Skip to content
Skip to navigation menu

Latest News

*Recent European FP7 Grant Application - DustPedia*

Jon Davies

The European Space Agency has invested heavily in two cornerstones missions; Herschel and Planck. These space observatories provide us with an unprecedented opportunity to study, at far infrared wavelengths, the cold Universe beyond our galaxy. As these missions come to an end (2013) they will leave a huge legacy data set that we intend to exploit. The data provides us with an opportunity to study cosmic dust in galaxies to answer fundamental questions about: the origin of the chemical elements, physical processes in the interstellar medium (ISM), its effect on stellar radiation, its relation to star formation and its relation to the cosmic far infrared background. In the course of our work we will develop tools and computer models that will help us relate observed cosmic dust emission to the physical properties of the dust (chemical composition, size distribution, temperature) and to the origins of dust (evolved stars, super novae, growth in the ISM) and the processes that destroy it (high energy collisions and shock heated gas). To help us interpret the data we will use our own, world leading, Monte Carlo photon tracing radiative transfer model of galaxies. To carry out this research we will need to combine the Herschel/Planck data with that from many other new and state-of-the-art databases that contain observations at other wavelengths, thus creating the DustPedia database. To maximise our spatial resolution and sensitivity to cosmic dust our intention is use 3045 local galaxies (v<3000 km/s) selected via their near infrared luminosity (stellar mass).

*Abstracts of proposed contributions to the National Astronomy Meeting 2013*

Chris Clark

The Herschel Astrophysical Terahertz Large Area Survey (H-ATLAS) is the largest open-time project undertaken by the Herschel Space Observatory, observing 550 square degrees of sky. We have used the 108 square degree Phase-1 internal data release to carry out the first dust-mass-selected volume-limited (z=0.003 to 0.01) investigation of the local universe. Previous far-infrared & sub-millimetre surveys of nearby galaxies have had to select their targets by means of properties derived from observations at other wavelengths that were expected to be proxies for dust mass (near-infrared K-band luminosity, IRAS hot dust luminosity, etc) or other criteria of interest; however, H-ATLAS finally permits us to perform an unbiased study. This allows us to both uncover the submillimetre characteristics of the local universe in an unrivalled manner, and also assess the effect that the selection criteria of previous surveys had upon their samples and results. We reveal surprising findings in both regards.

Chris Clark

Significant dust production in supernovae has been suggested as a means to account for the "dust budget crisis" in nearby galaxies and in high redshift sources. We present a sample of young Galactic supernova remnants observed with the Herschel Space Observatory in the far-infrared and submillimetre, including the Crab Nebula, a massive-star Type-II supernova, and Kepler and Tycho???s supernovae, both of type Type-Ia. For the Crab, synchrotron, spectral line and hot dust emission were removed, revealing a residual cool dust component of 0.11??0.01 Msol at 33??2 K (assuming amorphous carbon grains); the first unambiguous evidence that Type-II supernovae act as dust factories. In the remnants of both Tycho & Kepler???s supernovae, we detect small amounts of hot dust (0.0031 & 0.0083 Msol respectively), and show this is most likely dust swept-up from the surrounding interstellar material; we find no evidence of cold supernova dust in these canonical Type Ia remnants.

Chris Fuller

Nearby galaxy clusters Virgo, Coma, and Fornax provide us with a chance to study galaxy evolution in dense environments with high sensitivity and resolution. Due to the proximity of these three clusters they have been the focus of many multi-wavelength studies. Now with the Herschel Space Observatory we add the far infrared (FIR, 100, 160, 250, 350 and 500um), tracing cold dust. Dust is key for galaxy evolution as it has a critical role in the life cycle of a galaxy. For example, molecular hydrogen forms on the surface of dust grains, making it crucial for star formation. Like gas, dust is more easily stripped from galaxies in the cluster environment over the stellar component. Between these three clusters there is a wide range of cluster properties. Coma and Fornax are dynamically mature, with Virgo at an earlier epoch of formation. This is reflected in the higher proportion of early type galaxies in both Coma and Fornax. We also have a filament sample consisting of galaxies in the great wall connecting Coma and A1367. This acts as a good comparison sample with galaxy densities more typical of the general field. Our detection rates clearly show dust is a good tracer of environment as we have 53%, 34%, 15%, and 14% for the Filament, Virgo, Coma, and Fornax samples. These are galaxies that are detected in at least one Herschel band. Our detection rates show that the hostile Cluster environment has a strong affect on dust in its galaxies. We discuss FIR detection rates in the four different environments and the properties of the detected galaxies.

