Results of a spatial stability analysis and of numerical simulations of a “slab” jet in which optically thin radiative cooling is dynamically important are presented. Two different cooling curves are used. Unstable Kelvin-Helmholtz modes are significantly different from the adiabatic limit, and the form of the cooling function strongly affects the results. The numerical simulations are in excellent agreement with the linear stability analysis. In the non-linear regime growth of the surface wave at low frequencies results in sinusoidal oscillation which can disrupt the jet, while non-linear body waves produce low amplitude wiggles within the jet that can result in shocks within the jet. In cooling jets, these shocks can produce dense knots and filaments of cooling gas within the jet, and weak shock “spurs” in the ambient gas. Acceleration of ambient gas can be produced by these “spurs”, or by rapid entrainment if the jet is disrupted. For parameters typical of protostellar jets perturbations with a period of < 100 yrs should excite body waves which produce internal shocks and small amplitude wiggles. The lack of large amplitude wiggles in most observed systems is consistent with the suggestion that jets arise from the inner regions (r < 1 AU) of accretion disks.

I give an overview of a promising signature for radiative acceleration in quasars “the ghost of Lyα,” which is seen in the spectra of a few broad absorption line (BAL) QSOs. The ghost appears as an absorption hump in the C IV λ1549 BAL and is centered around 5,900 km s−1 to the blue of the corresponding broad emission line (BEL). This velocity separation is the same as the velocity separation between the N V λ1240 and Lyα lines. Thus, when the N V BAL and the C IV BAL are plotted as a function of velocity (where zero velocity corresponds to the peaks of these BELs), the hump in the C IV BAL appears at the same velocity as the Lyα BEL and qualitatively resembles it. The best examples and model fits are shown and the radiative dynamics explained.

I describe a model of energetic gamma ray and neutrino emission in blazars which is consistent with most of the gamma-ray observations, and use this model to estimate the diffuse intensity of high energy neutrinos from blazars.

VLBI provides the only direct imaging technique currently available that is able to resolve even the outer parts of accretion disks. Recent observations of water masers in NGC 4258 and of free-free absorption of counterjet radiation in NGC 1275 are the best examples of where VLBI has provided important data on the parameters of disks. Other observations are now being made of similar sources. In addition, recent VLBI observations of HI absorption promise to add yet more constraints on the outer disk regions.

One of the main uses of polarimetry over the last decade, has been to identify the nature of AGN that are normally hidden from direct view by an optically and geometrically thick torus, and thereby to unify different classes of AGN. Of growing importance is the role that polarimetry can play in our understanding of the properties and structure of AGN on a size scale which cannot be resolved directly. We review the progress being made in understanding the role of disks, extended scattering regions, and the obscuring torus in AGN.

We show how the effects of Faraday rotation and a detailed treatment of the radiative transfer can address various aspects of the polarization problem of AGN accretion disks. In particular, we show how accretion disks might explain the rises in polarization observed blueward of the Lyman edge in several quasars.

In a fashion similar to winds in O stars, the AGN disk winds may be responsible for the broad, blue-shifted absorption of resonance lines which characterize the Broad Absorption Line QSOs. The dynamical features of the disk wind may also account for many of the properties of the broad emission lines of AGN. Although the velocity field of the material responsible for the line emission is primarily that of the accretion disk, the presence of an accelerated radial wind influences the line formation so that single-peaked profiles are generic. Further, the recent reverberation mapping response of NGC 5548 is also qualitatively explained by these same radiative transfer effects.

HST images have led to the discovery that small (r ~ 1″ r ~ 100 – 200 pc), well-defined, gaseous disks are common in the nuclei of elliptical galaxies. Measurements of rotational velocities in the disks provide a means to measure the central mass and search for massive black holes in the parent galaxies. The minor axes of these disks are closely aligned with the directions of the large–scale radio jets, suggesting that it is angular momentum of the disk rather than that of the black hole that determines the direction of the radio jets. Because the disks are directly observable, we can study the disks themselves, and investigate important questions which cannot be directly addressed with observations of the smaller and unresolved central accretion disks. In this paper we summarize what has been learned to date in this rapidly unfolding new field.

New HST images and spectra of the nuclear disks in two Virgo Cluster E/S0 galaxies reveal that they were probably formed together with their parent galaxies. The two galaxies show very different color gradients. This is most likely caused by the escape of high metallicity gas from early stars in the lower mass galaxy. One galaxy shows strong kinematic evidence for a massive central black hole.

VLBI observations of H2O maser line emission from the central sub-parsec region of the galaxy NGC 4258 have recently revealed a remarkably well-defined, rapidly rotating molecular disk which is viewed nearly edge–on. We show that the peculiar clustering of the maser sources into several distinct clumps which are almost equally spaced apart from one another, and the striking asymmetry in the high-velocity maser spectrum can be naturally explained by spiral activity in the disk.

Stellar kinematic studies indicate the presence of a concentrated central mass at just under 2 × 106M⊙, in close agreement with the mass deduced from gas velocities measured with the [Ne II] line. Although this mass is most likely a black hole, it may be dominated by a tightly concentrated cluster of stellar remnants. If Sgr A*, a point radio source coincident with this central mass, is a massive black hole embedded in a region with strong gaseous outflows, as suggested by the observation of He I, Brα and Brγ line emission, it is accreting from its environment via the Bondi-Hoyle process. We discuss the consequences of this activity, including the expected mass and angular momentum accretion rate onto the black hole, and the resulting observable characteristics. The latest infrared images of this region appear to rule out the possibility that this large scale flow settles down into a standard α-disk at small radii. We discuss some possible scenarios that might account for this, including strong advection in the disk or the presence of a massive, fossilized disk. Not all of the gas affected in this way by Sgr A*’s strong gravitational field becomes bound. Some of it is redirected into a focused flow that in turn interacts with other coherent gas structures near the black hole. We suggest that the mini-cavity (to the south-west of Sgr A*) may be formed as a result of this activity, and argue that the characteristics of the mini-cavity lend some observational support for the presence of a concentrated mass near Sgr A*. We show, however, that as far as the mini-cavity is concerned, this concentrated mass need not be in the form of a point mass, but may instead be a highly concentrated cluster of stellar remnants.

