The morphology of the Milky Way - I. Reconstructing CO maps from simulations in fixed potentials

PublishedJournal ArticleWe present an investigation into the morphological features of the MilkyWay.We use smoothed particle hydrodynamics (SPH) to simulate the interstellar medium (ISM) in the Milky Way under the effect of a number of different gravitational potentials representing spiral arms and bars, assuming that the Milky Way is a grand design spiral in nature. The gas is subject to ISM cooling and chemistry, enabling us to track the evolution of molecular gas. We use a 3D radiative transfer code to simulate the emission from the SPH output, allowing for the construction of synthetic longitude-velocity (l-v) emission maps as viewed from the Earth. By comparing these maps with the observed emission in CO from the Milky Way, we infer the arm/bar geometry that provides a best fit to our Galaxy. We find that it is possible to reproduce nearly all features of the l-v diagram in CO emission. There is no model, however, that satisfactorily reproduces all of the features simultaneously. Models with two arms cannot reproduce all the observed arm features, while four armed models produce too bright local emission in the inner Galaxy. Our best-fitting models favour a bar pattern speed within 50-60 km s-1 kpc-1 and an arm pattern speed of approximately 20 km s-1 kpc-1, with a bar orientation of approximately 45° and arm pitch angle between 10°-15°.We thank an anonymous referee, whose comments and suggestions improved the paper. We also thank Tom Dame for providing access to the CO longitude–velocity data. The calculations for this paper were performed on the DiRAC Complexity machine, jointly funded by STFC and the Large Facilities Capital Fund of BIS, and the University of Exeter Supercomputer, a DiRAC Facility jointly funded by STFC, the Large Facilities Capital Fund of BIS and the University of Exeter. ARP is supported by an STFC-funded post-graduate studentship. CLD acknowledges funding from the European Research Council for the FP7 ERC starting grant project LOCALSTAR. DJP is supported by a Future Fellowship funded by the Australian Research Council (FT130100034). Figures showing SPH particle density were rendered using SPLASH (Price 2007). Datasets used in this paper are available at: http://hdl.handle.net/10871/15057

Synthetic CO, H2 and H I surveys of the second galactic quadrant, and the properties of molecular gas

articleWe present CO, H2, H I and HISA (H I self-absorption) distributions from a set of simulations of grand design spirals including stellar feedback, self-gravity, heating and cooling. We replicate the emission of the second galactic quadrant by placing the observer inside the modelled galaxies and post-process the simulations using a radiative transfer code, so as to create synthetic observations. We compare the synthetic data cubes to observations of the second quadrant of the Milky Way to test the ability of the current models to reproduce the basic chemistry of the Galactic interstellar medium (ISM), as well as to test how sensitive such galaxy models are to different recipes of chemistry and/or feedback. We find that models which include feedback and self-gravity can reproduce the production of CO with respect to H2 as observed in our Galaxy, as well as the distribution of the material perpendicular to the Galactic plane. While changes in the chemistry/feedback recipes do not have a huge impact on the statistical properties of the chemistry in the simulated galaxies, we find that the inclusion of both feedback and self-gravity are crucial ingredients, as our test without feedback failed to reproduce all of the observables. Finally, even though the transition from H2 to CO seems to be robust, we find that all models seem to underproduce molecular gas, and have a lower molecular to atomic gas fraction than is observed. Nevertheless, our fiducial model with feedback and self-gravity has shown to be robust in reproducing the statistical properties of the basic molecular gas components of the ISM in our Galaxy.We thank the referee, Ralf Klessen, for his comments that helped strengthen the paper. ADC and CLD acknowledge funding from the European Research Council for the FP7 ERC starting grant project LOCALSTAR. The calculations for this paper were performed on the DiRAC Complexity machine, jointly funded by STFC and the Large Facilities Capital Fund of BIS, and the University of Exeter Supercomputer, a DiRAC Facility jointly funded by STFC, the Large Facilities Capital Fund of BIS and the University of Exeter. Fig. 1 was produced using SPLASH (Price 2007). We acknowledge the use of NASA’s SkyView facility (http://skyview.gsfc.nasa.gov) located at NASA Goddard Space Flight Center. We also thank A. Rodrigues for providing high-resolution dust column density maps for benchmarking

