arXiv subjects

M. A. Thompson

Publications and source records attributed to M. A. Thompson.

At least 109 records · Page 6Linked to original sources

arXiv2010

Herschel-ATLAS: The far-infrared--radio correlation at z<0.5

We use data from the Herschel-ATLAS to investigate the evolution of the far-infrared--radio correlation over the redshift range 0 5sigma sources in the Herschel-ATLAS Science Demonstration Field and cross-matching these data with radio data from the Faint Images of the Radio Sky at Twenty-Centimetres (FIRST) survey and the NRAO VLA Northern Sky Survey (NVSS), we obtain 104 radio counterparts to the Herschel sources. With these data we find no evidence for evolution in the far-infrared--radio correlation over the redshift range 0<z<0.5, where the median value for the ratio between far-infrared and radio luminosity, $q_{\rm IR}$, over this range is $q_{\rm IR} = 2.40\pm 0.12$ (and a mean of $q_{\rm IR}=2.52 \pm 0.03$ accounting for the lower limits), consistent with both the local value determined from {\em IRAS} and values derived from surveys targeting the high-redshift Universe. By comparing the radio fluxes of our sample measured from both FIRST and NVSS we show that previous results suggesting an increase in the value of $q_{\rm IR}$ from high to low redshift may be the result of resolving out extended emission of the low-redshift sources with relatively high-resolution interferometric data, although AGN contamination could still play a significant role. We also find tentative evidence that the longer wavelength, cooler dust is heated by an evolved stellar population which does not trace the star-formation rate as closely as the shorter wavelength $\ltsim 250~μ$m emission or the radio emission, supporting suggestions based on detailed models of individual galaxies.

astro-ph.CO↗

arXiv2010

Variations of the spectral index of dust emissivity from Hi-GAL observations of the Galactic plane

Variations in the dust emissivity are critical for gas mass determinations derived from far-infrared observations, but also for separating dust foreground emission from the Cosmic Microwave Background (CMB). Hi-GAL observations allow us for the first time to study the dust emissivity variations in the inner regions of the Galactic plane at resolution below 1 degree. We present maps of the emissivity spectral index derived from the combined Herschel PACS 160 μm, SPIRE 250 μm, 350 μm, and 500 μm data, and the IRIS 100 μm data, and we analyze the spatial variations of the spectral index as a function of dust temperature and wavelength in the two Science Demonstration Phase Hi-GAL fields, centered at l=30° and l=59°. Applying two different methods, we determine both dust temperature and emissivity spectral index between 100 and 500 μm, at an angular resolution of 4'. Combining both fields, the results show variations of the emissivity spectral index in the range 1.8-2.6 for temperatures between 14 and 23 K. The median values of the spectral index are similar in both fields, i.e. 2.3 in the range 100-500 μm, while the median dust temperatures are equal to 19.1 K and 16.0 K in the l=30° and l=59° field, respectively. Statistically, we do not see any significant deviations in the spectra from a power law emissivity between 100 and 500 μm. We confirm the existence of an inverse correlation between the emissivity spectral index and dust temperature, found in previous analyses.

astro-ph.GA↗

arXiv2010

The 6-GHz methanol multibeam maser catalogue II: Galactic longitudes 6 to 20

We present the second portion of an unbiased survey of the Galactic plane for 6668-MHz methanol masers. This section of the survey spans the longitude range 6 degrees to 20 degrees. We report the detection of 119 maser sources, of which 42 are new discoveries. The masers are tightly constrained to the Galactic plane, with only four outside a latitude range of +/- 1 degree. This longitude region includes the brightest known 6668-MHz methanol maser, 9.621+0.196, as well as the two brightest newly discovered sources in the southern survey as a whole. We list all the sources associated with the 3-kpc arms within +/- 15 degrees longitude and consider further candidates beyond 15 degrees longitude. We identify three new sources associated with the Galactic bar and comment on the density of masers in relation to the bar orientation.

