Vistas in Astronomy最新文献

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The angular resolution of ∼ 0.15 arcsec (∼ 10 pc), achieved by MERLIN, is ideal for investigation of neutral gas in the nuclei of nearby Seyferts on scales of < 10 pc. We present, here, such observations of the radio source associated with the nucleus of NGC4151. We find that only the component containing the nucleus shows significant absorption, whereas no measurable absorption is seen against other components within the 4 arcsec radio jet. There is structure in the absorption on scales as small as 0.1 arcsec, with column densities in excess of 6 × 10²¹ atoms cm⁻² over parts of the radio nucleus. The neutral hydrogen column densities are much higher than UV absorption measurements which implies that the optical/UV continuum nucleus is not coincident with the strongest radio component. We suggest that the radio nucleus is associated with a weak VLBI component which is ∼ 0.1 arcsec (∼ 7 pc) to the west of the deepest absorption. We deduce that any neutral hydrogen torus, in which the nucleus is embedded, can be no thicker than 50 pc and may have a linear (E-W) extent of < 40 pc.

The angular asymmetries and relative separations of the hot-spots of FRII radio sources in a complete sample of 3CR sources have been used to study the effects of misalignment of the directions of ejection and the velocities of the hot-spots, in conjunction with a test of Barthel's models which unify radio galaxies and quasars. It is found that the observed distributions of asymmetry angles in radio galaxies and quasars are consistent with unified models, and with the source components being ejected uniformly within an angle of 6–7° about the source axis. The separation quotients are also consistent with the scheme if the velocities of the hot-spots have a broad distribution about a mean of 0.2c, with values extending at least up to 0.4c.

The hard-edged ionization cone seen by the HST imaging of NGC 4151 and the line-of-sight gas observed in some Seyfert 1 galaxies present a strong challenge for the simplest version of the Unified Model of AGN, which predicts that bi-conically shaped EELR can be observed only in type 2 objects and the line-of-sight to type 1 nucleus must be clear. To accommodate these rather contradictory requirements we suggest a modification of the shadowing/obscuration scenario. In our model the collimation of the ionizing radiation is provided by a partially neutral atmosphere surrounding the physically thin, dense torus. This atmosphere provides a natural way of producing wide angle ionization cones and at the same time allows for a relatively clear view of the central regions for some angles outside the cone edges. The model has an observationally testable predictions. Recently discovered nuclear disks in some elliptical galaxies hosting AGN may provide a natural way of supplying the gas for both the torus and the atmosphere surrounding it.

In the simple unified model for Seyfert nuclei, type 2 Seyferts are those where direct view of the central source is blocked by a dusty molecular torus. This simple model is able to explain many observed properties of Seyfert nuclei but fails to explain the UV excess observed in the continuum of many type 2 Seyferts, the relatively low level of continuum polarization, and the undiluted near IR stellar absorptions. The simplest solution is to postulate a continuum source on top of the scattered nuclear one. I discuss here some of the implications of having young stars in the torus providing the additional continuum.

We present preliminary results of a quasar survey based on cross-matching the Molongolo radio catalogue with unresolved optical objects on b_j plates. We find (i) a radio-optical correlation, with slope and dispersion consistent with unified scheme predictions; (ii) similar evolution at low z to radiogalaxies of comparable radio luminosity, consistent with an invariant torus opening angle; (iii) no evidence for a decline in the comoving number density of the most luminous radioquasars at z ≤ 2.5, in keeping with the current limits on the cutoff in radiogalaxies; (iv) possible evidence for a low quasar fraction in the z > 2 radio-loud AGN population.

This paper reviews the properties of so called “ionization cones”—extended regions of emission-line gas apparently photoionized by oppositely directed, bi-conical beams of radiation emitted by an active nucleus. Ionization cones have been found in about 15 Seyferts, mostly of type 2. When single (one-sided) cones are seen, they generally project against the far side of the galaxy disk, suggesting that the counter cone is present but obscured by dust in the disk. So far, clear examples of cones have not been seen in radio-loud AGN's. The emission-line cone and radio axes are very tightly aligned (to ≤ 6°), indicating that the ionizing photons and radio-emitting plasma are collimated by the same, or coplanar, disks. However, the radio ejecta are much more tightly collimated than the ionizing photons. Various lines of evidence point to strong obscuration, probably by a dusty torus, at the apex of the cones in some Seyfert 2's. The origin of the collimation of the ionizing photons remains unclear—both shadowing of an isotropic UV source by a dusty torus and direct radiation from an intrinsically anisotropic emitter (such as a radiation torus) remain viable models. Cones may be seen in a few Seyfert l's; possible explanations are reviewed. The observed opening angles of the emission-line cones are, in general, expected to differ from the true opening angles of the ionizing photon cones through simple projection effects.

We discuss optical broad band B-I maps of Seyfert galaxies, which show double or elongated blue morphological features. In NGC 1068, NGC 3227, Mrk 3 and Mrk 573 these features are roughly parallel to their extended radio emission and probably arise from scattering of nuclear light by extranuclear “mirrors”. In Mrk 78 and Mrk 348, there is a bluer region set against the red nucleus and host galaxy. In these cases the scatters may be either less efficient and/or further away from the nucleus than in the other galaxies. Our results demonstrate the usefulness of colour maps in enhancing the visibility of low contrast features.