arXiv subjects

M. Baes

Publications and source records attributed to M. Baes.

At least 181 records · Page 10Linked to original sources

arXiv2013

Towards understanding the relation between the gas and the attenuation in galaxies at kpc scales

[abridged] Aims. The aim of the present paper is to provide new and more detailed relations at the kpc scale between the gas surface density and the face-on optical depth directly calibrated on galaxies, in order to compute the attenuation not only for semi-analytic models but also observationally as new and upcoming radio observatories are able to trace gas ever farther in the Universe. Methods. We have selected a sample of 4 nearby resolved galaxies and a sample of 27 unresolved galaxies from the Herschel Reference Survey and the Very Nearby Galaxies Survey, for which we have a large set of multi-wavelength data from the FUV to the FIR including metallicity gradients for resolved galaxies, along with radio HI and CO observations. For each pixel in resolved galaxies and for each galaxy in the unresolved sample, we compute the face-on optical depth from the attenuation determined with the CIGALE SED fitting code and an assumed geometry. We determine the gas surface density from HI and CO observations with a metallicity-dependent XCO factor. Results. We provide new, simple to use, relations to determine the face-on optical depth from the gas surface density, taking the metallicity into account, which proves to be crucial for a proper estimate. The method used to determine the gas surface density or the face-on optical depth has little impact on the relations except for galaxies that have an inclination over 50d. Finally, we provide detailed instructions on how to compute the attenuation practically from the gas surface density taking into account possible information on the metallicity. Conclusions. Examination of the influence of these new relations on simulated FUV and IR luminosity functions shows a clear impact compared to older oft-used relations, which in turn could affect the conclusions drawn from studies based on large scale cosmological simulations.

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arXiv2013

GAMA/H-ATLAS: Linking the Properties of Sub-mm Detected and Undetected Early-Type Galaxies: I. z$\le$0.06 Sample

We present two large, nearby (0.013$\le$z$\le$0.06) samples of Early-Type Galaxies (ETGs): a visually classified sample of 220 ETGs, created using source-matched data from the Galaxy and Mass Assembly (GAMA) database with FIR/sub-mm detections from $Herschel$-ATLAS; and a visually classified sample of 551 ETGs which are undetected with $Herschel$-ATLAS. Active galactic nuclei (AGN) are removed from our samples using optical emission line diagnostics. These samples are scrutinised to determine characteristics of sub-mm detected versus undetected ETGs. We find similarities in the stellar mass distributions of the two ETG samples but testing other properties uncovers significant differences. The sub-mm detected sample is shown to have lower concentration and Sérsic indices than those in the undetected sample - a result which may be linked to the presence of dust in the former. Optical and UV-optical colours are also shown to be much bluer, indicating that the dust is linked with recent star formation. The intrinsic effective radii are on average 1.5 times larger for the sub-mm detected ETGs. Surface densities and groups data from the GAMA database are examined for the two samples, leading to the conclusion that dusty ETGs inhabit sparser environments than non-dusty ETGs in the nearby universe, although environments of the brightest ETGs are shown to differ the least. Modified Planck functions are fit to the H-ATLAS detected PACS and SPIRE fluxes for ETGs with sub-mm flux densities of at least 3$σ$ in the 350$μ$m SPIRE band, giving a resultant mean cold dust temperature of T$_{d}$=22.1K, with a range of 9-30K. The corresponding mean dust mass is 1.8$\times10^{7}$M$_{\odot}$, with a range of (0.08-35.0)$\times10^{7}$M$_{\odot}$. The dust masses calculated from these fits, normalised by stellar mass, are shown to increase with decreasing stellar mass and bluer colours.