astro-ph.GA↗

arXiv2010

The G305 star forming complex: Wide-Area molecular mapping of NH3 and H2O masers

We present wide area radio (12 mm) Mopra Telescope observations of the complex and rich massive star forming region G305. Our goals are to determine the reservoir for star formation within G305 using NH3 to trace the dense molecular content, and thus, the gas available to form stars; estimate physical parameters of detected NH3 clumps (temperature, column density, mass etc); locate current areas of active star formation via the presence of H2O and methanol masers and the distribution of YSOs and ultra compact HII regions associated with this region. This paper details the NH3 (J,K), (1,1), (2,2) and (3,3) inversion transition and 22 GHz H2O maser observations. We observed a \sim 1.5\circ x 1\circ region with \sim 2' angular resolution and a sensitivity of \sim 60 mK per 0.4 km s^-1 channel. We identify 15 NH3 (1,1), 12 NH3 (2,2) and 6 NH3 (3,3) clumps surrounding the central HII region. The sizes of the clumps vary between < 2.6 and 10.1 pc, the average kinetic temperature of the gas is 25 K. We calculate clump masses of > 10^4 M\odot and find the total molecular mass of the complex to be \sim 6x10^5 M\odot. We note the positions of 56 star formation tracers, and discover a high degree of correlation with detected NH3 clumps. We have detected 16 H2O masers, find they correlate with the detected ammonia clumps and in general are found closer to the NH3 clump cores than star formation tracers of later evolutionary stages.

astro-ph.GA↗

arXiv2010

Ammonia observations of bright-rimmed clouds: establishing a sample of triggered protostars

We observed 42 molecular condensations within previously identified bright-rimmed clouds in the ammonia rotational inversion lines NH3 (1,1), (2,2), (3,3) and (4,4) using the Green Bank Telescope in Green Bank, West Virginia. Using the relative peaks of the ammonia lines and their hyperfine satellites we have determined important parameters of these clouds, including rotational temperatures and column densities. These observations confirm the presence of dense gas towards IRAS point sources detected at submillimetre wavelengths. Derived physical properties allow us to refine the sample of bright-rimmed clouds into those likely to be sites of star formation, triggered via the process of radiatively-driven implosion. An investigation of the physical properties of our sources show that triggered sources are host to greater turbulent velocity dispersions, likely indicative of shock motions within the cloud material. These may be attributed to the passage of triggered shocks or simply the association of outflow activity with the sources. In all, we have refined the Sugitani et al. (1991) catalogue to 15 clouds which are clearly star-forming and influenced by external photoionisation-induced shocks. These sources may be said, with high confidence, to represent the best examples of triggering within bright-rimmed clouds.

astro-ph.GA↗

arXiv2010

The Herschel-ATLAS: Evolution of the 250um luminosity function out to z=0.5

We have determined the luminosity function of 250um-selected galaxies detected in the ~14 sq.deg science demonstration region of the Herschel-ATLAS project out to a redshift of z=0.5. Our findings very clearly show that the luminosity function evolves steadily out to this redshift. By selecting a sub-group of sources within a fixed luminosity interval where incompleteness effects are minimal, we have measured a smooth increase in the comoving 250um luminosity density out to z=0.2 where it is 3.6+1.4-0.9 times higher than the local value.

astro-ph.CO↗

arXiv2010

A search for debris disks in the Herschel ATLAS

Aims: We aim to demonstrate that the Herschel ATLAS (H-ATLAS) is suitable for a blind and unbiased survey for debris disks by identifying candidate debris disks associated with main sequence stars in the initial science demonstration field of the survey. We show that H-ATLAS reveals a population of far-infrared/sub-mm sources that are associated with stars or star-like objects on the SDSS main-sequence locus. We validate our approach by comparing the properties of the most likely candidate disks to those of the known population. Methods: We use a photometric selection technique to identify main sequence stars in the SDSS DR7 catalogue and a Bayesian Likelihood Ratio method to identify H-ATLAS catalogue sources associated with these main sequence stars. Following this photometric selection we apply distance cuts to identify the most likely candidate debris disks and rule out the presence of contaminating galaxies using UKIDSS LAS K-band images. Results: We identify 78 H-ATLAS sources associated with SDSS point sources on the main-sequence locus, of which two are the most likely debris disk candidates: H-ATLAS J090315.8 and H-ATLAS J090240.2. We show that they are plausible candidates by comparing their properties to the known population of debris disks. Our initial results indicate that bright debris disks are rare, with only 2 candidates identified in a search sample of 851 stars. We also show that H-ATLAS can derive useful upper limits for debris disks associated with Hipparcos stars in the field and outline the future prospects for our debris disk search programme.

astro-ph.EP↗

arXiv2010

Herschel-ATLAS: Dust temperature and redshift distribution of SPIRE and PACS detected sources using submillimetre colours

