Abstract

Magnetic forces seem likely to play the dominant role in both the acceleration and initial collimation of relativistic jets in AGNs. I describe recent developments in the theory of hydromagnetic jets and winds.

Introduction

Hydromagnetic propulsion as a mechanism for accelerating jets has become attractive largely through a process of elimination. Other mechanisms, such as acceleration by gas or radiation pressure, have been examined and found inadequate. The observational case for relativistic flow velocities, on both pc and kpc scales, continues to mount (see, e.g., and for recent reviews). Models involving acceleration by radiation pressure would have to be stretched to extremes in order to account for the Lorentz factors ∼ 2 – 10 which are needed to explain most instances of onesidedness and superluminal motion. Losses due to catastrophic cooling place even more severe constraints on acceleration by gas pressure. Recent observations of “intra-day” radio variability, may require Lorentz factors as high as ∼ 100 if catastrophic Compton losses are to be avoided.

Magmetic propulsion has other attractive features besides the ability to produce the high speeds indicated by observations. Chief among these is the tendency of magnetically driven flows to “self-collimate” due to the development of a predominantly toroidal magnetic field. Thus, it may not be necessary to invoke a “funnel” or external confining medium to explain the collimation of jets.

The Seyfert 1 galaxy NGC5548 has been monitored between 1988 December and 1989 October in the optical by an international collaboration (Peterson et al., 1991). Parallel to the optical monitoring, this galaxy was observed every 4 days with the IUE-satellite from Dec. to Aug. (Clavel et al., 1991). The internal calibration of the spectra was done by scaling with respect to narrow forbidden lines.

The optical emission lines Hα, Hβ, HeI5876, and HeII4686 show the same variability pattern as the UV continuum, the Lyα, and CIV1548 lines, but with different delays for the various lines (Fig. 1a — d) (Dietrich, Kollatschny, et al., 1992). We estimated the extent of the broad line region for different lines by cross-correlating the UV-continuum and the emission lines. For the Hα and Hβ lines we got lags of 17 and 19 days respectively. The helium lines originate closer to the central continuum source, as indicated by delays of 7 days (HeII4686) and 11 days (HeI5876).

In Figure 2a,b we have plotted the temporal evolution of the difference spectra of Hα and Hβ for several epochs with respect to that epoch when the galaxy was in the minimum state. The full width at zero intensity (FWZI) remained constant. This indicates a turbulent velocity field in the broad-line region of NGC5548. Furthermore, a blue component (vblue ≈ −2000kms−1) is visible in addition to the central component. This blue component varies independently and with a different amplitude than the central component (Kollatschny and Dietrich, 1991).

Introduction

The existence of microvariability at optical wavelengths has been clearly demonstrated for BL Lacertae objects by a number of groups during the past several years (Miller & Carini 1991, Carini & Miller 1992, Wagner et al. 1991). However, in no instance has the nature of the microvariability been investigated when a blazar was near a minimum in brightness in its long-term variability. Thus, the blazar AO 0235+164 was selected to be monitored with the goal of determining whether or not rapid variations are present when the object is near its minimum brightness level, based on its known historical variability.

Observations

The observations of AO 0235+164 reported here were obtained with the 42-inch telescope at Lowell Observatory equipped with a direct CCD camera. The observations were made through an R filter with an RCA CCD. Repeated exposures of typically 300 seconds were obtained for the star field containing AO 0235+164 and several standard stars. These standard stars, located on the same CCD frame as A0 0235+164, provided comparisons for use in the data reduction process. The observations were reduced using the method of Howell & Jacoby (1986). Each exposure is processed through an aperture photometry routine which reduces the data as if it were produced by a multi-star photometer. Differential magnitudes can then be computed for any pair of stars on the frame.

Abstract

Recent work on the quasar luminosity function at optical and X-ray wavelengths is reviewed. It is shown that the evolution of the quasar luminosity function in these regimes is marked by a strong and approximately similar power law increase in luminosity, L ∝ (1 + z)3±0.5, between the present epoch and z ∼ 2. At z > 2, a slow-down in the rate of quasar evolution is witnessed in both regimes with possible evidence for a decrease in the space density of quasars being seen amongst optically faint (MB > −27) QSOs at z > 3.5.

Introduction

The quasar luminosity function (LF) is one of the most fundamental statistics relating to the quasar population. Estimates of the quasar LF and its evolution with redshift are normally obtained from the statistical analysis of large unbiased quasar surveys with complete spectroscopic identification. As such, the rapid increase in the number of such surveys in recent years, particularly in the optical and X-ray regimes, has led to a dramatic improvement in our knowledge of the quasar LF and its evolution. The purpose of this review is to describe the current observational status of the quasar LF in the optical (4400Å) and X-ray (∼ 2keV ≡ 6.2Å) regimes.