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arXiv2013

GAMA/H-ATLAS: The Dust Opacity - Stellar Mass Surface Density Relation for Spiral Galaxies

We report the discovery of a well-defined correlation between B-band face-on central optical depth due to dust, τ^f_B, and the stellar mass surface density, μ_{*}, of nearby (z < 0.13) spiral galaxies: log(τ^f_B) = 1.12(+-0.11)log(μ_{*}/M_sol kpc^2)-8.6(+-0.8). This relation was derived from a sample of spiral galaxies taken from the Galaxy and Mass Assembly (GAMA) survey and detected in the FIR/submm in the Herschel-ATLAS survey. Using a quantitative analysis of the NUV attenuation-inclination relation for complete samples of GAMA spirals categorized according to μ_{*} we demonstrate that this correlation can be used to statistically correct for dust attenuation purely on the basis of optical photometry and S'ersic-profile morphological fits. Considered together with previously established empirical relationships between stellar mass, metallicity and gas mass, the near linearity and high constant of proportionality of the τ^f_B-μ_{*} relation disfavors a stellar origin for the bulk of refractory grains in spiral galaxies, instead being consistent with the existence of a ubiquitous and very rapid mechanism for the growth of dust in the ISM. We use the τ^f_B-μ_{*} relation in conjunction with the radiation transfer model for spiral galaxies of Popescu & Tuffs (2011) to derive intrinsic scaling relations between specific star formation rate (sSFR), stellar mass, and μ_{*}, in which the attenuation of the UV light used to measure the SFR is corrected on an object-to-object basis. A marked reduction in scatter in these relations is achieved which is demonstrably due to correction of both the inclination-dependent and face-on components of attenuation. Our results are consistent with a picture of spiral galaxies in which most of the submm emission originates from grains residing in translucent structures, exposed to UV in the diffuse interstellar radiation field.

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arXiv2013

H2O emission in high-z ultra-luminous infrared galaxies

Using IRAM PdBI we report the detection of H2O in six new lensed ultra-luminous starburst galaxies at high redshift, discovered in the Herschel H-ATLAS survey. The sources are detected either in the 2_{02}-1_{11} or 2_{11}-2_{02} H_2O emission lines with integrated line fluxes ranging from 1.8 to 14 Jy.km/s. The corresponding apparent luminosities are mu x L_H2O ~ 3-12 x 10^8 Lo, where mu is the lensing magnification factor (3 < mu < 12). These results confirm that H2O lines are among the strongest molecular lines in such galaxies, with intensities almost comparable to those of the high-J CO lines, and same profiles and line widths (200-900 km/s) as the latter. With the current sensitivity of PdBI, H2O can therefore easily be detected in high-z lensed galaxies (with F(500um) > 100 mJy) discovered in the Herschel surveys. Correcting the luminosities for lensing amplification, L_H2O is found to have a strong dependence on the IR luminosity, varying as ~L_IR^{1.2}. This relation which needs to be confirmed with better statistics, may indicate a role of radiative (IR) excitation of the H2O lines, and implies that high-z galaxies with L_IR >~ 10^13 Lo tend to be very strong emitters in H2O, that have no equivalent in the local universe.

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arXiv2013

Mid-J CO Emission in Nearby Seyfert Galaxies

We study for the first time the complete sub-millimeter spectra (450 GHz to 1550 GHz) of a sample of nearby active galaxies observed with the SPIRE Fourier Transform Spectrometer (SPIRE/FTS) onboard Herschel. The CO ladder (from Jup = 4 to 12) is the most prominent spectral feature in this range. These CO lines probe warm molecular gas that can be heated by ultraviolet photons, shocks, or X-rays originated in the active galactic nucleus or in young star-forming regions. In these proceedings we investigate the physical origin of the CO emission using the averaged CO spectral line energy distribution (SLED) of six Seyfert galaxies. We use a radiative transfer model assuming an isothermal homogeneous medium to estimate the molecular gas conditions. We also compare this CO SLED with the predictions of photon and X-ray dominated region (PDR and XDR) models.