Compact radio cores are not only common in radio galaxies and quasars but also in many nearby galaxies with low-active, supermassive black holes. One famous example is the Galactic Center source Sgr A*. Recent studies of proper motions and radial velocities of stars in the inner parsec of the Galaxy convincingly demonstrate the presence of a compact dark mass of 2.5 · 106M⊙ in the nucleus of the Milky Way. Millimeter VLBI and submm observations of Sgr A* thus probe a region of only a few Schwarzschild radii in diameter. In this paper I will review our current theoretical and observational knowledge of this source and compare it to some famous LINER galaxies like NGC 4258, NGC 3079, and NGC 6500. In all cases these radio cores can be well explained by a standard AGN jet model, and, with the exception of Sgr A*, large scale outflows are observed that have powers comparable to those inferred from the radio cores. Recent VLBI observations of radio-weak quasars and HST observations of Seyfert galaxies indicate that these AGN also produce powerful jets which, however, have relatively less luminous radio cores than radio-loud quasars and the LINERs discussed here. Therefore, jets and compact radio cores appear to be natural constituents of an AGN, but the reason why apparently some jets are radio-loud and others not remains a mystery.

We have analyzed the distribution of giant molecular clouds and their radial velocities in the inner ~ 300 pc of the Milky Way in order to deduce the characteristic properties of its large scale motion. We find radial inflow of matter towards the Galactic Center of ~ 10−2 M⊙/yr. Moreover, we have determined the position of individual clouds along the line of sight.

Langmuir turbulence plays an important role in electron heating and generation of plasma emission in accretion disks of active galactic nuclei. The nonlinear dynamical behavior of Langmuir turbulence and its relevance in the interpretation of AGN variability is discussed. In particular, we study nonlinear saturation of the Langmuir stimulated modulational instability, for which the low-frequency mode is a resonant ion-acoustic wave. The nonlinear system of coupled wave equations is shown to undergo transition from order to chaos via the route of quasiperiodicity. The periodic, quasiperiodic and chaotic variabilities in AGN emissions may be the electromagnetic signatures of the ordered and chaotic states of Langmuir turbulence in accretion disks or jets of AGN.

Recently, superluminal motions are observed not only from active galactic nuclei but also in our Galaxy. These phenomena are explained as relativistic jets propagating almost toward us with Lorentz factor more than 2. For the formation of such a relativistic jet, magnetically driven mechanism around a black hole is most promising. We have extended the 2.5D Newtonian MHD jet model (Shibata & Uchida 1986) to general relativistic regime. For this purpose, we have developed a general relativistic magnetohydrodynamic (GRMHD) numerical code and applied it to the simulation of the magnetized accretion disk around a black hole. We have found the formation of magnetically driven jets with 86 percent of light velocity (i.e. Lorentz factor ~ 2.0).

In the past, the interaction of AGN jets with the surrounding medium has mainly been considered important only insofar as it gave us some understanding of the properties of jets. The emphasis of the work summarised here is different. In a recent paper (Bicknell, Dopita and O’Dea, 1996) we have proposed that the key optical and radio properties of Gigahertz Peak Spectrum (GPS) and Compact Steep Spectrum (CSS) sources, both treated as examples of Compact Symmetric Objects (CSOs) can be explained in terms of the interaction of a jet with a dense ISM - presumably the result of a merger. We therefore suggest that these sources represent an important evolutionary phase in Active Galactic Nuclei in which the interaction with the ISM is dominant.

We present observations of the old nova HZ Pup (Nova Pup 1963 No. 1) showing it to be an intermediate polar with an exceptionally rich selection of photometric periodicities arising from spin and orbital variations and beating between these two.

The physical origin of associated absorption lines (zabs ≈ zqso) which are observed in radio-loud quasar is generally not well understood. Two observable properties, line variability and quasar optical polarization, can serve to constrain the possibilities. To this end, we have obtained high-resolution spectra (1 Å) and broad-band optical polarizations for over 30 radio-loud quasars with known CIV associated absorption. The quasars are at redshifts between about 1 and 2, and were previously observed spectroscopically at similar resolution between 1985 – 1986 by Foltz et al (1987). The associated CIV absorption systems include a variety of velocity profiles from weak single lines to very complex multi-component systems to the BAL-like profile of PHL 1157+0128. In this poster we show the first strong evidence for line variability in z > 1 radio-loud quasars. We find that in general the variability is less than ~ 30%. The optical polarization of CIV absorbed quasars is consistent with all radio-loud quasars.

It is by now evident that most young stars have associated disks and/or envelopes, which may be active (accreting) or passive (reprocessing stellar photons), or both. Knowing how such disks evolve is crucial to our understanding of how stars form, and may be relevant to other questions, such as the time available for the formation of planets in solar nebulae. In this poster we discuss the properties of pre-main sequence circumstellar disks in Lynds 1641 in Orion, the nearest giant molecular cloud complex.

ASCA observations of the LMXRB 4U1624–49 reveal the presence of extended emission surrounding the X-ray source probably due to a dust scattered halo. The halo spectrum can account for the low energy excess seen in the dip spectrum.