Astronomical instrumentation

Abstract:

We present mid-infrared images and an 8-13 μm spectrum of the southern H II region RCW 38. We determine the dust colour temperature from both our spectrum and images at 10 and 20 μm, and deduce the gas excitation from an image in the [S IV] fine-structure line, as well as spectra of the [Ar III], [S IV] and [Ne II] fine-structure lines. Our observations are consistent with a complex of sources associated with the RCW 38 IRS1 region, which represent knots of material in a shell, or ridge, surrounding a cavity of about 0.1 pc in radius, which is itself created by the stellar wind of the hot young source IRS2. The dust temperature does not peak closest to IRS2, but rather along the centre of the ridge, and is remarkably uniform over the extent of our image. From photoionization models for the observed line ratios at IRS1 we deduce a stellar effective temperature and gas density of about 43 000-48 000 K and 104 cm-3 respectively. Whilst the star, or star cluster, IRS2 is ultimately responsible for the observed thermal and ionic emission, the relatively uniform dust temperature implies that the bulk of the dust heating in the region is provided by resonantly trapped Lyman α photons, rather than direct stellar photons. This then also implies that the dust is depleted with respect to the gas by a factor of at least 100 from its normal interstellar value. The small-scale spatial variations in the continuum emission and temperature can be explained by changes in the density and/or gas-to-dust mass ratio.

Abstract:

We report first results from the Streaming Motions of Abell Clusters (SMAC) project, an all-sky Fundamental Plane survey of 699 early-type galaxies in 56 clusters between ∼3000 and ∼14,000 km s-1. For this sample, with a median distance of ∼8000 km s-1, we find a bulk flow of amplitude 630 ± 200 km s-1 toward l = 260 ± 15°, b = -1 ± 12° with respect to the cosmic microwave background. The flow is robust against the effects of individual clusters and data subsets, the choice of Galactic extinction maps, Malmquist bias, and stellar population effects. The direction of the SMAC flow is ∼90° away from the flow found by Lauer & Postman, but it is in good agreement with the gravity dipole predicted from the distribution of X-ray-luminous clusters. Our detection of a high-amplitude coherent flow on such a large scale argues for excess mass density fluctuation power at wavelengths λ ≳ 60 h-1 Mpc, relative to the predictions of currently popular cosmological models.

Abstract:

Spectropolarimetry of the 3.4 μm aliphatic C-H stretch feature, generally attributed to carbonaceous dust in the diffuse interstellar medium, has been carried out in the line of sight from the Galactic center source Sagittarius A IRS 7. The feature is unpolarized (Δp/Δτ < 0.2): the upper limit for polarization is well below that expected on the basis of a model in which the carrier molecules are associated with the aligned silicate component of interstellar dust, for example, as an organic or carbonaceous mantle on a silicate core. The simplest explanation is that the 3.4 μm carrier resides in a population of small, non-polarizing carbonaceous grains, physically separate from the silicates and sharing many characteristics with the carriers of the 217.5 nm extinction bump.