The evolution of giant molecular filaments

This is the final version. Available from OUP via the DOI in this recordIn recent years, there has been a growing interest in studying giant molecular filaments (GMFs), which are extremely elongated (> 100 pc in length) giant molecular clouds (GMCs). They are often seen as inter-arm features in external spiral galaxies, but have been tentatively associated with spiral arms when viewed in the Milky Way. In this paper, we study the time evolution of GMFs in a high-resolution section of a spiral galaxy simulation, and their link with spiral arm GMCs and star formation, over a period of 11 Myr. The GMFs generally survive the interarm passage, although they are subject to a number of processes (e.g. star formation, stellar feedback and differential rotation) that can break the giant filamentary structure into smaller sections. The GMFs are not gravitationally bound clouds as a whole, but are, to some extent, confined by external pressure. Once they reach the spiral arms, the GMFs tend to evolve into more substructured spiral arm GMCs, suggesting that GMFs may be precursors to arm GMCs. Here, they become incorporated into the more complex and almost continuum molecular medium that makes up the gaseous spiral arm. Instead of retaining a clear filamentary shape, their shapes are distorted both by their climbing up the spiral potential and their interaction with the gas within the spiral arm. The GMFs do tend to become aligned with the spiral arms just before they enter them (when they reach the minimum of the spiral potential), which could account for the observations of GMFs in the Milky Way.ADC and CLD acknowledge funding from the European Research Council for the FP7 ERC starting grant project LOCALSTAR. ADC also acknowledges the support of the UK STFC consolidated grant ST/N000706/1. This work used the DiRAC Complexity system, operated by the University of Leicester IT Services, which forms part of the STFC DiRAC HPC Facility (www.dirac.ac.uk). This equipment is funded by BIS National E-Infrastructure capital grant ST/K000373/1 and STFC DiRAC Operations grant ST/K0003259/1. DiRAC is part of the National E-Infrastructure. This work also used the University of Exeter Supercomputer, a DiRAC Facility jointly funded by STFC, the Large Facilities Capital Fund of BIS and the University of Exeter

The PdBI Arcsecond Whirlpool Survey (PAWS): The Role of Spiral Arms in Cloud and Star Formation

This is the final version of the article. Available from American Astronomical Society via the DOI in this record.The process that leads to the formation of the bright star-forming sites observed along prominent spiral arms remains elusive. We present results of a multi-wavelength study of a spiral arm segment in the nearby grand-design spiral galaxy M51 that belongs to a spiral density wave and exhibits nine gas spurs. The combined observations of the (ionized, atomic, molecular, dusty) interstellar medium with star formation tracers (H ii regions, young <10 Myr stellar clusters) suggest (1) no variation in giant molecular cloud (GMC) properties between arm and gas spurs, (2) gas spurs and extinction feathers arising from the same structure with a close spatial relation between gas spurs and ongoing/recent star formation (despite higher gas surface densities in the spiral arm), (3) no trend in star formation age either along the arm or along a spur, (4) evidence for strong star formation feedback in gas spurs, (5) tentative evidence for star formation triggered by stellar feedback for one spur, and (6) GMC associations being not special entities but the result of blending of gas arm/spur cross sections in lower resolution observations. We conclude that there is no evidence for a coherent star formation onset mechanism that can be solely associated with the presence of the spiral density wave. This suggests that other (more localized) mechanisms are important to delay star formation such that it occurs in spurs. The evidence of star formation proceeding over several million years within individual spurs implies that the mechanism that leads to star formation acts or is sustained over a longer timescale.S.E.M. and M.Q. acknowledge funding from the Deutsche Forschungsgemeinschaft (DFG) via grant SCHI 536/7-2 as part of the priority program SPP 1573 "ISM-SPP: Physics of the Interstellar Medium." C.L.D. acknowledges funding from the European Research Council for the FP7 ERC starting grant project LOCALSTAR. J.P. acknowledges support from the CNRS programme Physique et Chimie du Milieu Interstellaire (PCMI). M.Q. acknowledges the International Max Planck Research School for Astronomy and Cosmic Physics at the University of Heidelberg (IMPRS-HD). S.G.B. thanks support from Spanish grant AYA2012-32295. We acknowledge financial support to the DAGAL network from the People Programme (Marie Curie Actions) of the European Unions Seventh Framework Programme FP7/2007-2013/ under REA grant agreement number PITN-GA-2011-289313. E.S. thanks NRAO for their support and hospitality during her visits in Socorro. E.S. thanks the Kavli Institute for Theoretical Physics for hospitality during the writing of this paper. IRAM is supported by INSU/CNRS (France), MPG (Germany), and IGN (Spain)