*Recent Paper Accepted for Publication*

Ford et al., 2013, (arxiv1303.6284)- 'Herschel Exploitation of Local Galaxy Andromeda (HELGA) III: The Star Formation Law in M31'

We present a detailed study of how the Star Formation Rate (SFR) relates to the interstellar medium (ISM) of M31 at approx 140pc scales. The SFR is calculated using the far-ultraviolet and 24um emission, corrected for the old stellar population in M31. We find a global value for the SFR of 0.25+/-0.05 Msun/yr and compare this with the SFR found using the total far-infrared (FIR) luminosity. There is general agreement in regions where young stars dominate the dust heating. Atomic hydrogen (HI) and molecular gas (traced by carbon monoxide, CO) or the dust mass is used to trace the total gas in the ISM. We show that the global surface densities of SFR and gas mass place M31 amongst a set of low-SFR galaxies in the plot of Kennicutt (1998b). The relationship between SFR and gas surface density is tested in six radial annuli across M31, assuming a power law relationship with index, N. The star formation law using total gas traced by HI and CO gives a global index of N=2.03+/-0.04, with a significant variation with radius; the highest values are observed in the 10kpc ring. We suggest that this slope is due to HI turning molecular at about 10Msun/pc^2. When looking at H2 regions, we measure a higher mean SFR suggesting a better spatial correlation between H2 and SF. We find N~0.6 with consistent results throughout the disk - this is at the low end of values found in previous work and argues against a superlinear SF law on small scales.

*Recent Papers Published by the Galaxies Group*

Davies et al., 2013, MNRAS, 428, 834 - 'The Herschel Fornax Cluster Survey - I. The Bright Galaxy Sample'

We present Herschel Space Telescope observations of the nearby Fornax cluster at 100, 160, 250, 350 and 500um with a spatial resolution of 7 - 36 arc sec (10" is about 1 kpc at d_{Fornax}= 17.9 Mpc). We define a sample of eleven bright galaxies, selected at 500um, that can be directly compared with our past work on the Virgo cluster. We check and compare our results with previous observations made by IRAS and Planck, finding good agreement. The far-infrared luminosity density is higher, by about a factor of three, in Fornax compared to Virgo, consistent with the higher number density of galaxies. The 100um (42.5-122.5 um) luminosity is two orders of magnitude larger in Fornax than in the local field as measured by IRAS. We calculate stellar (L_{0.4-2.5}) and far-infrared (L_{100-500}) luminosities for each galaxy and use this to estimate a mean optical depth of tau=0.4+/-0.1 - the same value as we previously found for Virgo cluster galaxies. For ten of the eleven galaxies (NGC1399 excepted) we fit a modified blackbody curve (beta=2.0) to our observed flux densities to derive dust masses and temperatures of 10^{6.54-8.35} Mo and T=14.6-24.2K respectively, values comparable to those found for 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 broadly consistent with values for Virgo. Fornax is a mass overdensity in stars and dust of about 120 when 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. We consider in more detail two of the sample galaxies. As the brightest source in either Fornax and Virgo, NGC1365 is also detected by Planck. The Planck data fit the PACS/SPIRE SED out to 1382um 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 500um with the emission probably arising from the nuclear radio source rather than inter-stellar dust.

Davies - arXiv:1204.4649 'A heavy Baryonic Galactic Disc'

We investigate the possibility that the observed rotation of galaxies can be accounted for by invoking a massive baryonic disc with no need for non-baryonic dark matter or a massive halo. There are 5 primary reasons for suggesting this: 1. there are well known disc surface mass density distributions that naturally produce the observed rotation curves of galaxies. 2. there are a number of rotation curve `puzzles' that cannot be explained by a massive dark matter halo i.e. the success of maximum disc fitting, HI gas scaling to the observed rotation, the disc/halo conspiracy and the interpretation of the Tully-Fisher relation. 3. recent 21cm observations show an almost constant HI surface density and a distinct `cut-off' or edge to galactic discs. We explain this constant surface density in terms of either an optical depth effect or the onset of molecular gas formation and hence the possibility of considerably more gas existing in galaxies. We suggest that the HI cut-off does indeed mark the edge of the galactic disc. 4. there have been an increasing number of recent observations that imply that X_CO may be ten times higher in the outer Galaxy. This `dark' gas may provide adequate mass to account for galaxy rotation. 5. we show that the additional baryonic mass required to account for the rotation of galaxies is just that required to reconcile observed baryons with those predicted by big bang nucleosynthesis. Mestel discs can be used to straight forwardly explain the scaling laws of galaxies, particularly the observed relation between rotation velocity and radius and the oft used Tully-Fisher relation. We discuss observations of the baryonic content of galactic discs and where sufficient `hidden' baryons might be found to account for the rotation.

