Cosmology

Abstract:

The Very Small Array (VSA) has been used to survey the ℓ ∼ 27° to region of the Galactic plane at a resolution of 13 arcmin. This ℓ-range covers a section through the Local, Sagittarius and the Cetus spiral arms. The survey consists of 44 pointings of the VSA, each with an rms sensitivity of ∼90 mJy beam-1. These data are combined in a mosaic to produce a map of the area. The majority of the sources within the map are H ii regions. The main aim of the programme was to investigate the anomalous radio emission from the warm dust in individual H ii regions of the survey. This programme required making a spectrum extending from GHz frequencies to the far-infrared (FIR) IRAS frequencies for each of nine strong sources selected to lie in unconfused areas. It was necessary to process each of the frequency maps with the same u, v coverage as was used for the VSA 33 GHz observations. The additional radio data were at 1.4, 2.7, 4.85, 8.35, 10.55, 14.35 and 94 GHz in addition to the 100, 60, 25 and 12 μm IRAS bands. From each spectrum the free-free, thermal dust and anomalous dust emission were determined for each H ii region. The mean ratio of 33 GHz anomalous flux density to FIR 100 μm flux density for the nine selected H ii regions was ΔS(33 GHz)/S(100 μm) = 1.10 ± 0.21 × 10-4. When combined with six H ii regions previously observed with the VSA and the Cosmic Background Imager, the anomalous emission from warm dust in H ii regions is detected with a 33 GHz emissivity of 4.65 ± 0.40 μK (MJy sr-1)-1 (11.5σ). This level of anomalous emission is 0.3 to 0.5 of that detected in cool dust clouds. A radio spectrum of the H ii region anomalous emission covering GHz frequencies is constructed. It has the shape expected for spinning dust composed of very small grains. The anomalous radio emission in H ii regions is on average 41 ± 10 per cent of the radio continuum at 33 GHz. Another result is that the excess (i.e. non-free-free) emission from H ii regions at 94 GHz correlates strongly with the 100 μm emission; it is also inversely correlated with the dust temperature. Both these latter results are as expected for very large grain dust emission. The anomalous emission on the other hand is expected to originate in very small spinning grains and correlates more closely with the 25 μm emission. © 2010 The Authors. Journal compilation © 2010 RAS.

Abstract:

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. © 2010 ESO.

Abstract:

To exploit synergies between the Herschel Space Observatory and next generation radio facilities, we have extended the semi-empirical extragalactic radio continuum simulation of Wilman et al. to the mid- and far-infrared. Here, we describe the assignment of infrared spectral energy distributions (SEDs) to the star-forming galaxies and active galactic nuclei, using Spitzer 24, 70 and 160 μm and SCUBA 850 μm survey results as the main constraints.Star-forming galaxies dominate the source counts, and a model in which their far-infrared-radio correlation and infrared SED assignment procedure are invariant with redshift underpredicts the observed 24 and 70 μm source counts. The 70 μm deficit can be eliminated if the star-forming galaxies undergo stronger luminosity evolution than originally assumed for the radio simulation, a requirement which may be partially ascribed to known non-linearity in the far-infrared-radio correlation at low luminosity if it evolves with redshift. At 24 μm, the shortfall is reduced if the star-forming galaxies develop SEDs with cooler dust and correspondingly stronger polycyclic aromatic hydrocarbon emission features with increasing redshift at a given far-infrared luminosity, but this trend may reverse at z > 1 in order not to overproduce the submillimetre source counts. The resulting model compares favourably with recent Balloon-borne Large Aperture Submillimetre Telescope (BLAST) results, and we have extended the simulation data base to aid the interpretation of Herschel surveys. Such comparisons may also facilitate further model refinement and revised predictions for the Square Kilometre Array and its precursors. © 2010 The Authors. Journal compilation © 2010 RAS.

Abstract:

We present the results of our search for high-redshift Lyman-break galaxies over the GOODS-South field. We use Hubble Space Telescope (HST)-ACS data in B, V, i′ & z′, Very Large Telescope (VLT)-ISAAC J and Ks, Spitzer-Infrared Array Camera (IRAC) 3.6, 4.5, 5.8 and 8.0 μm data in conjunction with the new HAWK-I Y-band science verification data to search for dropout galaxies in the redshift range 6 < z < 9. We survey ≈119 arcmin2 to YAB = 25.7 (5σ), of which 37.5 arcmin2 reaches YAB = 25.9. Candidate z′ and Y dropouts were selected on the basis of a colour cut of (Y - J)AB > 0.75 mag and (z′ - Y)AB > 1.0 mag, respectively. We find no robust Y-drops (z ≈ 9) brighter than JAB < 25.4. In our search for z′-band dropouts (z ≈ 6.5-7.5), we identify four possible candidates, two with z′-drop colours and clear Spitzer-IRAC detections and two less likely candidates. We also identify two previously known Galactic T-dwarf stellar contaminants with these colours, and two likely transient objects seen in the Y-band data. The implications if all or none of our candidates is real on the ultraviolet galaxy luminosity functions at z > 6.5 are explored. We find our number of z′-drop candidates to be insufficient based on the expected number of z′ drops in a simple no-evolution scenario from the z = 3 Lyman-break galaxy luminosity function but we are consistent with the observed luminosity function at z ≈ 6 (if all our candidates are real). However, if one or both of our best z′-drop candidates are not z > 6.5 galaxies, this would demand evolution of the luminosity function at early epochs, in the sense that the number density of ultraviolet luminous star- forming galaxies at z > 7 is less than at z ~ 6. We show that the future surveys to be conducted with the European Southern Observatory VISTA telescope over the next 5 yr will be able to measure the bulk of the luminosity function for both z′ and Y dropouts and thus provide the strongest constraints on the level of star-formation within the epoch of reionization. © 2010 The Authors. Journal compilation. © 2010 RAS.

Abstract:

We present the results of our investigation into the stellar populations of 24 radio galaxies at z ≃ 0.5 drawn from four complete, low-frequency-selected radio surveys. We use the strength of the 4000-Å break as an indicator of recent star formation and compare this with radio luminosity, optical spectral classification and morphological classification. We find evidence of different star formation histories for high- and low-luminosity radio sources; our group of low radio luminosity sources (typically Fanaroff-Riley type I sources) has systematically older stellar populations than the higher radio luminosity group. Our sample is also fairly well divided by optical spectral classification. We find that galaxies classified as having low excitation spectra (LEGs) possess older stellar populations than high excitation line objects (HEGs), with the HEGs showing evidence for recent star formation. We also investigate the link between radio morphology, as used by Owen and Laing, and the stellar populations. We find that there is a preference for the 'fat-double' sources to have older stellar populations than the 'classical double' sources, although this is also linked to these sources lying predominantly in the LEG and HEG categories, respectively. These results are consistent with the hypothesis that HEGs are powered by accretion of cold gas, which could be supplied, for example, by recent mergers, secular instabilities or filamentary cold flows. These processes could also trigger star formation in the host galaxy. The host galaxies of the LEGs do not show evidence for recent star formation and an influx of cold gas and are consistent with being powered by the accretion of the hot phase of the interstellar medium. © 2010 The Authors. Journal compilation © 2010 RAS.