Gravitational torques imply molecular gas inflow towards the nucleus of M 51

PublishedJournal Article© 2016 ESO.The transport of gas towards the centre of galaxies is critical for black hole feeding and, indirectly, it can control active galactic nucleus (AGN) feedback. We have quantified the molecular gas inflow in the central R< 1 kpc of M 51 to be 1 M⊙/yr, using a new gravitational torque map and the molecular gas traced by the Plateau de Bure Interferometer Arcsecond Whirlpool Survey (PAWS). The nuclear stellar bar is responsible for this gas inflow. We also used torque profiles to estimate the location of dynamical resonances, and the results suggest a corotation for the bar CRbar ∼ 20″, and a corotation for the spiral CRsp ∼ 100″. We demonstrate how important it is to correct 3.6 μm images for dust emission when gravitational torques are to be computed, and we examine further sources of uncertainty. Our observational measurement of gas inflow can be compared with nuclear molecular outflow rates and provide useful constraints for numerical simulations.The authors would like to thank the anonymous referee for a helpful report, as well as Daniela Calzetti, Nick Z. Scoville and Mari Polletta for making the HST/F190N mosaic available to us. We also appreciate valuable comments from Françoise Combes and Sebastian Haan. We acknowledge financial support to the DAGAL network from the People Programme (Marie Curie Actions) of the European Union’s Seventh Framework Programme FP7/2007- 2013/ under REA grant agreement number PITN-GA-2011-289313. M.Q. acknowledges the International Max Planck Research School for Astronomy and Cosmic Physics at the University of Heidelberg (IMPRS-HD). S.G.B. thanks support from Spanish grant AYA2012-32295. J.P. acknowledges support from the CNRS programme “Physique et Chimie du Milieu Interstellaire” (PCMI). M.Q., S.E.M., D.C. and A.H. acknowledge funding from the Deutsche Forschungsgemeinschaft (DFG) via grants SCHI 536/7-2,SCHI 536/5-1, and SCHI 536/7-1 as part of the priority program SPP 1573 “ISM-SPP: Physics of the Interstellar Medium”

Short GMC lifetimes: an observational estimate with the PdBI Arcsecond Whirlpool Survey (PAWS)