Davies et al., 2012, MNRAS, 419, 3505 - 'The Herschel Virgo Cluster Survey - VIII. The Bright Galaxy Sample'

We describe the Herschel Virgo Cluster Survey and the first data that cover the complete survey area (four 4x4 deg^2 regions). We use these data to measure and compare the global far-infrared properties of 78 optically bright galaxies that are selected at 500µm and detected in all five far-infrared bands. We show that our measurements and calibration are broadly consistent with previous data obtained by the IRAS, ISO, Spitzer and Planck. We use SPIRE and PACS photometry data to produce 100-, 160-, 250-, 350- and 500µm cluster luminosity distributions. These luminosity distributions are not power laws, but 'peaked', with small numbers of both faint and bright galaxies. We measure a cluster 100-500µm far-infrared luminosity density of 1.6(7.0) +/- 0.2 ? 10^9 Lsun Mpc^-3. This compares to a cluster 0.4-2.5 µm optical luminosity density of 5.0(20.0) ? 10^9 Lsun Mpc^-3, some 3.2(2.9) times larger than the far-infrared. A 'typical' photon originates from an optical depth of 0.4 ± 0.1. Most of our sample galaxies are well fitted by a single modified blackbody (beta= 2), leading to a mean dust mass of log M_Dust= 7.31 M_sun and temperature of 20.0 K. We also derive both stellar and atomic hydrogen masses from which we calculate mean values for the star-to-gas (atomic) and gas (atomic)-to-dust mass ratios of 15.1 and 58.2, respectively. Using our derived dust, atomic gas and stellar masses, we estimate cluster mass densities of 8.6(27.8) x 10^6, 4.6(13.9) x 10^8 and 7.8(29.7) x 10^9 M_sun Mpc^-3 for dust, atomic gas and stars, respectively. These values are higher than those derived for field galaxies by factors of 39(126), 6(18) and 34(129), respectively. In the above, the luminosity/mass densities are given using the whole sample with the values in brackets using just those galaxies that lie between 17 and 23 Mpc. We provide a data table of flux densities in all the Herschel bands for all 78 bright Virgo Cluster galaxies.

Auld et al., 2012, MNRAS, 420, 1882 - 'Herschel observations of Cen A: stellar heating of two extragalactic dust clouds'

We present the first results of a multiwavelength survey, incorporating Herschel, Spitzer, GALEX and Australian Telescope Compact Array (ATCA) observations, of a 1x1 sq deg field centred on Centaurus A. As well as detecting the inner lobes of the active galactic nucleus (AGN) jet and counterjet, we have found two clouds, bright at sub-millimetre wavelengths, approximately 15 kpc from the centre of Cen A that are co-aligned with the jets. Flux measurements at Herschel wavelengths have proved vital in constraining fits to the spectral energy distributions (SEDs). The clouds are well fitted by a single-temperature, modified blackbody spectrum (beta=2) indicating that we are looking at two cold dust clouds on the outskirts of Cen A. The temperature and masses of the clouds are: T_{north} = 12.6^{+1.1}_{-1.2} K, T_{south} = 15.1^{+1.7}_{-1.6} K; log(M_{north} / M_o) = 5.8^{+0.2}_{-0.2}, log(M_{south} / M_o) = 5.6^{+0.2}_{-0.2} and the gas-dust ratio for both clouds is about 100. The measured values for the northern dust cloud are consistent with previous measurements from ISO while the southern cloud is a new submillimetre detection. The two dust clouds are located at the termini of the partial HI ring that surrounds Cen A which is also where the gas column density peaks. The Herschel survey encompasses the partial HI ring yet we find no evidence of dust emission in any other part of the ring. Assuming that the gas-dust ratio is the same in the rest of the ring, dust mass upper limits in the H I ring are consistent with low column density dust being present but falling below the SPIRE detection limit. We have discussed the origin of these clouds and various possible heating mechanisms. The observations favour a scenario in which the gas and dust were once part of a late-type galaxy, which has since merged with Cen A. The dominant heating mechanism which adequately explains the observed temperatures in both clouds is heating from the evolved stellar population within Cen A.

Eales et al., 2012, arXiv1202.0547- 'Can dust emission be used to map the interstellar medium in high-redshift galaxies? Results from the Herschel Reference Survey'

It has often been suggested that an alternative to the standard CO/21-cm method for estimating the mass of the interstellar medium (ISM) in a galaxy might be to estimate the mass of the ISM from the continuum dust emission. In this paper, we investigate the potential of this technique using Herschel observations of ten galaxies in the Herschel Reference Survey and in the Herschel Virgo Cluster Survey. We show that the emission detected by Herschel is mostly from dust that has a temperature and emissivity index similar to that of dust in the local ISM in our galaxy, with the temperature generally increasing towards the centre of each galaxy. We calibrate the dust method using the CO and 21-cm observations to provide an independent estimate of the mass of hydrogen in each galaxy, solving the problem of the uncertain `X factor' for the molecular gas by minimizing the dispersion in the ratio of the masses estimated using the two methods. With the calibration for the dust method and the estimate of the X-factor produced in this way, the dispersion in the ratio of the two gas masses is 30%, which gives an upper limit on the fundamental accuracy of the dust method. The calibration we obtain for the dust method is very similar to an independent Herschel measurement for M31 and to the calibration for the Milky Way from Planck measurements.