We present colour-colour diagrams of detected sources in the Herschel-ATLAS Science Demonstration Field from 100 to 500 microns using both PACS and SPIRE. We fit isothermal modified black bodies to the spectral energy distribution (SED) to extract the dust temperature of sources with counterparts in Galaxy And Mass Assembly (GAMA) or SDSS surveys with either a spectroscopic or a photometric redshift. For a subsample of 330 sources detected in at least three FIR bands with a significance greater than 3 $σ$, we find an average dust temperature of $(28 \pm 8)$K. For sources with no known redshift, we populate the colour-colour diagram with a large number of SEDs generated with a broad range of dust temperatures and emissivity parameters, and compare to colours of observed sources to establish the redshift distribution of this sample. For another subsample of 1686 sources with fluxes above 35 mJy at 350 microns and detected at 250 and 500 microns with a significance greater than 3$σ$, we find an average redshift of $2.2 \pm 0.6$.

astro-ph.CO↗

arXiv2010

A Herschel study of YSO evolutionary stages and formation timelines in two fields of the Hi-GAL survey

We present a first study of the star-forming compact dust condensations revealed by Herschel in the two 2 \times 2 \degr Galactic Plane fields centered at [l;b] = [30\degr; 0 \degr] and [l;b] = [59\degr; 0 \degr], respectively, and observed during the Science Demonstration Phase for the Herschel infrared Galactic Plane survey (Hi-GAL) Key-Project. Compact source catalogs extracted for the two fields in the five Hi-GAL bands (70, 160, 250, 350 and 500 $μ$m) were merged based on simple criteria of positional association and spectral energy distribution (SED) consistency into a final catalog which contains only coherent SEDs with counterparts in at least three adjacent Herschel bands. These final source lists contain 528 entries for the l = 30\degr field, and 444 entries for the l = 59\degr field. The SED coverage has been augmented with ancillary data at 24 $μ$m and 1.1 mm. SED modeling for the subset of 318 and 101 sources (in the two fields, respectively) for which the distance is known was carried out using both a structured star/disk/envelope radiative transfer model and a simple isothermal grey-body. Global parameters like mass, luminosity, temperature and dust properties have been estimated. The Lbol/Menv ratio spans four orders of magnitudes from values compatible with the pre-protostellar phase to embedded massive zero-age main sequence stars. Sources in the l = 59\degr field have on average lower L/M, possibly outlining an overall earlier evolutionary stage with respect to the sources in the l = 30\degr field. Many of these cores are actively forming high-mass stars, although the estimated core surface densities appear to be an order of magnitude below the 1 g cm$^{-2}$ critical threshold for high-mass star formation.

astro-ph.GA↗

arXiv2010

A multiwavelength study of star formation in the vicinity of Galactic HII region Sh2-100

We present multiwavelength investigation of morphology, physical-environment, stellar contents and star formation activity in the vicinity of star-forming region Sh 2-100. It is found that the Sh 2-100 region contains seven HII regions of ultracompact and compact nature. The present estimation of distance for three HII regions, along with the kinematic distance for others, suggests that all of them belong to the same molecular cloud complex. Using NIR photometry, we identified the most probable ionizing sources of six HII regions. Their approximate photometric spectral type estimates suggest that they are massive early-B to mid-O ZAMS stars and agree well with radio continuum observations at 1280 MHz. The morphology of the complex shows a non-uniform distribution of warm and hot dust, well mixed with the ionized gas, which correlates well with the variation of average visual extinction (~ 4.2 - 97 mag) across the region. We estimated the physical parameters of ionized gas with the help of radio continuum observations. We detected an optically visible compact nebula located to the south of the 850 micron emission associated with one of the HII regions and the diagnostic of the optical emission line ratios gives electron density and electron temperature of ~ 0.67 x 10^3 cm^-3 and ~ 10^4 K, respectively. The physical parameters suggest that all the HII regions are in different stages of evolution, which correlate well with the probable ages in the range ~ 0.01 - 2 Myr of the ionizing sources. The positions of IR excess stars, ultracompact and compact HII regions at the periphery of an HI shell, possibly created by a WR star, indicate that star formation in Sh 2-100 region might have been induced by an expanding HI shell.

astro-ph.GA↗

arXiv2010

The 6-GHz methanol multibeam maser catalogue I: Galactic Centre region, longitudes 345 to 6

