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Kirk J.M.,University of Central Lancashire | Kirk J.M.,University of Cardiff | Gear W.K.,University of Cardiff | Fritz J.,Ghent University | And 21 more authors.
Astrophysical Journal | Year: 2015

In this paper we present a catalog of giant molecular clouds (GMCs) in the Andromeda (M31) galaxy extracted from the Herschel Exploitation of Local Galaxy Andromeda (HELGA) data set. GMCs are identified from the Herschel maps using a hierarchical source extraction algorithm. We present the results of this new catalog and characterize the spatial distribution and spectral energy properties of its clouds based on the radial dust/gas properties found by Smith et al. A total of 326 GMCs in the mass range 104-107 M⊙ are identified; their cumulative mass distribution is found to be proportional to M-2.34, in agreement with earlier studies. The GMCs appear to follow the same correlation of cloud mass to LCO observed in the Milky Way. However, comparison between this catalog and interferometry studies also shows that the GMCs are substructured below the Herschel resolution limit, suggesting that we are observing associations of GMCs. Following Gordon et al., we study the spatial structure of M31 by splitting the observed structure into a set of spiral arms and offset rings. We fit radii of 10.3 and 15.5 kpc to the two most prominent rings. We then fit a logarithmic spiral with a pitch angle of 8.°9 to the GMCs not associated with either ring. Last, we comment on the effects of deprojection on our results and investigate the effect different models for M31's inclination will have on the projection of an unperturbed spiral arm system. © 2015. The American Astronomical Society. All rights reserved.

Remy-Ruyer A.,University Paris - Sud | Madden S.C.,University Paris - Sud | Galliano F.,University Paris - Sud | Hony S.,University Paris - Sud | And 20 more authors.
Astronomy and Astrophysics | Year: 2013

Context. We present new photometric data from our Herschel guaranteed time key programme, the Dwarf Galaxy Survey (DGS), dedicated to the observation of the gas and dust in low-metallicity environments. A total of 48 dwarf galaxies were observed with the PACS and SPIRE instruments onboard the Herschel Space Observatory at 70, 100, 160, 250, 350, and 500 μm. Aims. The goal of this paper is to provide reliable far-infrared (FIR) photometry for the DGS sample and to analyse the FIR/submillimetre (submm) behaviour of the DGS galaxies. We focus on a systematic comparison of the derived FIR properties (FIR luminosity, LFIR, dust mass, Mdust, dust temperature, T, emissivity index, β) with more metal-rich galaxies and investigate the detection of a potential submm excess. Methods. The data reduction method is adapted for each galaxy in order to derive the most reliable photometry from the final maps. The derived PACS flux densities are compared with the Spitzer MIPS 70 and 160 μm bands. We use colour-colour diagrams to analyse the FIR/submm behaviour of the DGS galaxies and modified blackbody fitting procedures to determine their dust properties. To study the variation in these dust properties with metallicity, we also include galaxies from the Herschel KINGFISH sample, which contains more metal-rich environments, totalling 109 galaxies. Results. The location of the DGS galaxies on Herschel colour-colour diagrams highlights the differences in dust grain properties and/or global environments of low-metallicity dwarf galaxies. The dust in DGS galaxies is generally warmer than in KINGFISH galaxies (TDGS ~ 32 K and TKINGFISH ~ 23 K). The emissivity index, β, is ~1.7 in the DGS, however metallicity does not make a strong effect on β. The proportion of dust mass relative to stellar mass is lower in low-metallicity galaxies: Mdust/Mstar ~ 0.02% for the DGS versus 0.1% for KINGFISH. However, per unit dust mass, dwarf galaxies emit about six times more in the FIR/submm than higher metallicity galaxies. Out of the 22 DGS galaxies detected at 500 μm, about 41% present an excess in the submm beyond the explanation of our dust SED model, and this excess can go up to 150% above the prediction from the model. The excess mainly appears in lower metallicity galaxies (12 + log(O/H) 8.3), and the strongest excesses are detected in the most metal-poor galaxies. However, we also stress the need for observations longwards of the Herschel wavelengths to detect any submm excess appearing beyond 500 μm. © ESO, 2013.