Gomez et al., 2012, MNRAS, 420, 3557 - 'Dust in historical Galactic Type Ia supernova remnants with Herschel'

The origin of interstellar dust in galaxies is poorly understood, particularly the relative contributions from supernovae and the cool stellar winds of low-intermediate-mass stars. Recently, large masses of newly formed dust have been discovered in the ejecta of core-collapse supernovae. Here, we present Herschel Photodetector Array Camera and Spectrometer (PACS) and Spectral and Photometric Imaging Receiver (SPIRE) photometry at 70-500µm of the historical, young supernova remnants: Kepler and Tycho, both thought to be the remnants of Type Ia explosion events. We detect a warm dust component in Kepler's remnant with T = 82 K and mass 0.0031 M_sun; this is spatially coincident with thermal X-ray emission and optical knots and filaments, consistent with the warm dust originating in the circumstellar material swept up by the primary blast wave of the remnant. Similarly for Tycho's remnant, we detect warm dust at 90 K with mass 0.0086 M_sun. Comparing the spatial distribution of the warm dust with X-rays from the ejecta and swept-up medium, and Hα emission arising from the post-shock edge, we show that the warm dust is swept up interstellar material. We find no evidence of a cool (25-50 K) component of dust with mass >0.07 M_sun as observed in core-collapse remnants of massive stars. Neither the warm or cold dust components detected here are spatially coincident with supernova ejecta material. We compare the lack of observed supernova dust with a theoretical model of dust formation in Type Ia remnants which predicts dust masses of 88(17) x 10^-3 M_sun for ejecta expanding into ambient surrounding densities of 1(5) cm^-3. The model predicts that silicon- and carbon-rich dust grains will encounter, at most, the interior edge of the observed dust emission at approximately 400 years, confirming that the majority of the warm dust originates from swept-up circumstellar or interstellar grains (for Kepler and Tycho, respectively). The lack of cold dust grains in the ejecta suggests that Type Ia remnants do not produce substantial quantities of iron-rich dust grains and has important consequences for the 'missing' iron mass observed in ejecta. Finally, although, we cannot completely rule out a small mass of freshly formed supernova dust, the Herschel observations confirm that significantly less dust forms in the ejecta of Type Ia supernovae than in the remnants of core-collapse explosions.

Smith et al., 2012, ApJ, 748, 123 - The Herschel Reference Survey: Dust in Early-Type Galaxies and Across the Hubble Sequence'

We present Herschel observations of 62 Early-Type Galaxies (ETGs), including 39 galaxies morphologically classified as S0+S0a and 23 galaxies classified as ellipticals using SPIRE at 250, 350 and 500 microns (and PACS 100 and 160 microns for 19 sources) as part of the volume-limited Herschel Reference Survey. We detect dust emission in 24% of the ellipticals and 62% of the S0s. The mean temperature of the dust is 23.9+/-0.8 K, warmer than that found for late-type galaxies in the Virgo Cluster. Including the non-detections, the mean dust mass is log(Mdust) = 5.9+/-0.1 and 5.2+/-0.1 Msun for the S0s and elliptical galaxies respectively. The mean dust-to-stellar mass is log(Mdust/Mstar) = -4.4+/-0.1 (S0s) and -5.8+/-0.1 (ellipticals). Virtually all the galaxies lie close to the red sequence yet the large number of detections of cool dust, the gas-to-dust ratios and the ratios of far-infrared to radio emission all suggest that many ETGs contain a cool interstellar medium similar to that in late-type galaxies. The mean dust-to-stellar mass ratio for S0s is approximatly a factor of ten less than for early-type spirals and the sizes of the dust sources in the S0s are also much smaller. We show that the difference cannot be explained by either the different bulge-to-disk ratios or environmental effects such as ram-pressure stripping. The wide range in the dust-to-stellar mass ratio for ETGs and the lack of a correlation between dust mass and optical luminosity suggest that much of the dust in the ETGs detected by Herschel has been acquired as the result of gravitational interactions; these interactions are unlikely to have had a major effect on the stellar masses of the ETGs. The Herschel observations tentatively suggest that in the most massive ETGs, the mass of the interstellar medium is unconnected to the evolution of the stellar populations.