Astronomical instrumentation

Abstract:

The Very Small Array (VSA) has been used to survey the ℓ∼ 27° to forumla 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.

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.