This paper gives an overview of the global properties of Active Galactic Nuclei, introducing and putting into context those areas that will be more fully described later in this volume. For the new entrant into AGN research, a semi-historical description of how the field developed is also given; showing how far we have come in a relatively short time. The pioneering work is now done, the big picture of how AGNs work is understood, we now embark on the search for the precise details of the emission mechanisms and their interrelations.

The HST has made many contributions to all areas of research in the field of AGN, and I have selected three topics where major progress in our understanding has been made over the last two years. The study of the NLR is key to understanding to what extent the unified model for AGN is applicable. In particular, understanding how the NLR is ionized and how its morphology is defined makes an important contribution to clarify the differences between Seyfert 1 and Seyfert 2. Work by Macchetto et al., Capetti et al., Axon et al., and Wilson et al. has helped clarify the picture. Our work on NGC 1068, Mrk 3, Mrk 7, Mrk 348, Mrk 6, and Mrk 573 has shown the following important properties: a) all AGN with a linear radio jet show emission-line morphology ([O III], [O II], Hα) which is aligned along the jet in a surrounding cocoon; b) those AGN with radio lobes show emission-line morphology which is filamentary and coincident with the position of the lobes; and c) in NGC 1068 and in Mrk 3 we have measured transverse velocities to the radio-jet as large as 1700 km s−1. These velocities measured at different positions across the radio-jet show an almost perfect velocity ellipsoid, indicating that the cocoon around the jet is expanding, compresses the ISM and shocks and ionizes the region.

Infrared observations of complete samples of active galactic nuclei (AGN) have shown that a substantial fraction of their bolometric luminosity is emitted at wavelengths ˜8–1000μm. In radio-loud and Blazar-like objects much of this emission appears to be direct non-thermal synchrotron radiation. However, in the much larger numbers of radio-quiet AGN it is now clear that thermal dust emission is responsible for the bulk of radiation from the near-infrared through submillimeter wavelengths. Luminous infrared-selected AGN are often surrounded by powerful nuclear starbursts, both of which appear to be fueled by enormous supplies of molecular gas and dust funneled into the nuclear region during the strong interaction/merger of gas rich disks. All-sky surveys in the infrared show that luminous infrared AGN are at least as numerous as optically-selected AGN of comparable bolometric luminosity, suggesting that AGN may spend a substantial fraction of their lifetime in a dust-enshrouded phase. The space density of luminous infrared AGN at high redshift may be sufficient to account for much of the X-Ray background, and for a substantial fraction of the far-infrared background as well. These objects plausibly represent a major epoch in the formation of spheroids and massive black holes (MBH).

We discuss the compact radio sources associated with quasars and AGN with particular emphasis on the implications of their structure and kinematics for relativistic beaming models.

Megamaser emission provides an unprecedented view of the inner nuclear regions of active galaxies. The characteristics of OH, H2CO, and H2O sources identify different physical environments in the nuclei of megamaser host galaxies. Various classification schemes suggest a dominant starburst nature for the OH and H2CO sources providing the NIR/FIR radiation field as a pump. The molecular emission originates in a 50–100 parsec disk/torus, which is not yet completely observed. For H2O masers in relatively dust-free LINER/Seyfert 2 sources, the AGN provides the X-ray radiation field for pumping in a compact sub-parsec thin disk.

We present a new diagnostic tool based on mid-infrared spectra (5–16μm) for distinguishing the emission triggered by the active galactic nucleus (AGN) and by the star formation activity. We show that the AGN spectra, contrary to the starburst spectra, present an important continuum below 9μm and an absence of Unidentified Infrared Bands (UIBs).

Luminous Infrared Galaxies (LIGs) are locally more numerous than normal galaxies, AGN, and QSOs above L ˜ 1011L⊙ and may be the evolutionary precursors of classical radio-quiet quasars. VLBI observations of a complete sample show that high-Tb radio cores are common, perhaps universal among LIGs. VLBI imaging shows that these radio cores may be produced by intense starbursts which generate luminous radio supernovae, as in the case of Arp 220 (Smith et al. 1998), or by a classical AGN core, as in the case of Mrk 231, which we interpret as a newly formed QSO emerging from a starburst. Compact OH 1667MHz maser emission appears to be common in LIGs and may be related to AGN activity. These results lend further support to the scenario suggested by Sanders et al (1988) in which mergers of gas-rich galaxies lead first to luminous starbursts which evolve into radio-quiet quasars.

The WFPC2 observations made by J. Westphal, in July 1995, allow us to confirm the existence of condensations O', O1 and O2; they are located at intersections between the radio jet and filaments emitting in the continuum and in the lines. The depolarization of the radio jet at the position of knot R1 is related to the interaction of the jet with a strongly ionized region of the galaxy. We also detect additional knots closer than 2.5” from the nucleus. The high angular resolution brings precise measurements in position and flux. The optical and radio positions are in good agreement and the radio/optical spectral indices of the knots are as expected in the case of synchrotron radiation.