We have conducted a Galactic plane survey of methanol masers at 6668 MHz using a 7-beam receiver on the Parkes telescope. Here we present results from the first part, which provides sensitive unbiased coverage of a large region around the Galactic Centre. Details are given for 183 methanol maser sites in the longitude range 345$^{\circ}$ through the Galactic Centre to 6$^{\circ}$. Within 6$^{\circ}$ of the Centre, we found 88 maser sites, of which more than half (48) are new discoveries. The masers are confined to a narrow Galactic latitude range, indicative of many sources at the Galactic Centre distance and beyond, and confined to a thin disk population; there is no high latitude population that might be ascribed to the Galactic Bulge. Within 2$^{\circ}$ of the Galactic Centre the maser velocities all lie between -60 and +77 \kms, a range much smaller than the 540 \kms range observed in CO. Elsewhere, the maser with highest positive velocity (+107 \kms) occurs, surprisingly, near longitude 355$^{\circ}$ and is probably attributable to the Galactic bar. The maser with the most negative velocity (-127 \kms) is near longitude 346$^{\circ}$, within the longitude-velocity locus of the near side of the `3-kpc arm'. It has the most extreme velocity of a clear population of masers associated with the near and far sides of the 3-kpc arm. Closer to the Galactic Centre the maser space density is generally low, except within 0.25 kpc of the Centre itself, the `Galactic Centre Zone', where it is 50 times higher, which is hinted at by the longitude distribution, and confirmed by the unusual velocities.

astro-ph.GA↗

arXiv2009

VISTA Variables in the Via Lactea (VVV): The public ESO near-IR variability survey of the Milky Way

We describe the public ESO near-IR variability survey (VVV) scanning the Milky Way bulge and an adjacent section of the mid-plane where star formation activity is high. The survey will take 1929 hours of observations with the 4-metre VISTA telescope during five years (2010-2014), covering ~10^9 point sources across an area of 520 deg^2, including 33 known globular clusters and ~350 open clusters. The final product will be a deep near-IR atlas in five passbands (0.9-2.5 microns) and a catalogue of more than 10^6 variable point sources. Unlike single-epoch surveys that, in most cases, only produce 2-D maps, the VVV variable star survey will enable the construction of a 3-D map of the surveyed region using well-understood distance indicators such as RR Lyrae stars, and Cepheids. It will yield important information on the ages of the populations. The observations will be combined with data from MACHO, OGLE, EROS, VST, Spitzer, HST, Chandra, INTEGRAL, WISE, Fermi LAT, XMM-Newton, GAIA and ALMA for a complete understanding of the variable sources in the inner Milky Way. This public survey will provide data available to the whole community and therefore will enable further studies of the history of the Milky Way, its globular cluster evolution, and the population census of the Galactic Bulge and center, as well as the investigations of the star forming regions in the disk. The combined variable star catalogues will have important implications for theoretical investigations of pulsation properties of stars.

astro-ph.GA↗

arXiv2009

The Herschel ATLAS

The Herschel ATLAS is the largest open-time key project that will be carried out on the Herschel Space Observatory. It will survey 510 square degrees of the extragalactic sky, four times larger than all the other Herschel surveys combined, in five far-infrared and submillimetre bands. We describe the survey, the complementary multi-wavelength datasets that will be combined with the Herschel data, and the six major science programmes we are undertaking. Using new models based on a previous submillimetre survey of galaxies, we present predictions of the properties of the ATLAS sources in other wavebands.

astro-ph.CO↗

arXiv2009

The P Cygni supergiant [OMN2000] LS1 - implications for the star formation history of W51

We investigated the nature of the massive star [OMN2000] LS1 and used these results to constrain the history of star formation within the host complex W51. A combination of near-IR spectroscopy and non-LTE model atmosphere analysis was used to derive the physical properties of [OMN2000] LS1 and a combination of theoretical evolutionary calculations and Monte Carlo simulations to apply limits on the star formation history of W51. The spectrum of [OMN2000] LS1 is consistent with that of a P Cygni supergiant, but with a temperature of 13.2-13.7kK and log(L/L_sun)<5.75, it is significantly cooler, less luminous, and massive than proposed by previous authors. The presence of such a star within W51 shows that star formation has been underway for at least 3Myr, while the formation of massive O stars is still on going. We find no evidence of internally triggered, sequential star formation within W51, and favour the suggestion that star formation has proceeded at multiple indepedent sites within the GMC. [ABRIDGED]

