Observations (Espey et al. 1989, Carswell et al. 1991, Marziani et al. 1996) at high resolution have shown relative shifts between the Balmer lines and the C iv line in a number of QSOs. In this paper, we suggest that in the BLR there are many cloudlets with dense gas that is optically thick, and the BLR is not only illuminated by UV-X radiation, but also by relativistic electrons. Therefore the Čerenkov line radiation has to be taken into consideration, and we calculate the Čerenkov line redshifts and the profiles.

The strength of optical Fe II lines seems to be connected to both X-ray spectral slope and X-ray loudness, as well as other interesting properties such as emission-line width and the presence of a low-ionization broad absorber. The reason for this cluster of correlations is far from clear.

For NGC 3516 we find that the X-ray warm absorption and the broad UV associated absorption features can be produced by the same absorbing material. We argue that the evolution of the XUV absorber from pre-1992 to 1995 is consistent with expectation for an expanding, outflowing material.

Combining information on absorbing material in AGN from X-ray and the UV creates a powerful investigative tool. Here we give examples from both low and high redshift.

Recent HST spectra indicate that a large fraction of Seyfert 1 galaxies have UV absorption lines that are intrinsic to the active nucleus or its environment. IUE monitoring or multiple HST observations of several sources show that many absorption lines are variable, either as a result of mass motion across the line-of-sight or of changing ionization within the gas in response to variations in the ionizing continuum. The UV absorbers may be related to the warm absorbers seen in the X-ray spectra of many Seyfert galaxies, but a direct connection between the two has not been made. Some of the absorption lines may arise in the emission-line regions in the nucleus or host galaxy, providing valuable clues to their nature.

We describe a warm absorber/emitter model that is used to model the complex X-ray spectrum of Mkn 290. The best fit to the data also includes a contribution from an ionized reflector.

We have studied the warm absorbers in several Seyfert galaxies using both PI and archival ROSAT data. The parameters that govern the emissivity of the ionized material, ionization parameter U, warm-absorber column density Nwa, and the source intrinsic power-law photon index Γx, are determined from X-ray spectral fits. These are then used to predict the expected IR to UV line emission of the ionized material in the individual objects. In particular, the possibility of a warm-absorber origin of one of the known emission-line regions in AGN is assessed. Good ‘candidate components’ are, e.g., the coronal-line region, and/or the high-ionization component of the NLR, or the component responsible for broad wings in the Balmer lines. All calculations have been carried out using the code CLOUDY (Ferland 1993).

This paper reports on AGN with extremely soft X-ray spectra observed with ROSAT. From their optical emission lines, these objects are classified as narrow-line Seyfert 1 galaxies (NLSl), almost all with extremely large Fe II/Hβ flux ratios and relatively narrow optical lines of hydrogen. NLSl have generally steeper soft X-ray continuum slopes than normal Seyfert 1s, and there may exist an anticorrelation between 0.1–2.4 keV continuum slope and the FWHM of the Hβ line. Objects with steep 0.1–2.4 keV continuum slopes and Hβ FWHM > 3000 km s−1 are clearly discriminated against by nature. When simple power-law models are fit to the data, photon indices reach values up to about 5, much higher than is usually seen in Seyfert 1s. Models with smaller-mass black holes and/or higher accretion rates show some promise to explain the relation between the FWHM of the Hβ line and the X-ray continuum slope. We further report evidence for persistent giant and rapid variability in the ultrasoft narrow-line Seyfert 1 galaxy IRAS 13224–3809.

According to photoionization calculations, the broad emission-line (BEL) spectrum depends not only on the physical conditions of line-emitting gas, but also on the spectral shape of incident ionizing continuum, especially from the UV to X-ray. Analysis of emission-line spectra and their correlations with the continuum, therefore, provide a way of probing the anisotropy of ionizing continuum on the BLR scale, which is predicted by current models. Previous works have concentrated on explaining the Baldwin effect, an inverse correlation between equivalent width and continuum luminosity. In this contribution, we present the results of an analysis of 75 AGNs which have well-determined soft X-ray spectral parameters from ROSAT and UV line and continuum measurements.

The correlation between the FWHM of Lyα, C IV and Hβ lines and the photon index of the soft X-ray continuum is investigated for a sample of PG quasars. Only the FWHM of Hβ line is correlated with the photon index and this correlation depends on the sample of the AGNs considered.

We present some of the results of a preliminary statistical study of 20 QSOs. We compare equivalent widths and widths at 40–80% of flux maximum of the Lyα, Si IVλ1400, C IV λ1549, and C III] λ1909 lines between radio-loud and radio-quiet QSO subsamples and with previously reported findings.

Recent investigations of the broad UV lines in luminous QSOs identified strong correlations involving emission-line widths, shifts, equivalent widths, and ratios (Francis et al. 1992; Wills et al. 1993; Brotherton et al. 1994a, b). A simple model developed to explain these trends approximates UV broad lines as emission from two regions, an intermediate-line region (ILR), and a very broad-line region (VBLR), together comprising the traditional broad-line region (BLR). The observed and derived properties for the ILR and VBLR are summarized in Table 1, along with typical values for the narrow-line region (NLR).

