We are able to measure the chemical compositions of hundreds of planetary nebulae in our own Galaxy and in the Magellanic Clouds. Why, therefore, do we need to expend the effort to observe much more difficult targets in other Local Group galaxies? A severe lack of distances does not allow us to place Galactic planetary nuclei on the log L-log T plane with any degree of accuracy, so we cannot properly examine composition differences relative to core mass and state of evolution. We can perform such tasks for Magellanic Cloud objects, but do not know how the Clouds’ low-metallicities affect the results, and thus do not know how this sample of planetaries relates to the nebulae in our own system. We know, for example, that in the Clouds, nitrogen enrichment begins above a core mass of about 0.68 M⊙ (Kaler and Jacoby 1990), but other than a clear link between core mass and chemical enrichment in the Galaxy, do not know at what point it becomes important. To study such relationships, we therefore have to go to other galaxies with a variety of initial conditions for which distances are secure, namely those of the Local Group.

Currently over 2000 Planetary Nebulae (PN) have been detected in Local Group E and S0 galaxies by on/off band [O III]5007Å CCD photometry. PN can currently be detected out to Fornax and Virgo clusters.

The magnitude of the brightest PN in the nearest elliptical galaxy (NGC 205) is m5007 20.7 (Ciardullo et al, 1989); for NGC 5128 (Cen-A) it is 23.5 (Hui et al, 1993a) and ≥ 25.6 for Virgo ellipticals (Jacoby et al, 1990). Given the relation between m5007 and line flux:

m5007 = −2.5 log F5007 − 13.74

the typical Hβ fluxes are ≤1.5×10-16 ergs cm−2 s−1 for NGC 5128 PN and ≤ 2.5×10−17 for Virgo PN. Obtaining spectra of these faint emisison lines is clearly a challenge. The only PN spectra studied in Local Group galaxies are 1 PN in M32 (NGC 221) (Log F(H/β) = −14.9; Jenner et al, 1979) and recently 15 in NGC 205 (Jacoby et al, 1995). None of the spectra are of high quality and the Te diagnostic line [O III]4363Å was not detected. There is evidence of possible high N abundance in the M32 PN.

Some topics on composition gradients are discussed. One of the most important processes responsible for the abundance gradients is the variation of the low-mass end of the IMF with O/H.

We present new results from spectro-photometric models for elliptical galaxies in which detailed descriptions of the star formation history and chemical enrichment are taken into account. In particular, we focus on the important role played stars of different chemical composition evolving along the red giant branch (RGB), horizontal branch (HB), asymptotic giant branch (AGB), and finally planetary nebula (PN) stages in relation to the sources of UV flux in elliptical galaxies. A few important applications of these new galaxy models are briefly outlined.

Far UV observations and optical studies of planetary nebula luminosity functions (PNLFs) offer complementary views of the late phases of stellar evolution in elliptical galaxies and spiral galaxy bulges. UV spectroscopy reveals that the hot stellar population is composite, with a mix of temperatures that varies from galaxy to galaxy. This changing mix is most likely due to changes in the relative numbers of stars that channel through the Post-Asymptotic Giant Branch (PAGB), Post-Early-AGB (PEAGB) and Extreme Horizontal Branch (EHB) phases of evolution. EHB stars appear to dominate the integrated λ < 2000 Å flux from galaxies with the strongest far-UV emission, but are too faint to resolve individually in even the nearest galaxies. Far UV images of M31 and M32 reveal a population of hot stars that are much brighter, but do not account for the majority of the far-UV flux. The sources detected are most likely low-mass PAGB stars (0.55 < M/M⊙ < 0.59). In contrast, the PNLF probes the PAGB star mass function at values greater than ∼ 0.6 M⊙. For a given galaxy the relative numbers of stars in these different branches of evolution are determined by the age and chemical evolution of the galaxy and by the physics of mass loss on the red giant branch. We review current constraints on the mass function of hot evolved stars in elliptical galaxies, highlight a few puzzles, and outline where future observations might contribute.

Nowadays it is possible to detect planetary nebulae, and measure their [O III] λ5007 fluxes, in many galaxies. Given this information it is relatively straightforward to build the [O III] λ5007 PNLF for any galaxy, because we can always assume all its PNs to be essentially at the same distance from us.

Of course the situation is much more difficult for PNs in the solar neighborhood. We have two problems here: (1) the individual PN distances are poorly known, except for ∼ 25 objects; (2) in the literature there is no agreement on which are the 25 PNs with reliable distances. Clearly it was a wise decision to calibrate the PNLF method of extragalactic distance determinations using the distance to M 31 (Ciardullo et al. 1989, Jacoby et al. 1992). In these short notes I review if it may be possible to build a PNLF for our Galaxy (the answer is: yes, probably more than one) and discuss if it would be worth the trouble (the answer is: yes, it would) and what can be done to achieve that elusive goal.

Planetary nebula luminosity functions provide very accurate distances to galaxies as judged by comparisons with other techniques, including Cepheids; systematic effects cannot be significant. The method is empirical and is a consequence of the shallow initial-final mass relationship predicted by stellar evolution and directly measured. From distances to ∼ 30 galaxies, the PNLF yields H0 = 79, in excellent agreement with recent Cepheid discoveries in Virgo.

The planetary nebulae (hereafter PN) distance criterion (Ciardullo et al. 1989a) relies on the shape adopted for the luminosity function (hereafter LF) and its adequate fit to the observed data. Crucial hypothesis concerning the LF are its universal shape with an universal value of the absolute magnitude cut-off and concerning the sample, its completeness and the selection of the rejected data.