The Optical Luminosity Function

As a result of the recent improvement in quasar statistics at B > 20, it has become increasingly clear (Koo 1983, Marshall 1987, Boyle et al. 1988, Koo & Kron 1988) that the low redshift (z < 3) quasar optical LF (OLF) exhibits a significant break in its power law slope at faint absolute magnitudes.

Introduction

The intense X–ray emission of AGN (active galactic nuclei) can heat the gas in these objects to high temperatures, driving a wind from regions in which the thermal velocity is comparable to or greater than the escape velocity (Begelman et al. 1983). Other mechanisms, such as heating due to dissipation of magnetic fields, or acceleration by rotating magnetic fields or radiation pressure, can also produce winds in AGN; thus, X–ray heated winds may be considered to be the minimum required by observation. These winds are important both because they can alter the accretion rate onto the central object by extracting mass, and because they provide important diagnostics of the distribution and dynamics of gas in AGN (Begelman and McKee 1983).

The nature of the wind is determined by the geometry of the gas relative to the source of the X–rays. The variability of the X–ray emission in AGN indicates that the source of the emission is compact (e.g., Turner and Pounds 1988). The gas may be distributed around this compact source in several possible ways: First, it could be in an accretion disk, although direct observational evidence for this assumption is lacking at present; by contrast, there is good evidence for accretion disks in many binary X–ray sources in the Galaxy. A wind will be driven from an accretion disk either if the disk flares (as it does in the standard α disk—Shakura and Sunyaev 1973) or if the source of the X–rays is above the disk (as in Compton reflection models—Fabian, this volume).

Abstract

The broad emission lines of active galactic nuclei exhibit a wide variety of profile shapes and widths. A simple model is used to illustrate the possible significance of this diversity and to gain some insight into its physical origin.

Introduction: from igloos to Eiffel Towers

The broad emission lines are often the most prominent features in the optical-UV spectra of active galactic nuclei. Understanding how they are formed is therefore a key problem, not least because the emitting gas is thought to be closely associated with the fundamental energy source and so must be strongly influenced by its radiation field and the dynamical forces it produces. It is generally believed that the great widths of the line profiles arise mainly from large bulk velocities of the emitting gas. However, the structure and dynamics of the broad line region (BLR) are not understood in detail, although many possible models have been proposed (Section 2).

The shape of the emission line profile is one of the principle observational constraints on theories of the BLR. Initially, it was thought that the observations were adequately described by logarithmic profiles (Lλ∞–ln[Δλ]), as expected for radiation pressure acceleration (Blumenthal & Matthews 1975). However, as the quality and quantity of data have increased, it is becoming clear that other forms (e.g. power laws—van Groningen 1983; Penston et al. 1990) are often more appropriate.

Abstract

We fail to detect a polarized broad line in the 2% polarized narrow line radio galaxy Cygnus A. The possible explanations are that a diluting dichroic component is present (in which case more sensitive observations should show the polarized broad line), that a hidden broad line nucleus is being scattered by electrons which smear out the broad line, or that Cygnus A really is a narrow line radio galaxy and contains no “hidden quasar”.

Introduction

There is now considerable evidence that some apparently narrow emission line objects contain hidden broad line regions. The first discovery in this field was the observation (Antonucci and Miller 1985) that the Seyfert 2 (narrow line) galaxy NGC 1068 had a broad line region in polarized light, which they interpreted as a scattered image of a broad line nucleus whose direct line of sight was blocked by an obscuring torus. Further such discoveries in Seyfert galaxies (Miller and Goodrich 1990), in one broad line radio galaxy (3C234, Antonucci 1984) and in one low–power narrow line radio galaxy (Inglis et al 1992) lend support to the idea that at least some narrow line objects would appear as broad line objects if seen from a suitable angle.

Most of the objects so far shown to contain polarized broad lines have been low–luminosity objects. Proposals that narrow–line and broad line high–luminosity, radio loud objects can be unified by a scenario such as that of Antonucci and Miller have been made over the last few years (Scheuer 1987, Peacock 1987, Barthel 1989).

Abstract

We use the lack of a significant silicate or silicon carbide emission feature in bright AGNs at λ ∼ 10 µm to constrain the properties of the dust. We first calculate the optical properties of grains in the 0.005 – 10 µm size range, over the 1000 µm – 1Å wavelength range. We use these grain models to calculate the emission of optically thin and of optically thick dust, incorporating both absorption and scattering in the radiative transfer. A galactic dust composition in any configuration which is optically thin at 10 µm produces a very strong silicate emission feature and is clearly ruled out. We list what grain compositions, grain size distributions, and dust optical depths are consistent with the absence of a 10 µm feature. Independent dynamical arguments lead to a very similar set of constraints on the dust properties. We finally list the implications of these constraints on dust reddening, and line emission from AGNs.

