sample is more likely to be self-gravitating, whereas gas at lower pressure appears more influenced by ambient pressure and/or external gravity. Furthermore, we show that the ratio between $P_\mathrm{turb}$ and the observed SFR surface density, $Σ_\mathrm{SFR}$, is compatible with stellar feedback-driven momentum injection in most cases, while a subset of the regions may show evidence of turbulence driven by additional sources. The correlation between $Σ_\mathrm{SFR}$ and kpc-scale $P_\mathrm{DE}$ in galaxy disks is consistent with the expectation from self-regulated star formation models. Finally, we confirm the empirical correlation between molecular-to-atomic gas ratio and kpc-scale $P_\mathrm{DE}$ reported in previous works.

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arXiv2019

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

It remains a major challenge to derive a theory of cloud-scale ($\lesssim100$ 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 ($\sim100$ 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 10-30 Myr, and exhibit environmental variation, between and within galaxies. At kpc-scale molecular gas surface densities $Σ_{\rm H_2}\geqslant8$M$_{\odot}$pc$^{-2}$, the GMC lifetime correlates with time-scales for galactic dynamical processes, whereas at $Σ_{\rm H_2}\leqslant8$M$_{\odot}$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% of the cloud lifetime), GMCs disperse within just 1-5 Myr 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% These results show that galactic star formation is governed by cloud-scale, environmentally-dependent, dynamical processes driving rapid evolutionary cycling. GMCs and HII regions are the fundamental units undergoing these lifecycles, with mean separations of 100-300 pc in star-forming discs. Future work should characterise the multi-scale physics and mass flows driving these lifecycles.

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arXiv2019

The headlight cloud in NGC 628: An extreme giant molecular cloud in a typical galaxy disk

Cloud-scale surveys of molecular gas reveal the link between molecular clouds properties and star formation (SF) across a range of galactic environments. Cloud populations in galaxy disks are considered to be representative of the `normal' SF. At high resolution, however, clouds with exceptional gas properties and SF activity may also be observed in normal disk environments. In this paper, we study the brightest cloud traced in CO emission in the disk of NGC628. The cloud is spatially coincident with an extremely bright HII region. We characterize its molecular gas properties and investigate how feedback and large-scale processes influence the properties of the molecular gas. High resolution CO ALMA observations are used to characterize its mass and dynamical state, which are compared to other clouds in NGC628. A LVG analysis is used to constrain the beam-diluted density and temperature of the molecular gas. We analyze the MUSE spectrum using Starburst99 to characterize the young stellar population associated with the HII region. The cloud is massive ($1-2\times10^7$M$_{\odot}$), with a beam-diluted density of $n_{\rm H_2}=5\times10^4$ cm$^{-3}$. It has a low virial parameter, suggesting that its CO emission may be overluminous due to heating by the HII region. A young ($2-4$ Myr), massive $3\times10^{5}$ M$_{\odot}$ stellar population is associated. We argue that the cloud is currently being destroyed by feedback from young massive stars. Due to the cloud's large mass, this phase of the cloud's evolution is long enough for the impact of feedback on the excitation of the gas to be observed. Its high mass may be related to its location at a spiral co-rotation radius, where gas experiences reduced galactic shear compared to other regions of the disk, and receives a sustained inflow of gas that can promote the cloud's mass growth.

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arXiv2019

A dynamically young, gravitationally stable network of filaments in Orion B

Filaments are a key step on the path that leads from molecular clouds to star formation. However, their characteristics are heavily debated, and the exact processes that lead to their formation and fragmentation into dense cores still remain to be fully understood. We aim at characterising the mass, kinematics, and stability against gravitational collapse of a statistically significant sample of filaments in the Orion B molecular cloud, which is renown for its very low star formation efficiency. We characterise the gas column densities and kinematics over a field of 1.9 deg$^2$, using C$^{18}$O(J=1-0) data from the IRAM-30m large programme ORION-B. Using two different Hessian-based filters, we extract and compare two filamentary networks, each containing over 100 filaments. Independent of the extraction method, the filaments have widths of 0.12$\pm$0.04 pc, and show a wide range of linear (1 - 100 $M_{\odot}$pc$^{-1}$) and volume densities (2.10$^3$ - 2.10$^5$ cm$^{-3}$). Compared to previous studies, the filament population is dominated by low-density, thermally sub-critical structures, suggesting that most of the identified filaments are not collapsing to form stars. In fact, only ~1% of the Orion B cloud mass covered by our observations can be found in super-critical, star-forming filaments, explaining the low star formation efficiency of the region. The velocity profiles observed across the filaments show quiescence in the centre, and coherency in the plane of the sky, despite being mostly supersonic. The filaments in Orion B apparently belong to a continuum which contains a few elements comparable to already studied star-forming filaments as well as many lower-density, gravitationally unbound structures. This comprehensive study of the Orion B filaments shows that the mass fraction in super-critical filaments is a key factor in determining star formation efficiency.

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