C-BASS

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:

Luminous infrared galaxies (LIR > 1011 L⊙) are often associated with interacting galactic systems and are thought to be powered by merger-induced starbursts and/or dust-enshrouded active galactic nucleus. In such systems, the evolution of the dense, star-forming molecular gas as a function of merger separation is of particular interest. Here, we present observations of the CO(3-2) emission from a sample of luminous infrared galaxy mergers that span a range of galaxy-galaxy separations. The excitation of the molecular gas is studied by examining the CO(3-2)/CO(1-0) line ratio, r31, as a function of merger extent. We find these line ratios, r31, to be consistent with kinetic temperatures of Tk = (30-50) K and gas densities of We also find weak correlations between r31 and both merger progression and star formation efficiency [LFIR/LCO(1-0)]. These correlations show a tendency for gas excitation to increase as the merger progresses and the star formation efficiency rises. To conclude, we calculate the contributions of the CO(3-2) line to the 850-μm fluxes measured with SCUBA (Submillimetre Common-User Bolometer Array), which are seen to be significant (∼22 per cent). © 2010 The Authors. Journal compilation © 2010 RAS.

Abstract:

Lynds dark cloud LDN1622 represents one of the best examples of anomalous dust emission, possibly originating from small spinning dust grains. We present Cosmic Background Imager (CBI) 31-GHz data of LDN1621, a diffuse dark cloud to the north of LDN1622 in a region known as Orion East. A broken ring-like structure with diameter ≈20 arcmin of diffuse emission is detected at 31 GHz, at ≈20–30 mJy beam−1 with an angular resolution of ≈5 arcmin. The ring-like structure is highly correlated with far-infrared (FIR) emission at 12–100 μm with correlation coefficients of r≈ 0.7–0.8, significant at ∼10σ. The FIR-correlated emission at 31 GHz therefore appears to be mostly due to radiation associated with dust. Multifrequency data are used to place constraints on other components of emission that could be contributing to the 31-GHz flux. An analysis of the GB6 survey maps at 4.85 GHz yields a 3σ upper limit on free–free emission of 7.2 mJy beam−1 (≲30 per cent of the observed flux) at the CBI resolution. The bulk of the 31-GHz flux therefore appears to be mostly due to dust radiation. Aperture photometry, at an angular resolution of 13 arcmin and with an aperture of diameter 30 arcmin, allowed the use of IRAS maps and the Wilkinson Microwave Anisotropy Probe 5-yr W-band map at 93.5 GHz. A single modified blackbody model was fitted to the data to estimate the contribution from thermal dust, which amounts to ∼10 per cent at 31 GHz. In this model, an excess of 1.52 ± 0.66 Jy (2.3σ) is seen at 31 GHz. Correlations with the IRAS 100 μm gave a coupling coefficient of 18.1 ± 4.4 μK (MJy sr−1)−1, consistent with the values found for LDN1622.