Astronomical instrumentation

Abstract:

The optical backscatter of the 4W CW laser used to produce a mesospheric sodium-layer laser guide star for the MPE adaptive optics system (ALFA) has been observed from a neighbouring 2.2 m telescope. The observations, taken at the Max Planck Observatory in Calar-Alto (Spain), in August 1998, had two aims: study the Na plume (altitude and profile variations) and the Rayleigh cone to achieve Rayleigh scattering measurements. In the framework of the network, 'Laser Guide Star for 8m class telescopes', a program of the European Commission, ESO, MPE and NUI, Galway are collaborating on studying the light pollution due to the MPE ALFA laser. The light pollution of the observatory is due to Rayleigh and Mie scattering, including water scattering, of the laser beam in the low atmosphere. With the increase of Laser Guide Star Adaptive Optics system use, there is a need for a laser management policy. In order to optimise observing time, it is important to evaluate exactly when to stop the LGS, as the line of sight of one telescope gets near the laser beam, and as a function of other telescope observing programme. In this perspective, not only the single and multiple Rayleigh scattering by atmospheric molecules but also Mie scattering by aerosols has to be taken into account. Modeling of these phenomena needs an experimental calibration in realistic circumstances, and precise measurements of single and multiple light scattering in the surroundings of the beam at various altitudes. We report here on the result of such experiments.

Abstract:

We present ISOSWS, ISOPHOT_S and 8-13 μm observations of Centaurus A which show prominent PAH and silicate features. These and other data are used to construct a model for the infrared continuum. We find that, in a small nuclear aperture (∼4 arcsec, ∼60 pc), the spectral energy distribution is characteristic of emission from a starburst and dusty AGN torus; in larger apertures, additional components of cirrus and starburst emission are required. The model components are based on the radiative transfer models of Efstathiou et al. which include multiple scattering and the radiative effects of a dust-embedded source with a distribution of grain species and sizes. The torus component is modelled in terms of a tapered dusty disc centrally illuminated by a quasar-like source. The cirrus and starburst components are, respectively, modelled in terms of diffuse dust illuminated by the interstellar medium and an ensemble of optically thick molecular clouds centrally illuminated by hot stars. These latter components additionally include emission from small graphite particles and PAHs. Based on our overall model, the torus diameter is estimated to be 3.6pc and the best inclination angle of the torus is 45°. We present independent observational evidence for this structure. This result has implications for the detectability of tori in low-power AGN and for the use of the IRAS 60/25-μm flux ratio as an indicator of the torus inclination.

Abstract:

We compare the bulk flow of the SMAC sample to the predictions of popular cosmological models and to other recent large-scale peculiar velocity surveys. Both analyses account for aliasing of small-scale power due to the sparse and non-uniform sampling of the surveys. We conclude that the SMAC bulk flow is in marginal conflict with flat COBE-normalized Lambda-CDM models which fit the cluster abundance constraint. However, power spectra which are steeper shortward of the peak are consistent with all of the above constraints. When recent large-scale peculiar velocity surveys are compared, we conclude that all measured bulk flows (with the possible exception of that of Lauer & Postman) are consistent with each other given the errors, provided the latter allow for `cosmic covariance'. A rough estimate of the mean bulk flow of all surveys (except Lauer & Postman) is ~400 km/s towards l=270, b=0.