Foster J.B.,Boston University | Foster J.B.,Yale University | Mandel K.S.,Imperial College London | Pineda J.E.,Regional Center Node | And 3 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2013

We investigate the shape of the extinction law in two 1{ring operator} square fields of the Perseus molecular cloud complex. We combine deep red-optical (r, i and z band) observations obtained using Megacam on the MMT with UKIRT (United Kingdom Infrared Telescope) Infrared Deep Sky Survey near-infrared (J, H and K band) data to measure the colours of background stars. We develop a new hierarchical Bayesian statistical model, including measurement error, intrinsic colour variation, spectral type and dust reddening, to simultaneously infer parameters for individual stars and characteristics of the population. We implement an efficient Markov chain Monte Carlo algorithm utilizing generalized Gibbs sampling to compute coherent probabilistic inferences. We find a strong correlation between the extinction (AV) and the slope of the extinction law (parametrized by RV). Because the majority of the extinction towards our stars comes from the Perseus molecular cloud, we interpret this correlation as evidence of grain growth at moderate optical depths. The extinction law changes from the 'diffuse' value of RV ~ 3 to the 'dense cloud' value of RV ~ 5 as the column density rises from AV = 2 to 10 mag. This relationship is similar for the two regions in our study, despite their different physical conditions, suggesting that dust grain growth is a fairly universal process. © 2012 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

Avison A.,University of Manchester | Avison A.,Regional Center Node | Peretto N.,University of Cardiff | Fuller G.A.,University of Manchester | And 4 more authors.
Astronomy and Astrophysics | Year: 2015

Aims. Recent ALMA observations identified one of the most massive star-forming cores yet observed in the Milky Way: SDC335-MM1, within the infrared dark cloud SDC335.579-0.292. Along with an accompanying core MM2, SDC335 appears to be in the early stages of its star formation process. We aim to constrain the properties of the stars forming within these two massive millimetre sources. Methods. Observations of SDC335 at 6, 8, 23 and 25 GHz were made with the Australia Telescope Compact Array. We report the results of these continuum measurements, which combined with archival data, allow us to build and analyse the spectral energy distributions (SEDs) of the compact sources in SDC335. Results. Three hyper-compact Hii regions within SDC335 are identified, two of which are within the MM1 core. For each HCHii region, we fit a free-free emission curve to the data, providing the derivation of the sources' emission measure, ionising photon flux, and electron density. Using these physical properties we assign each HCHii region a zero-age main sequence (ZAMS) spectral type, finding two protostars with characteristics of spectral type B1.5 and one with a lower limit of B1-B1.5. Ancillary data from infrared to mm wavelength are used to construct free-free component subtracted SEDs for the mm-cores, which allows us to calculate the bolometric luminosities and revise the previous gas mass estimates. Conclusions. The measured luminosities for the two mm-cores are lower than expected from accreting sources displaying characteristics of the ZAMS spectral type assigned to them. The protostars are still actively accreting, suggesting that a mechanism is limiting the accretion luminosity. We present the case for two different mechanisms capable of causing lower than expected accretion luminosity. Finally, using the ZAMS mass values as lower limit constraints, a final stellar population for SDC335 was synthesised finding SDC335 is likely to be in the process of forming a stellar cluster comparable to the Trapezium cluster and NGC 6334 I(N). © ESO, 2015.

Peretto N.,University Paris Diderot | Peretto N.,University of Cardiff | Fuller G.A.,University of Manchester | Fuller G.A.,Regional Center Node | And 15 more authors.
Astronomy and Astrophysics | Year: 2013

The relative importance of primordial molecular cloud fragmentation versus large-scale accretion still remains to be assessed in the context of massive core/star formation. Studying the kinematics of the dense gas surrounding massive-star progenitors can tell us the extent to which large-scale flow of material impacts the growth in mass of star-forming cores. Here we present a comprehensive dataset of the 5500(±800) M· infrared dark cloud SDC335.579-0.272 (hereafter SDC335), which exhibits a network of cold, dense, parsec-long filaments. Atacama Large Millimeter Array (ALMA) Cycle 0 observations reveal two massive star-forming cores, MM1 and MM2, sitting at the centre of SDC335 where the filaments intersect. With a gas mass of 545( -385 +770) M· contained within a source diameter of 0.05 pc, MM1 is one of the most massive, compact protostellar cores ever observed in the Galaxy. As a whole, SDC335 could potentially form an OB cluster similar to the Trapezium cluster in Orion. ALMA and Mopra single-dish observations of the SDC335 dense gas furthermore reveal that the kinematics of this hub-filament system are consistent with a global collapse of the cloud. These molecular-line data point towards an infall velocity Vinf = 0.7(± 0.2) km s-1, and a total mass infall rate M inf ‰ 2.5(±1.0) × 10-3 M · yr-1 towards the central pc-size region of SDC335. This infall rate brings 750(±300) M· of gas to the centre of the cloud per free-fall time (tff = 3 × 10 5 yr). This is enough to double the mass already present in the central pc-size region in 3.5-1.0 +2.2 × t ff. These values suggest that the global collapse of SDC335 over the past million year resulted in the formation of an early O-type star progenitor at the centre of the cloud's gravitational potential well. © 2013 ESO.

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