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arXiv2012

H-ATLAS: The cosmic abundance of dust from the far-infrared background power spectrum

We present a measurement of the angular power spectrum of the cosmic far-infrared background (CFIRB) anisotropies in one of the extragalactic fields of the Herschel Astrophysical Terahertz Large Area Survey (H-ATLAS) at 250, 350 and 500 μm bands. Consistent with recent measurements of the CFIRB power spectrum in Herschel-SPIRE maps, we confirm the existence of a clear one-halo term of galaxy clustering on arcminute angular scales with large-scale two-halo term of clustering at 30 arcminutes to angular scales of a few degrees. The power spectrum at the largest angular scales, especially at 250 μm, is contaminated by the Galactic cirrus. The angular power spectrum is modeled using a conditional luminosity function approach to describe the spatial distribution of unresolved galaxies that make up the bulk of the CFIRB. Integrating over the dusty galaxy population responsible for the background anisotropies, we find that the cosmic abundance of dust, relative to the critical density, to be between Ω_dust=10^{-6} and 8 x 10^{-6} in the redshift range z ~ 0-3. This dust abundance is consistent with estimates of the dust content in the Universe using quasar reddening and magnification measurements in the SDSS.

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arXiv2012

Herschel-ATLAS/GAMA: a difference between star-formation rates in strong-line and weak-line radio galaxies

We have constructed a sample of radio-loud objects with optical spectroscopy from the Galaxy and Mass Assembly (GAMA) project over the Herschel-ATLAS Phase 1 fields. Classifying the radio sources in terms of their optical spectra, we find that strong-emission-line sources (`high-excitation radio galaxies') have, on average, a factor ~4 higher 250-micron Herschel luminosity than weak-line (`low-excitation') radio galaxies and are also more luminous than magnitude-matched radio-quiet galaxies at the same redshift. Using all five H-ATLAS bands, we show that this difference in luminosity between the emission-line classes arises mostly from a difference in the average dust temperature; strong-emission-line sources tend to have comparable dust masses to, but higher dust temperatures than, radio galaxies with weak emission lines. We interpret this as showing that radio galaxies with strong nuclear emission lines are much more likely to be associated with star formation in their host galaxy, although there is certainly not a one-to-one relationship between star formation and strong-line AGN activity. The strong-line sources are estimated to have star-formation rates at least a factor 3-4 higher than those in the weak-line objects. Our conclusion is consistent with earlier work, generally carried out using much smaller samples, and reinforces the general picture of high-excitation radio galaxies as being located in lower-mass, less evolved host galaxies than their low-excitation counterparts.

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arXiv2012

The Herschel Fornax Cluster Survey I: The Bright Galaxy Sample

We present Herschel observations of the Fornax cluster at 100, 160, 250, 350 and 500u with a spatial resolution of 7 - 36 arc sec (10" = 1 kpc at d_Fornax=17.9 Mpc). We define a sample of 11 bright galaxies, selected at 500u, directly comparable with our past work on Virgo. We find good agreement with previous observations made by IRAS and Planck. The FIR luminosity density is higher (factor of three) in Fornax compared to Virgo. The 100u (42.5-122.5u) luminosity is two orders of magnitude larger in Fornax than in the local field as measured by IRAS. Using stellar (L_{0.4-2.5}) and FIR (L_{100-500}) luminosities we estimate a mean optical depth of tau=0.4+/-0.1 - the same value as Virgo. For 10 of the 11 galaxies (NGC1399 excepted) we fit a modified blackbody curve (beta=2.0) to the SEDs to derive dust masses and temperatures of 10^{6.54-8.35} M_0 and T=14.6-24.2K respectively, comparable to Virgo. The derived stars-to-gas(atomic) and gas(atomic)-to-dust ratios vary from 1.1-67.6 and 9.8-436.5 respectively, again consistent with Virgo. Fornax is a mass overdensity in stars and dust of about 120 compared to the local field (30 for Virgo). Fornax and Virgo are both a factor of 6 lower over densities in gas(atomic) than in stars and dust indicating loss of gas, but not dust and stars, in the cluster environment. As the brightest FIR source in either Fornax and Virgo, NGC1365 is detected by Planck. The Planck data fit the PACS/SPIRE SED out to 1382u with no evidence of other sources of emission ('spinning dust', free-free, synchrotron). At the opposite end of the scale NGC1399 is detected only at 500$μ$m with the emission probably arising from the nuclear radio source rather than inter-stellar dust.

