Vistas in Astronomy最新文献

We report on a detailed study by the Lovers of Active Galaxies collaboration of the gas and dust surrounding the active nucleus in NGC4151. The ionization structure of the extended narrow line region (ENLR) is broadly consistent with photoionization by the AGN radiation field. The jet-like morphology of the ENLR, and its misalignment with the arcsecond-scale radio source, can be explained if ionizing radiation emerges from the nucleus in a broad inclined cone and illuminates the galactic disk at grazing incidence. The overall distribution of gas and dust suggests that bar-driven gas flows are important in the circum-nuclear region and that these are intimately linked to the material collimating the ionizing radiation field.

To overcome the problem of noise amplification during the image reconstruction process, we present a new method based on wavelet decomposition. After decomposing the raw image to be reconstructed into several wavelet planes and a residual image, we reconstruct each one independently using an iterative Maximum Likelihood algorithm. To control the processs, we stop the reconstruction of each one of the wavelet planes and the residual image at a different number of iterations. The inverse problem is then transformed into a multi-channel reconstruction problem. The method has been applied to a real image of the planet Saturn obtained by the non-refurbished Hubble Space Telescope.

The origin of the discoveries, both on the Sun and in the laboratory, of the action of a magnetic field on spectral lines—the so-called Zeeman effect—is studied. The paper embraces the period from 1866, first date of which the author is aware of observed evidences about the widening of spectral lines in sunspots (as compared to those formed in the photosphere), until 1908, year in which the magnetic filed in sunspots is definitely discovered. The interval between 1896–1897, and 1908 is mainly dealt with from an astrophysical standpoint, although there are plenty of important contributions from laboratory experiments. The reason is two-fold: on the one hand, the significant role played by the Zeeman effect on the development of quantum mechanics has suggested major historical studies that have already appeared in the literature and that are mainly concerned with laboratory—but not with astrophysical—spectroscopy; on the other hand, the understanding of the sizeable delay between Zeeman's and Hale's discoveries (12 years) seems to be of concern after accounting for the fact that the findings by the first author were soon brought to the notice of the astrophysical community.

A modified version of the code of Efstathiou and Rowan-Robinson (1990), that solves accurately the axially symmetric radiative transfer problem in dust clouds, is used to model the infrared emission from dust in Active Galactic Nuclei. The method takes into account a distribution of grain species and sizes and includes treatment of multiple scattering from grains. Arguments are presented supporting the idea that tapered discs (discs whose height increases with distance from the central source but tapers off to a constant height in their outer part) with steep density gradients are the most successful in satisfying the observational constraints.

We present HST polarization observations of NGC 1068 obtained with the Faint Object Camera in the ultraviolet (λ ∼ 2700–3700 Å), and the Wide Field Planetary Camera in the visual (λ ∼ 5000–6000 Å), at a resolution of 0.″06 and 0.″08, respectively. The UV continuum polarization is very high, peaking at ∼ 60% in the vicinity of the emission line knot 4″.5 NE of the nucleus. To a high degree of precision the polarization vectors show the centro-symmetric pattern expected from scattering from a point source. By locating the centre of symmetry of this pattern we have determined the location of the hidden nuclear source, to an accuracy of ±0.″05, which lies 0.″65 South of the emission peak and 0.″35 South of the 12.4μ peak. A pair of highly polarized clouds (P ∼ 45% in the UV) lies close to the position of the scattered nuclear source and correspond to the “twin cresent” object seen in FOC [OIII] images. The WF/PC-I polarization images contain contributions from both the optical continuum and emission lines. Generally, the large scale polarization structure is dominated by scattered [O III] emission from knot B, except at the edge of the cavity when other NLR knots contribution. However, scattered continuum from the nucleus dominates the polarization in a narrow linear region stetching from the “hidden nucleus” to knot B, suggesting that there is a component to the nuclear light which is highly collimated.

We present subarcsecond MERLIN 21cm neutral hydrogen absorption line measurements of a 4 × 3 arcseconds (300 × 200 pc) region associated with the active nucleus of the Sc galaxy NGC3079. Broad (200–400 km s⁻¹) and deep (τ ∼ 0.5–1) absorption is seen over much of the region, corresponding to column densities as high as 3 × 10²² atoms cm⁻². The absorption shows a velocity gradient which could be consistent with rotation about a central mass of ∼ 10⁹ M_⊙, however significant non-circular motions are also present which may be due to gas motions in a non-axisymetric potential.

Recent experience is helping to sharpen our understanding of how the electronic world differs from the centuries-old, paper-based communication methods. The electronic future is highly interlinked, with all different types of information being included in the web of available resources. The enhanced ability to interchange information electronically improves the effectiveness of communication. At this time the author community does not make full use of the new capabilities. Most astronomers writing for electronic distribution have been slow to adopt new styles of presentation which exploit the new capabilities inherent in the electronic environment. By working together and sharing the burden, the community's resources can be multiplied. A coordinated distributed effort can yield a much more valuable product than any single person or group. Long term archiving and site maintenance are important responsibilities for publishers of electronic scholarly information. Ensuring access to electronic information into the foreseeable future takes planning from the beginning, and requires cooperation between the publishers, libraries, and other scholarly institutions.

The paper is concerned with two of main research activities currently carried on at the Computer Science and Artificial Intelligence lab of DIE. The first part deals with hybrid artificial vision models, intended to provide object recognition and classification capabilities to an autonomous intelligen system. In this framework, a system recovering 3-D shape information from grey-level images of a scene, building a geometric representation of the scene in terms of superquadrics at the geometric level, and reasoning about the scene at the symbolic level is described. In the second part, attention is focused on automatic indexing of image databases. JACOB, a prototypal system allowing for the automatic extraction from images of salient features like colour and texture, and for content-based browsing and querying in image and video databases is briefly described.

A wide-field astronomical image is often considered as a set of quasi point-like sources spread on a slow-varying backgound. With this model, the image is described as a set of connected fields. We have to code the field positions, the field boundaries and their pixel values. It exists different methods for coding this information, they are mainly connected to the Mathematical Morphology. The fields may be coded from their contours, their binary skeletons or the grey-tone ones. The morphological skeleton transformation in general gives us the best results. The H-transform is a two-dimensional generalization of the Haar transform often used for compressing astronomical images. Blocking effects appear in the restored image. The quality of the restoration is improved by introducing an a priori knowledge on the solution. These two different approaches lead to high compression rates on classical astronomical images. The best compression technique is directly connected to the image modelling.