PublishedJournal ArticleWe describe and execute a novel approach to observationally estimate the lifetimes of giant molecular clouds (GMCs). We focus on the cloud population between the two main spiral arms in M51 (the inter-arm region) where cloud destruction via shear and star formation feedback dominates over formation processes. By monitoring the change in GMC number densities and properties across the inter-arm, we estimate the lifetime as a fraction of the inter-arm travel time. We find that GMC lifetimes in M51's inter-arm are finite and short, 20-30 Myr. Over most of the region under investigation shear appears to regulate the lifetime. As the shear timescale increases with galactocentric radius, we expect cloud destruction to switch primarily to feedback at larger radii. We identify a transition from shear- to feedback-dominated disruption, finding that shear is more efficient at dispersing clouds, whereas feedback transforms the population, e.g., by fragmenting high-mass clouds into lower mass pieces. Compared to the characteristic timescale for molecular hydrogen in M51, our short lifetimes suggest that gas can remain molecular while clouds disperse and reassemble. We propose that galaxy dynamics regulates the cycling of molecular material from diffuse to bound (and ultimately star-forming) objects, contributing to long observed molecular depletion times in normal disk galaxies. We also speculate that, in extreme environments like elliptical galaxies and concentrated galaxy centers, star formation can be suppressed when the shear timescale is short enough that some clouds will not survive to form stars.We thank the IRAM staff for their support during the observations with the Plateau de Bure interferometer and the 30 m telescope. S.E.M., D.C., and A.H. acknowledge funding from the Deutsche Forschungsgemeinschaft (DFG) via grants SCHI 536/7-2, SCHI 536/5-1, and SCHI 536/7-1 as part of the priority program SPP 1573 “ISM-SPP: Physics of the Interstellar Medium.” C.L.D. acknowledges funding from the European Research Council for the FP7 ERC starting grant project LOCALSTAR. T.A.T. acknowledges support from NASA grant number NNX10AD01G. J.P. acknowledges support from the CNRS program “Physique et Chimie du Milieu Interstellaire” (PCMI). M.Q. acknowledges financial support to the DAGAL network from the People Programme (Marie Curie Actions) of the European Union’s Seventh Framework Programme FP7/2007-2013/ under REA grant agreement number PITN-GA-2011-289313

Photoionizing feedback in spiral arm molecular clouds

This is the author accepted manuscript. The final version is available from Oxford University Press via the DOI in this recordWe present simulations of a 500 pc2 region, containing gas of mass 4 × 106 M⊙, extracted from an entire spiral galaxy simulation, scaled up in resolution, including photoionising feedback from stars of mass > 18 M⊙. Our region is evolved for 10 Myr and shows clustered star formation along the arm generating ≈ 5000 cluster sink particles ≈ 5% of which contain at least one of the ≈ 4000 stars of mass > 18 M⊙. Photoionisation has a noticeable effect on the gas in the region, producing ionised cavities and leading to dense features at the edge of the HII regions. Compared to the no-feedback case, Photoionisation produces a larger total mass of clouds and clumps, with around twice as many such objects, which are individually smaller and more broken up. After this we see a rapid decrease in the total mass in clouds and the number of clouds. Unlike studies of isolated clouds, our simulations follow the long range effects of ionisation, with some already-dense gas, becoming compressed from multiple sides by neighbouring HII regions. This causes star formation that is both accelerated and partially displaced throughout the spiral arm with up to 30% of our cluster sink particle mass forming at distances > 5 pc from sites of sink formation in the absence of feedback. At later times, the star formation rate decreases to below that of the no-feedback case.European Union Horizon 2020European Union FP

The PdBI Arcsecond Whirlpool Survey (PAWS): Multi-phase cold gas kinematic of M51