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arXiv2012

The Herschel Virgo Cluster Survey XII: FIR properties of optically-selected Virgo Cluster galaxies

The Herschel Virgo Cluster Survey (HeViCS) is the deepest, confusion-limited survey of the Virgo Cluster at far-infrared (FIR) wavelengths. The entire survey at full depth covers $\sim$55 sq. deg. in 5 bands (100-500 \micron), encompassing the areas around the central dominant elliptical galaxies (M87, M86 & M49) and extends as far as the NW cloud, the W cloud and the Southern extension. The survey extends beyond this region with lower sensitivity so that the total area covered is 84 sq. deg. In this paper we describe the data, the data acquisition techniques and present the detection rates of the optically selected Virgo Cluster Catalogue (VCC). We detect 254 (34%) of 750 VCC galaxies found within the survey boundary in at least one band and 171 galaxies are detected in all five bands. For the remainder of the galaxies we have measured strict upper limits for their FIR emission. The population of detected galaxies contains early- as well as late-types although the latter dominate the detection statistics. We have modelled 168 galaxies, showing no evidence of a strong synchrotron component in their FIR spectra, using a single-temperature modified blackbody spectrum with a fixed emissivity index ($β= 2$). A study of the $χ^2$ distribution indicates that this model is not appropriate in all cases, and this is supported by the FIR colours which indicate a spread in $β$=1--2. Statistical comparison of the dust mass and temperature distributions from 140 galaxies with $χ^2_{dof=3} < 7.8$ (95% confidence level) shows that late-types have typically colder, more massive dust reservoirs; the early-type dust masses have a mean of ${\rm log}( / M_{\sun}) = 6.3 \pm 0.3 $, while for late-types ${\rm log}( / M_{\sun}) =7.1 \pm 0.1$... (abridged)

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arXiv2012

Herschel-ATLAS/GAMA: spatial clustering of low-redshift sub-mm galaxies

We have measured the clustering properties of low-redshift (z < 0.3) sub-mm galaxies detected at 250 micron in the Herschel-ATLAS Science Demonstration Phase (SDP) field. We selected a sample for which we have high-quality spectroscopic redshifts, obtained from reliably matching the 250-micron sources to a complete (for r < 19.4) sample of galaxies from the GAMA database. Both the angular and spatial clustering strength are measured for all z < 0.3 sources as well as for five redshift slices with thickness delta z=0.05 in the range 0.05 < z < 0.3. Our measured spatial clustering length r_0 is comparable to that of optically-selected, moderately star-forming (blue) galaxies: we find values around 5 Mpc. One of the redshift bins contains an interesting structure, at z = 0.164.

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arXiv2012

The nature of the interstellar medium of the starburst low-metallicity galaxy Haro11: a multi-phase model of the infrared emission

(abridged) Our goal is to describe the multi-phase ISM of the IR bright low-metallicity galaxy Haro 11, dissecting the photoionised and photodissociated gas components. We present observations of the mid- and far-IR fine-structure cooling lines obtained with the Spitzer/IRS and Herschel/PACS spectrometers. We use the spectral synthesis code Cloudy to methodically model the ionised and neutral gas from which these lines originate. We find that the mid- and far-IR lines account for ~1% of the total IR luminosity L_TIR. Haro 11 is undergoing a phase of intense star formation, as traced by the brightest line [OIII] 88um, with L_[OIII]/L_TIR ~0.3%, and high ratios of [NeIII]/[NeII] and [SIV]/[SIII]. Due to their different origins, the observed lines require a multi-phase modeling comprising: a compact HII region, dense fragmented photodissociation regions (PDRs), a diffuse extended low-ionisation/neutral gas which has a volume filling factor of at least 90%, and porous warm dust in proximity to the stellar source. For a more realistic picture of the ISM of Haro 11 we would need to model the clumpy source and gas structures. We combine these 4 model components to explain the emission of 17 spectral lines, investigate the global energy balance of the galaxy through its spectral energy distribution, and establish a phase mass inventory. While the ionic emission lines of Haro 11 essentially originate from the dense HII region component, a diffuse low-ionisation gas is needed to explain the [NeII], [NII], and [CII] line intensities. The [OIII] 88um line intensity is not fully reproduced by our model, hinting towards the possible presence of yet another low-density high-ionisation medium. The [OI] emission is consistent with a dense PDR of low covering factor, and we find no evidence for an X-ray dominated component. The PDR component accounts for only 10% of the [CII] emission.