Introduction

About a third of the bolometric luminosity of quasars and bright Seyfert 1 galaxies, is emitted in the 1 – 100 µm range (Sanders et al. 1989). The emission mechanism is not yet established, but an attractive possibility is thermal reprocessing of the optical to far UV emission from the central continuum source by nearby dust. This hypothesis is supported by the IR spectral shape, its low amplitude variability, lack of significant polarization, and the near IR response to UV variations (e.g. Clavel, Wamsteker & Glass 1989).

Abstract

We present highlights of a multi-band optical imaging study of the ENLRs in seven Seyfert galaxies. Flux-calibrated narrowband emission-line and continuum images obtained in subarcsecond seeing conditions with the 3.6m CFHT reveal a remarkable wealth of emission-line and optical continuum structures. We show an example from our data of an apparent mechanical interaction between radio plasma ejecta and the ENLR, and describe an extended near-UV continuum component, which appears to be scattered nuclear continuum light, that is seen in all of the five Seyfert 2s, but is conspicuously absent in the two extended-emission Seyfert 1s.

Introduction

Many Seyfert nuclei are surrounded by kpc-scale regions of highly ionized gas known as the “extended narrow-line region” (ENLR). The energetics, ionization state, and morphology of the ENLR have been one of our most powerful probes of the interaction between a Seyfert nucleus and its host galaxy environment. Traditionally, the ENLR in Seyferts has been mapped using CCD images taken through narrow interference filters isolating the bright emission lines of Hα+[N II] and [O III]λ5007. These lines bracket a wide range of ionization potential, and so provide a simple, qualitative diagnostic of the ionization state of the gas. Two-band imaging surveys have had a fair amount of success so far at revealing extended emission in Seyferts and other AGN [1,2]. Often, however, one wants more quantitative information such as a particular line ratio which is diagnostic of a nebular parameter.

Abstract

The Hubble Space Telescope (HST) has been producing data from regularly scheduled observations for some time now. At this juncture, it is an appropriate time to review some of the areas where instruments such as the Faint Object Spectrograph (FOS), when coupled with ground-based data, may improve studies of Active Galactic Nuclei (AGN) and, hopefully, stir the imaginations of the theorists! We show examples of objects where the data indicate that the emission line properties of high- and low-luminosity objects are similar, lending support to relatively simple models in which the emission line regions of AGN scale homologously. We note, however, that the definition of what the ‘normal’ properties of AGN are rather than individual peculiarities will have to await complete samples of objects with data covering as large a wavelength range as possible.

Line Ratios.

It has become feasible in recent years to study the Balmer lines of high redshift QSOs in some detail and to determine velocity-resolved ratios such as those of Lyα/Hα – see, for example,. Even more recently, it has become possible to regularly obtain UV data with the HST and to perform more detailed analysis than was possible using IUE spectra. As an example, in Figure 1 we show a comparison between the Lyα/Hα line ratios in two radio-loud objects of similar linewidth and velocity shifts.

The 3C273 data are from [4] and the PKS 1448 – 232 are taken from [2].

Abstract

We analyse the properties of a sample of polarized radio galaxies with a wide range of redshifts. Our results suggest that a large fraction of UV light in distant radio galaxies is scattered anisotropic nuclear radiation.

Introduction

High redshift radio galaxies (HZRGs) have elongated morphologies (emission lines and continuum) aligned with the radio axis. The alignment of the continuum is strong in the ultraviolet and weak in the infrared around 1µ (rest frame) [2]. While the IR round component can be explained with an evolved stellar population, the nature of the UV aligned component is still a matter of debate. Two origins have been proposed for the UV light : (a) star formation induced in the ISM by the radio jet (b) dust/electron scattering of anisotropic nuclear light. We present new data which show that at least a considerable fraction of the UV light is not stellar.

The sample and the correlations

We have collected a sample of 41 radio galaxies with 0.1≤z≤1.2 taken from our survey ([1]) and from the literature. We find that the degree of linear polarization P increases with redshift and with the total radio power at 178 MHz (Pr), while it decreases with the rest frame wavelength of the observation (λrest). The main problem in the interpretation of these correlations is that the quantities z−Pr−λrest are interdependent and it is not possible to clarify which correlations are intrinsic and which derived.