We report the detection of linear polarization from Sgr A* at 750, 850, 1350, and 2000 µm which confirms the contribution of synchrotron radiation. From the lack of polarization at longer wavelengths, it appears to arise in the millimeter/submillimeter excess. There are large position angle changes between the millimeter and submillimeter results, and these are discussed in terms of a polarized dust contribution in the submillimeter and various synchrotron models. In the model that best explains the data, the synchrotron radiation from the excess is self-absorbed in the millimeter region and becomes optically thin in the submillimeter. This implies that the excess arises in an extremely compact source of approximately 2 Schwarzschild radii.

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We present the results of a search for bright high-redshift galaxies in two large Submillimetre Common-User Bolometer Array (SCUBA) scan-maps of Galactic regions. A Mexican hat wavelet technique was used to locate point sources in these maps, which suffer high foreground contamination as well as typical scan-map noise signatures. A catalogue of point-source objects was selected and observed again in the submillimetre continuum, and in HCO + (3 → 2) at zero redshift to rule out Galactic sources. No extragalactic sources were found. Simulations show that the survey was sensitive to sources with fluxes 50 mJy, depending on the local background. These simulations result in upper limits on the 850-µm counts of SCUBA galaxies of 53 per deg 2 at 50 mJy and 2.9 per deg 2 at 100 mJy.

We report the results of a submillimetre continuum emission survey targeted towards 78 star formation regions, 72 of which are devoid of methanol maser and UC H II regions, identified in the Swedish ESO Submillimetre Telescope (SEST)/SEST IMaging Bolometer Array (SIMBA) millimetre continuum survey of Hill et al. At least 45 per cent of the latter sources, dubbed 'mmonly', detected in this survey are also devoid of the mid-infrared MSX emission. The 450-and 850-μm continuum emission was mapped using the Submillimetre Common User Bolometer Array (SCUBA) instrument on the James Clerk Maxwell Telescope (JCMT). Emission is detected towards 97 per cent of the 78 sources targeted as well as towards 28 other SIMBA sources lying in the SCUBA fields.In total, we have identified 212 cores in this submillimetre survey, including 106 previously known from the SIMBA survey. Of the remaining 106 sources, 53 result from resolving a SIMBA source into multiple submillimetre components, whilst the other 53 sources are submillimetre cores, not seen in the SIMBA. Additionally, we have identified two further mmonly sources in the SIMBA images. Of the total 405 sources identified in the SIMBA survey, 255 are only seen at millimetre wavelengths.We concatenate the results from four (sub)millimetre continuum surveys of massive star formation, together with the Galactic plane map of Pierce-Price et al. in order to determine the dust grain emissivity index β for each of the sources in the SIMBA source list. We examine the value of β with respect to temperature, as well as for the source classes identified in the SIMBA survey, for variation of this index. Our results indicate that β is typically 2, which is consistent with previous determinations in the literature, but for a considerably larger sample than previous work.

Abstract.We have analysed SCUBA 850 µm images of the (near) face-on spiral galaxy NGC 6946, paying particular attention to the subtraction of sky signal. A comparison with both 21 cm HI and 12 CO(2-1) intensity maps reveals a tight correlation between dust thermal emission and molecular gas at the kiloparsec level. By means of a Monte Carlo radiative transfer model, we convert a B−K colour image of NGC 6946 into a map of visual optical depth. The model yields maximum opacities since we assume that any increase in B−K colour, with respect to the disk edge, is attributable solely to extinction by dust. The resultant map of visual optical depth relates well to the distribution of neutral gas (HI+H2) and implies a global gas-to-dust ratio of 90 (this value is a lower limit). There is no significant radial variation of this ratio: this can be understood, since the gas content is dominated by far by the molecular gas. The latter is estimated through the CO emission tracer, which is itself dependent on metallicity, similarly to dust emission. In the absence of a more objective tracer, it is not possible to derive the true gas-to-dust ratio. By comparing the radial profile of our visual optical depth map with that of the SCUBA image, we infer an emissivity (dust absorption coefficient) at 850 µm that is 3 times lower than the value measured by COBE in the Milky Way, and 9 times lower than in NGC 891. We view this very much as a lower estimate, however, given our initial assumptions in deriving the visual opacity, and the possibility of underestimating the large-scale submm emission, the effect being more severe for the nearly face-on orientation of NGC 6946. A decomposition of the spiral structure half way out along the disk of NGC 6946 suggests an interarm optical depth of between 1 and 2. These surprisingly high values represent 40-80% of the visual opacity that we measure for the arm region.