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arXiv2012

Herschel-ATLAS: the far-infrared properties and star-formation rates of broad absorption line quasi-stellar objects

We have used data from the Herschel-ATLAS at 250, 350 and 500 μm to determine the far-infrared (FIR) properties of 50 Broad Absorption Line Quasars (BAL QSOs). Our sample contains 49 high-ionization BAL QSOs (HiBALs) and 1 low-ionization BAL QSO (LoBAL) which are compared against a sample of 329 non-BAL QSOs. These samples are matched over the redshift range 1.5 \leq z < 2.3 and in absolute i-band magnitude over the range -28 \leq M_{i} \leq -24. Of these, 3 BAL QSOs (HiBALs) and 27 non-BAL QSOs are detected at the > 5 sigma level. We calculate star-formation rates (SFR) for our individually detected HiBAL QSOs and the non-detected LoBAL QSO as well as average SFRs for the BAL and non-BAL QSO samples based on stacking the Herschel data. We find no difference between the HiBAL and non-BAL QSO samples in the FIR, even when separated based on differing BAL QSO classifications. Using Mrk 231 as a template, the weighted mean SFR is estimated to be \approx240\pm21 M_{\odot} yr^{-1} for the full sample, although this figure should be treated as an upper limit if AGN-heated dust makes a contribution to the FIR emission. Despite tentative claims in the literature, we do not find a dependence of {\sc C\,iv} equivalent width on FIR emission, suggesting that the strength of any outflow in these objects is not linked to their FIR output. These results strongly suggest that BAL QSOs (more specifically HiBALs) can be accommodated within a simple AGN unified scheme in which our line-of-sight to the nucleus intersects outflowing material. Models in which HiBALs are caught towards the end of a period of enhanced spheroid and black-hole growth, during which a wind terminates the star-formation activity, are not supported by the observed FIR properties.

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arXiv2012

The Herschel Exploitation of Local Galaxy Andromeda (HELGA). I: Global far-infrared and sub-mm morphology

We have obtained Herschel images at five wavelengths from 100 to 500 micron of a ~5.5x2.5 degree area centred on the local galaxy M31 (Andromeda), our nearest neighbour spiral galaxy, as part of the Herschel guaranteed time project "HELGA". The main goals of HELGA are to study the characteristics of the extended dust emission, focusing on larger scales than studied in previous observations of Andromeda at an increased spatial resolution, and the obscured star formation. In this paper we present data reduction and Herschel maps, and provide a description of the far-infrared morphology, comparing it with features seen at other wavelengths. We use high--resolution maps of the atomic hydrogen, fully covering our fields, to identify dust emission features that can be associated to M31 with confidence, distinguishing them from emission coming from the foreground Galactic cirrus. Thanks to the very large extension of our maps we detect, for the first time at far-infrared wavelengths, three arc-like structures extending out to ~21, ~26 and ~31 kpc respectively, in the south-western part of M31. The presence of these features, hosting ~2.2e6 Msol of dust, is safely confirmed by their detection in HI maps. Overall, we estimate a total dust mass of ~5.8e7 Msol, about 78% of which is contained in the two main ring-like structures at 10 and 15 kpc, at an average temperature of 16.5 K. We find that the gas-to-dust ratio declines exponentially as a function of the galacto-centric distance, in agreement with the known metallicity gradient, with values ranging from 66 in the nucleus to ~275 in the outermost region. [Abridged]