Comparison of the emission-line and continuum parameters of 48 low-redshift QSOs reveals the asymmetry of the C IV λ1549 broad emission line to be strongly correlated with ultraviolet continuum luminosity, such that increasing luminosity produces increasing redward asymmetry. A similar correlation has been found for broad Hβ. Redward profile asymmetries can be modeled as the result of the gravitational redshift of line photons from the very broad-line region (VBLR) by 109–1010M⊙ black holes, but blueward profile asymmetries require a competing effect such as electron scattering.

We recently completed the first comparison of high- and low-ionization broad lines in a sample of 52 AGN. We summarize here the principal results of a correlation analysis for this sample. We then briefly outline a model for the broad-line structure in radio-quiet AGN that is suggested by the correlation analysis.

This paper reviews the morphology, excitation, and ionization of the gas in the narrow and extended narrow line regions (NLRs and ENLRs) in AGNs. Many high-resolution images of the NLRs of Seyfert galaxies are now available from HST. In some galaxies, these images reveal straight, sharp-edged V-shaped structures (believed to be the projections of 3-dimensional cones), while in others there is a close association between the ionized gas and the radio components. The latter effect arises through radiative shocks driven into the surrounding interstellar gas by the outwardly moving radio components or jets. Photoionization models of ionization-bounded clouds are quite successful in reproducing the line intensity ratios, but a number of problems remain. These problems may be resolved if matter-bounded clouds are also present. Differences between the excitation levels of different NLRs and ENLRs may reflect variations in the fraction of matter-bounded clouds as well as variations in the ionization parameter. I discuss the issue of whether the radiation responsible for photoionizing the gas in Seyfert 2 galaxies originates from a hidden, compact type 1 Seyfert nucleus or from photoionizing shocks within the NLR. These two scenarios are hard to distinguish between. The photoionizing-shock model requires surprisingly high pre-shock densities, raising the question of whether the shocks are sufficiently energetic to power the observed line emission.

High-excitation extended emission-line nebulae around active galactic nuclei probably result either from photoionization of the interstellar medium by beamed continuum radiation, or dynamical interactions between radio sources and the ambient gas, or both. Here I discuss the models based on these mechanisms, and their relative importance in radio-quiet and radio-loud active galaxies.

Recent observations show that the CLR is small (a few tens of parsecs), has large column densities (N ≈ 1022 cm−2), and lies close to and is probably photoionized by the AGN. Coronal lines give information on Ne, Mg, Si, S, Ca, and Fe abundances, on the dust content and dynamics of the CLR, and can be used to trace hidden AGN. If they are powered by photoionization, coronal lines could be used to reconstruct the AGN spectrum at 100-400 eV. However, a proper understanding of the results requires careful use of photoionization codes, since the ionization parameter at the inner face is probably meaningless for CLR models, and a reliable convergence of the ‘highly variable’ atomic parameters for coronal lines. A shock model for the CLR in NGC 1068 requires collisional ionization of ≥ 600 M⊙ yr−1 of dense (n > 104 cm −3) gas within a region no larger than 30 pc. Maintaining a shock-excited CLR for a reasonable time requires > 1010M⊙ of fuel, which must be continuously supplied to the CLR.

We discuss kinematical evidence that highly ionized (‘coronal’) lines are emitted from gas sharing the same velocity field as the BLR, NLR, and extended NLR. A close correlation between ionization potential and line width and blueshift with respect to systemic velocity suggests a hot outflow being responsible for the emission of coronal lines. This outflow is probably connected to the warm absorber observed in X-rays and the outflowing radio plasma. The high degree of ionization is dominantly due to photons from the blue bump but contributions from shocks are indicated in at least some of the sources observed.

We present infrared (IR) long-slit spectroscopy of two galaxies with Seyfert 2 nuclei, NGC 2110 and the Circinus galaxy, both known from previous studies to exhibit anisotropic high-excitation gaseous emission. In the “unified models” for Seyferts, this anisotropy is due to collimation of the ionizing nuclear radiation by an optically thick molecular torus which surrounds the nucleus. We have looked for signatures of this torus and its collimating effect in the IR (J and K bands), taking advantage of the reduced relative attenuation at these wavelengths. The two galaxies are found to show extended emission in the lines [Fe II] λl.257 µm, Paβ, H2λ2.12 °m, and Brγ, both along the collimation axis as well as perpendicular to it. The [Fe II] line is found to trace the high-excitation emission, and, in the case of NGC 2110, is more extended along the collimation axis. The H2 emission, although consistent with the presence of a molecular torus, extends for hundreds of parsecs and the derived kinematics is consistent with circular rotation in the plane of the galaxies. The emission-line ratios [Fe II]/Paβ and H2/Brγ are much smaller in Circinus (respectively ~ 0.5 and 1) than in NGC 2110 (~ 7 and ≥ 4, respectively), and indicate the presence of a nuclear starburst in the former galaxy. IR rotation curves were determined for both galaxies. In the case of NGC 2110, the IR rotation curve shows different behavior than the rotation curve determined from optical lines, which shows the importance of obscuration effects in the study of the gas kinematics.

Simultaneous optical and infrared monitoring of a sample of 50 Seyfert 1 galaxies and quasars has revealed an outburst of the active galaxy Markarian 744. Cross-correlation analysis indicates that the IR lags the optical light curve by roughly one month. A simple model developed by the author can fully explain the IR variability of this object in terms of thermal dust reverberation.