astro-ph.SR↗

arXiv2009

Infrared Dark Cloud Cores in the SCUBA Legacy Catalogue

We present an investigation of candidate Infrared Dark Cloud cores as identified by Simon et al. (2006) located within the SCUBA Legacy Catalogue. After applying a uniform noise cut to the Catalogue data we identify 154 Infrared Dark Cloud cores that were detected at 850um and 51 cores that were not. We derive column densities for each core from their 8um extinction and find that the IRDCs detected at 850um have higher column densities (a mean of 1.7x10^22 cm-2) compared to those cores not detected at 850um (a mean of 1.0x10^22 cm-2). Combined with sensitivity estimates, we suggest that the cores not detected at 850um are low mass, low column density and low temperature cores that are below the sensitivity limit of SCUBA at 850um. For a subsample of the cores detected at 850um those contained within the MIPSGAL area) we find that two thirds are associated with 24um sources. Cores not associated with 24um emission are either ``starless'' IRDC cores that perhaps have yet to form stars, or contain low mass YSOs below the MIPSGAL detection limit. We see that those ``starless'' IRDC cores and the IRDC cores associated with 24um emission are drawn from the same column density population and are of similar mass. If we then assume the cores without 24um embedded sources are at an earlier evolutionary stage to cores with embedded objects we derive a statistical lifetime for the quiescent phase of a few 10^3-10^4 years. Finally, we make conservative predictions for the number of observed IRDCs that will be observed by the Apex Telescope Galactic Plane Survey (ATLASGAL), the Herschel Infrared Galactic Plane Survey (Hi-GAL), the JCMT Galactic Plane Survey (JPS) and the SCUBA-2 ``All Sky'' Survey (SASSy).

astro-ph.GA↗

arXiv2009

Supersonic turbulence in the cold massive core JCMT 18354-0649S

An example of a cold massive core, JCMT 18354-0649S, a possible high mass analogue to a low mass star forming core is studied. Line and continuum observations from JCMT, Mopra Telescope and Spitzer are presented and modelled in detail using a 3D molecular line radiative transfer code. In almost every way JCMT 18354-0649S is a scaled-up version of a typical low mass core with similar temperatures, chemical abundances and densities. The difference is that both the infall velocity and the turbulent width of the line profiles are an order of magnitude larger. While the higher infall velocity is expected due to the large mass of JCMT 18354-0649S, we suggest that the dissipation of this highly supersonic turbulence may lead to the creation of dense clumps of gas that surround the high mass core.

astro-ph.GA↗

arXiv2009

Accounting for the Foreground Contribution to the Dust Emission towards Kepler's Supernova Remnant

Whether or not supernovae contribute significantly to the overall dust budget is a controversial subject. Submillimetre (submm) observations, sensitive to cold dust, have shown an excess at 450 and 850 microns in young remnants Cassiopeia A (Cas A) and Kepler. Some of the submm emission from Cas A has been shown to be contaminated by unrelated material along the line of sight. In this paper we explore the emission from material towards Kepler using submm continuum imaging and spectroscopic observations of atomic and molecular gas, via HI, 12CO (J=2-1) and 13CO (J=2-1). We detect weak CO emission (peak TA* = 0.2-1K, 1-2km/s fwhm) from diffuse, optically thin gas at the locations of some of the submm clumps. The contribution to the submm emission from foreground molecular and atomic clouds is negligible. The revised dust mass for Kepler's remnant is 0.1--1.2 solar masses, about half of the quoted values in the original study by Morgan et al. (2003), but still sufficient to explain the origin of dust at high redshifts.

astro-ph.GA↗

arXiv2009

Observational study of sites of triggered star formation: CO and mid-infrared observations

(Abridged) Bright-rimmed clouds (BRCs) are isolated molecular clouds located on the edges of evolved HII regions where star formation is thought may have been triggered. In this paper we investigate the current level of star formation within a sample of BRCs and evaluate to what extent star formation may have been induced. We present the results of a programme of position-switched CO observations towards 45 southern BRCs. The 12CO, 13CO and C18O (J=1-0) were simultaneously observed using the 22m Mopra telescope. We complement these observations with archival mid-IR submm and radio data. Analysis of the CO, mid-IR and radio data result in the clouds being divided into three distinct groups. We refer to these groups as spontaneous, triggered, and zapped clouds, respectively. Comparing the physical parameters of spontaneous and triggered samples we find striking differences in luminosity, surface temperature and column density with all three quantities significantly enhanced for the clouds considered to have been triggered. Furthermore, we find strong evidence for star formation within the triggered sample by way of methanol and H_2O masers, embedded mid-IR point sources and CO wings, however, we find evidence of ongoing star formation within only two of the spontaneous sample. We have used CO, mid-IR and radio data to identify 24 of the 45 southern BRCs that are undergoing a strong interaction with their HII region. We can therefore exclude ~50% from future studies. 14 of the 24 interacting BRCs are found to be associated with embedded mid-IR point sources and we find strong evidence of that these clouds are forming stars. The absence of mid-infrared sources towards the remaining ten clouds leads us to conclude that these represent an earlier evolutionary stage of star formation.

astro-ph.GA↗