Low Ionization Nuclear Emission Line Regions (LINERs) are found in ˜30% of all bright galaxies. The nuclear luminosities in these objects are such that they can be produced by a number of mechanisms and there have been heated debates on the nature of ionizing sources in LINERs. The variety of ionizing mechanisms suggested are low luminosity AGNs, starbursts, shocks, or any combination of these. We have studied Hubble Space Telescope (HST) ultraviolet (UV) spectra of seven LINERs having compact nuclear UV sources.

The picture emerging from this comparison is that the compact source observed in these LINER galaxies, at least in some cases, is a nuclear star cluster rather than a low-luminosity active galactic nucleus (AGN). In these cases, the UV luminosity is driven by tens of thousands of O-type stars, depending on the assumed extinction for these objects. The O-stars could be the high-mass end of a bound stellar population, similar to those seen in super star clusters. Our data do not exclude the possibility that a similar stellar continuum source could dominate in all the LINERs. Alternatively, there may be two types of UV-bright LINERs: those where the UV continuum is produced by a starburst, and those where it is nonstellar.

The “clearly-stellar”, weak [O I] emitters, LINERs have relatively weak X-ray emission, and their stellar populations probably provide enough ionizing photons to explain the observed optical emission-line flux. The other LINERs, strong [O I] emitters, have severe ionizing photon deficits, for reasonable extrapolations of their UV spectra beyond the Lyman limit, but have an X-ray/UV power ratio that is higher by two orders of magnitudes than that of the “clearly-stellar” LINERs. A component which emits primarily in the extreme-UV may be the main photoionizing agent in these objects.

Recent results show that nuclear-starburst and quasar-like activity are often intermingled. Our results extend this result to the lower luminosities of the LINERs.

We present J and K band imaging polarimetry and optical and K band spectropolarimetry of the Circinus galaxy. The imaging polarimetry shows a bipolar scattering cone in the J band and a more compact structure in the K band. The spectropolarimetry shows broad polarised hydrogen alpha in the optical, however broad lines are not detected in the K band. Analysis of the observations show that galactic and stellar processes dominate in the optical total and polarised flux whilst the nucleus dominates in the near-IR polarised flux. Modelling of the observations show that the small fraction of nuclear polarisation in the optical is due to electron scattering whilst the K band polarisation is dominated by dichroism.

Narrow Line Seyfert 1 galaxies (NLS1s) are an important subclass of radio quiet AGN having extreme optical and X-ray spectroscopic properties. Their relationship to other types of Seyferts remains unclear. NLS1s exhibit many characteristics of Seyfert 1 AGN, but their optical spectra show narrow permitted lines, and some high ionization species. This may result from us viewing them close to ‘pole-on' orientation with respect to an inner torus geometry. We present comparisons of the radio and X-ray properties of NLS1s with those of the 12μm and CfA Seyfert samples.

We present HSTSpace Telescope Imaging Spectrograph (STIS) slitless spectroscopy of the NGC4151 narrow line region (NLR) as a probe of the kinematic stucture of the extended emission-line gas emanating from the nucleus. Using slitless spectroscopy at two roll angles (with a spatial resolution of 0.051 ″/pixel and a point source spectral resolution of 0.55 Å) augmented with narrow band images, we have mapped the velocity field of the NLR as defined by ˜60 discrete cloud structures in [OIII]. Flux measurements of [OII], Hβ, [OIII], [OI], and [SII] were made for individual cloud structures wherever possible.

The main observational properties of AGNs at centimeter-decimeter wavelengths –- spectra, variability on various time scales, polarization and milli-arcsecond maps –- are in agreement with calculations in a jet model with a strong quasi radial magnetic field. Typical spectra and VLBI structures can be explained by a simple case of a straight line jet, unlike the degree of the linear polarization. Curved jets in the model allow decreasing the polarization to observational values as well.

The Seyfert 1.5 galaxy Akn564 [1] is a well known X-ray active galactic nucleus (AGN) included in our list of selected barred AGN. The galaxy was observed during the August 1996 season at the 2-m RCC telescope of the Astronomical Observatory “Rozhen” of the Bulgarian Academy of Sciences. ST-6 with standard Schott V, R, I filters were used. MIDAS'96 package was used for the data reduction with teh Richter's expansion for the surface photometry [2]. Most of the basic data for the galaxy are shown in the Table 1 below.

Akn 564 (α1950 = 22h40m18.3s, δ1950 = 29°27′47″) is a Sy1.5G SBb type galaxy. According to Zwicky (1966) it has a photographic magnitude mp = 14.4 and a redshift of 0.025. The spectra of the galaxy were obtained at the 2.6-m telescope of the Crimean Astrophysical Observatory with a spectrograph having a dispersion of 100 A mm−1. They were processed with the help of SPEC and LONG packages integrated in MIDAS. As a result of the spectrophotometry we obtain the fluxes at λλ 4363, 4959, 5007 A: I(4363), I(4959), I(5007). The spectrum of the galaxy in λλ 4000–7000 is shown in Figure 1. We use the relation of the fluxes of those narrow forbidden emission lines: sensitive at a greater extent to the electron temperature Te than to the electron density ne. The value of R = 74.3 we got, having a typical value of ne = 5×105 cm−3 for the NLR (Narrow Line Region), leads to the estimation of a typical temperature of Te = 104 K.