This is the final version of the article. Available from the publisher via the DOI in this record.The kinematic complexity and the favorable position of M51 on the sky make this galaxy an ideal target to test different theories of spiral arm dynamics. Taking advantage of the new high-resolution PdBI Arcsecond Whirlpool Survey data, we undertake a detailed kinematic study of M51 to characterize and quantify the origin and nature of the non-circular motions. Using a tilted-ring analysis supported by several other archival data sets, we update the estimation of M51's position angle (P.A. = (173 ± 3)°) and inclination (i = (22 ± 5)°). Harmonic decomposition of the high-resolution (∼40 pc) CO velocity field shows the first kinematic evidence of an m = 3 wave in the inner disk of M51 with a corotation at R CR, m = 3 = 1.1 ± 0.1 kpc and a pattern speed of Ωp, m = 3 ≈ 140 km s -1 kpc-1. This mode seems to be excited by the nuclear bar, while the beat frequencies generated by the coupling between the m = 3 mode and the main spiral structure confirm its density-wave nature. We observe also a signature of an m = 1 mode that is likely responsible for the lopsidedness of M51 at small and large radii. We provide a simple method to estimate the radial variation of the amplitude of the spiral perturbation (V sp) attributed to the different modes. The main spiral arm structure has 〈V sp〉 = 50-70 km s-1, while the streaming velocity associated with the m = 1 and m = 3 modes is, in general, two times lower. Our joint analysis of H I and CO velocity fields at low and high spatial resolution reveals that the atomic and molecular gas phases respond differently to the spiral perturbation due to their different vertical distribution and emission morphology. © 2014. The American Astronomical Society. All rights reserved.We thank our anonymous referee for thoughtful comments that improved the quality of the paper. We thank the IRAM staff for their support during the observations with the Plateau de Bure interferometer and the 30 m telescope. D.C. and A.H. acknowledge funding from the Deutsche Forschungsgemeinschaft (DFG) via grant SCHI 536/5-1 and SCHI 536/7-1 as part of the priority program SPP 1573 “ISM-SPP: Physics of the Interstellar Medium.” C.L.D. acknowledges funding from the European Research Council for the FP7 ERC starting grant project LOCALSTAR. T.A.T. acknowledges support from NASA grant No. NNX10AD01G. During this work, J.P. was partially funded by the grant ANR-09-BLAN-0231-01 from the French Agence Nationale de la Recherche as part of the SCHISM project (http://schism.ens.fr/). E.S., A.H. and D.C. thank NRAO for their support and hospitality during their visits in Charlottesville. E.S. thanks the Aspen Center for Physics and the NSF grant No. 1066293 for hospitality during the development and writing of this paper. D.C. thanks Glenn van de Ven for the useful discussion and the help with the harmonic decomposition code. S.G.B. acknowledges economic support from Junta de Andalucia grant P08 TIC 03531. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc

The PdBI Arcsecond Whirlpool Survey (PAWS): Environmental dependence of giant molecular cloud properties in M51

This is the final version of the article. Available fromAmerican Astronomical Society / IOP Publishing via the DOI in this recordUsing data from the PdBI Arcsecond Whirlpool Survey (PAWS), we have generated the largest extragalactic giant molecular cloud (GMC) catalog to date, containing 1507 individual objects. GMCs in the inner M51 disk account for only 54% of the total 12CO(1-0) luminosity of the survey, but on average they exhibit physical properties similar to Galactic GMCs. We do not find a strong correlation between the GMC size and velocity dispersion, and a simple virial analysis suggests that ∼30% of GMCs in M51 are unbound. We have analyzed the GMC properties within seven dynamically motivated galactic environments, finding that GMCs in the spiral arms and in the central region are brighter and have higher velocity dispersions than inter-arm clouds. Globally, the GMC mass distribution does not follow a simple power-law shape. Instead, we find that the shape of the mass distribution varies with galactic environment: the distribution is steeper in inter-arm region than in the spiral arms, and exhibits a sharp truncation at high masses for the nuclear bar region. We propose that the observed environmental variations in the GMC properties and mass distributions are a consequence of the combined action of large-scale dynamical processes and feedback from high-mass star formation. We describe some challenges of using existing GMC identification techniques for decomposing the 12CO(1-0) emission in molecule-rich environments, such as M51's inner disk. © 2014. The American Astronomical Society. All rights reserved

The role of collision speed, cloud density, and turbulence in the formation of young massive clusters via cloud–cloud collisions

This is the final version. Available from Oxford University Press via the DOI in this record Young massive clusters (YMCs) are recently formed astronomical objects with unusually high star formation rates. We propose the collision of giant molecular clouds (GMCs) as a likely formation mechanism of YMCs, consistent with the YMC conveyor-belt formation mode concluded by other authors. We conducted smoothed particle hydrodynamical simulations of cloud–cloud collisions and explored the effect of the clouds’ collision speed, initial cloud density, and the level of cloud turbulence on the global star formation rate and the properties of the clusters formed from the collision. We show that greater collision speed, greater initial cloud density and lower turbulence increase the overall star formation rate and produce clusters with greater cluster mass. In general, collisions with relative velocity ≳ 25 km s−1, initial cloud density ≳ 250 cm−3, and turbulence of ≈2.5 km s−1 can produce massive clusters with properties resembling the observed Milky Way YMCs.European Commissio