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arXiv2012

Herschel-ATLAS: Multi-wavelength SEDs and physical properties of 250 micron-selected galaxies at z < 0.5

We present a pan-chromatic analysis of an unprecedented sample of 1402 250 micron-selected galaxies at z < 0.5 (mean z = 0.24) from the Herschel-ATLAS survey. We complement our Herschel 100-500 micron data with UV-K-band photometry from the Galaxy And Mass Assembly (GAMA) survey and apply the MAGPHYS energy-balance technique to produce pan-chromatic SEDs for a representative sample of 250 micron selected galaxies spanning the most recent 5 Gyr of cosmic history. We derive estimates of physical parameters, including star formation rates, stellar masses, dust masses and infrared luminosities. The typical H-ATLAS galaxy at z < 0.5 has a far-infrared luminosity in the range 10^10 - 10^12 Lsolar (SFR: 1-50 Msolar/yr) thus is broadly representative of normal star forming galaxies over this redshift range. We show that 250 micron-selected galaxies contain a larger mass of dust at a given infra-red luminosity or star formation rate than previous samples selected at 60 micron from IRAS. We derive typical SEDs for H-ATLAS galaxies, and show that the emergent SED shape is most sensitive to specific star formation rate. The optical-UV SEDs also become more reddened due to dust at higher redshifts. Our template SEDs are significantly cooler than existing infra-red templates. They may therefore be most appropriate for inferring total IR luminosities from moderate redshift submillimetre selected samples and for inclusion in models of the lower redshift submillimetre galaxy populations.

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arXiv2012

Analytical shear and flexion of Einasto dark matter haloes

N-body simulations predict that dark matter haloes are described by specific density profiles on both galactic- and cluster-sized scales. Weak gravitational lensing through the measurements of their first and second order properties, shear and flexion, is a powerful observational tool for investigating the true shape of these profiles. One of the three-parameter density profiles recently favoured in the description of dark matter haloes is the Einasto profile. We present exact expressions for the shear and the first and second flexions of Einasto dark matter haloes derived using a Mellin-transform formalism in terms of the Fox H and Meijer G functions, that are valid for general values of the Einasto index. The resulting expressions can be written as series expansions that permit us to investigate the asymptotic behaviour of these quantities. Moreover, we compare the shear and flexion of the Einasto profile with those of different mass profiles including the singular isothermal sphere, the Navarro-Frenk-White profile, and the Sérsic profile. We investigate the concentration and index dependences of the Einasto profile, finding that the shear and second flexion could be used to determine the halo concentration, whilst for the Einasto index the shear and first and second flexions may be employed. We also provide simplified expressions for the weak lensing properties and other lensing quantities in terms of the generalized hypergeometric function.

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arXiv2012

A Detailed Gravitational Lens Model Based on Submillimeter Array and Keck Adaptive Optics Imaging of a Herschel-ATLAS Sub-millimeter Galaxy at z=4.243

We present high-spatial resolution imaging obtained with the Submillimeter Array (SMA) at 880um and the Keck Adaptive Optics (AO) system at Ks-band of a gravitationally lensed sub-millimeter galaxy (SMG) at z=4.243 discovered in the Herschel-Astrophysical Terahertz Large Area Survey. The SMA data (angular resolution ~0.6") resolve the dust emission into multiple lensed images, while the Keck AO Ks-band data (angular resolution ~0.1") resolve the lens into a pair of galaxies separated by 0.3". We present an optical spectrum of the foreground lens obtained with the Gemini-South telescope that provides a lens redshift of z_lens = 0.595 +/- 0.005. We develop and apply a new lens modeling technique in the visibility plane that shows that the SMG is magnified by a factor of mu = 4.1 +/- 0.2 and has an intrinsic infrared (IR) luminosity of L_IR = (2.1 +/- 0.2) x 10^13 Lsun. We measure a half-light radius of the background source of r_s = 4.4 +/- 0.5 kpc which implies an IR luminosity surface density of Sigma_IR = (3.4 +/- 0.9) x 10^11 Lsun kpc^-2, a value that is typical of z > 2 SMGs but significantly lower than IR luminous galaxies at z~0. The two lens galaxies are compact (r_lens ~ 0.9 kpc) early-types with Einstein radii of theta_E1 = 0.57 +/- 0.01 and theta_E2 = 0.40 +/- 0.01 that imply masses of M_lens1 = (7.4 +/- 0.5) x 10^10 Msun and M_lens2 = (3.7 +/- 0.3) x 10^10 Msun. The two lensing galaxies are likely about to undergo a dissipationless merger, and the mass and size of the resultant system should be similar to other early-type galaxies at z~0.6. This work highlights the importance of high spatial resolution imaging in developing models of strongly lensed galaxies discovered by Herschel.

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arXiv2012

The Herschel Exploitation of Local Galaxy Andromeda (HELGA) II: Dust and Gas in Andromeda

We present an analysis of the dust and gas in Andromeda, using Herschel images sampling the entire far-infrared peak. We fit a modified-blackbody model to ~4000 quasi-independent pixels with spatial resolution of ~140pc and find that a variable dust-emissivity index (beta) is required to fit the data. We find no significant long-wavelength excess above this model suggesting there is no cold dust component. We show that the gas-to-dust ratio varies radially, increasing from ~20 in the center to ~70 in the star-forming ring at 10kpc, consistent with the metallicity gradient. In the 10kpc ring the average beta is ~1.9, in good agreement with values determined for the Milky Way (MW). However, in contrast to the MW, we find significant radial variations in beta, which increases from 1.9 at 10kpc to ~2.5 at a radius of 3.1kpc and then decreases to 1.7 in the center. The dust temperature is fairly constant in the 10kpc ring (ranging from 17-20K), but increases strongly in the bulge to ~30K. Within 3.1kpc we find the dust temperature is highly correlated with the 3.6 micron flux, suggesting the general stellar population in the bulge is the dominant source of dust heating there. At larger radii, there is a weak correlation between the star formation rate and dust temperature. We find no evidence for 'dark gas' in M31 in contrast to recent results for the MW. Finally, we obtained an estimate of the CO X-factor by minimising the dispersion in the gas-to-dust ratio, obtaining a value of (1.9+/-0.4)x10^20 cm^-2 [K kms^-1]^-1.

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arXiv2012

Spitzer IRAC identification of Herschel-ATLAS SPIRE sources

We use spitzer-IRAC data to identify near-infrared counterparts to submillimeter galaxies detected with Herschel-SPIRE at 250um in the Herschel Astrophysical Terahertz Large Area Survey (H-ATLAS). Using a likelihood ratio analysis we identify 146 reliable IRAC counterparts to 123 SPIRE sources out of the 159. We find that, compared to the field population, the SPIRE counterparts occupy a distinct region of 3.6 and 4.5um color-magnitude space, and we use this property to identify a further 23 counterparts to 13 SPIRE sources. The IRAC identification rate of 86% is significantly higher than those that have been demonstrated with wide-field ground-based optical and near-IR imaging of Herschel fields. We estimate a false identification rate of 3.6%, corresponding to 4 to 5 sources. Among the 73 counterparts that are undetected in SDSS, 57 have both 3.6 and 4.5um coverage. Of these 43 have [3.6] - [4.5]> 0 indicating that they are likely to be at z > 1.4. Thus, ~ 40% of identified SPIRE galaxies are likely to be high redshift (z > 1.4) sources. We discuss the statistical properties of the IRAC-identified SPIRE galaxy sample including far-IR luminosities, dust temperatures, star-formation rates, and stellar masses. The majority of our detected galaxies have 10^10 to 10^11 L_sun total IR luminosities and are not intense starbursting galaxies as those found at z ~ 2, but they have a factor of 2 to 3 above average specific star-formation rates compared to near-IR selected galaxy samples.

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