Because the completeness is achieved only in the bright part of the LF, the brighest PN play a crucial role in the distance determination and it is well known that the brightest objects (with intrinsic scatter) in a giant galaxy are brighter than in a dwarf galaxy. Thus more luminous galaxies are given smaller distances. Because giant galaxies are rare they are not seen at small distances; on the contrary, they are typically observed at large distances. This effect induces a progressive compression of the distance scale.

Until now, [O III] λ5007 surveys of extragalactic planetary nebulae (PN) have been aimed at determining distances and identifying test particles for future (cinematica] studies. However, PN observations are useful in another area of astrophysics - the investigation of stellar populations. Because the production rate of [O III] bright PN in a galaxy can vary by an order of magnitude, this number is potentially a powerful discriminant between age and metallicity in an evolved stellar population. A strong inverse correlation exists between galaxy absolute magnitude and bolometric luminosity-specific PN density (α2.5), with the small metal-poor ellipticals having values of α2.5 in exact accordance with the predictions of stellar evolution theory. This correlation, along with the direct dependence of α2.5 on galactocentric radius, and the scatter in α2.5 seen in “young” galaxies, argues strongly that elliptical galaxies are old systems which follow a metallicity sequence. It also suggests that the combination of absorption line spectroscopy, UV photometry, and PN measurements can be a powerful probe of elliptical galaxy stellar populations.

ASCA capability of X-ray imaging spectroscopy up to 10 keV. allows us to examine the emission and absorption feature from AGN. Warm absorber, low energy lines and broad iron K lines are confirmed from Sy I’s. High sensitivity in broad energy band makes it possible to distinguish multiple components emerged by different processes. Detection of X-rays from faint sources tells us various galaxies may harbor AGN sometimes with obscuration tori. They might have considerable contribution to CXB.

The extreme ultraviolet (EUV) wavelength interval (commonly defined to constitute the range 68 — 912 Å, or 13.6 — 182 eV) hes between the intensively studied FUV (912 - 3000 Å) and X-ray (1 - 68 Å) spectral regimes, but until recently has not been subjected to systematic investigations. This lack of information has been particularly acute in the case of active galactic nuclei (AGNs), where the very ability to carry out successful EUV observations has been seriously questioned. This has stemmed from the fact that, for a Galactic H I column density NG(H I) = 1 × 1020 cm−2, the interstellar transmission factor is 0.42 at 60 Å, but a factor of 10 smaller at 100 Å. In turn, there are no known regions of the interstellar medium (ISM) that have [NG(H I)/1020 cm−2] < 0.6 (resp., 0.8) for positive (resp., negative) latitudes (e.g., Heiles 1991).

There are, however, several reasons why EUV observations of AGNs could provide important information about the nature of these objects. In the case of radio-quiet QSOs, the peak power is known to be emitted in the EUV range (several hundred A). Furthermore, in these sources there is evidence for a distinct soft X-ray component (representing a “soft excess” over an extrapolation of the ISM-absorbed hard X-ray component), whose properties are at present controversial. In particular, the first results of ROSAT PSPC observations of a complete sample of optically selected QSOs (Laor et al. 1994) have pointed to a spectral index αx = 1.50 ± 0.40 in the 0.2 − 2 keV range that flattens by Δαx ≈ 0.5 above ∽ 2 keV. This study has concluded that steep-αx sources are characterized by a week hard component. A similar distribution of αx has been inferred for soft X-ray-bright QSOs and Seyfert 1 galaxies observed in the ROSAT all-sky survey. On the other hand, previous observations (enumerated in Laor et al. 1994) by the Einstein and EXOSAT satellites yielded different soft X-ray spectral slopes and led to an alternative explanation of the steep-αχ objects. These questions could in principle be clarified by EUV measurements, which would then also help determine the soft X-ray emission mechanism.

This is a brief review of the recent progress made on this domain, earlier work is nicely summarised either in the review of Barnes & Hernquist (1992a), or in Proceedings of Rencontres de Moriond in spring 1992 (e.g. Thuan et al 1993), or of Lexington Conference in spring 1993 (e.g. Shlosman 1994).

The dynamical evidence for black holes (BHs) in galactic nuclei is reviewed, with emphasis on recent improvements in spatial resolution, methods for analyzing galaxy spectra and dynamical modeling. M31, M32 and M87 are discussed in some detail.

VLBI images of the H2O megamaser in NGC 4258 and a time series of spectra taken over several years combine to make a compelling case that there is a compact molecular disk associated with a supermassive object in the nucleus of this galaxy. The images of the maser in the velocity rangenear the systemic velocity show a highly elongated structure with a major axis of about 0.009 pc, along which the gradient in line-of-sight velocity is essentially a constant of 7970 ± 40 km s-1 pc-1. The observed acceleration of these spectral features by about 6-11 km s-1 yr-1, the presence of high-velocity maser satellite emission, and the VLBI results suggest emission from a disk of diameter0.2 pc, rotating with a velocity of ~800 Km s-1, bound by a mass of 1.5×107 M⊙ and denisty of ≳3.6×109 M⊙ pc-3

Using the Faint Object Spectrograph (FOS) on Hubble Space Telescope (HST), Harms et al. (1994, H94) have recently reported the spectroscopy of central region of the elliptical galaxy M87. Ford et al. 1994 (hereafter F94), using Wide Field Planetary Camera-2 have imaged the region around the nucleus in Hα+[NII] and find an ionized disk with spiral structures of mainly two arms. From the kinematical argument, based on the Doppler shifts of several lines emitted from the disk, and assuming a Keplerian motion of the emitting gas, they conclude that the mass of the disk plus the nucleus: Mc(R < 18pc) = (2.4± 0.7)× 109M⊙ and the inclination angle of the disk with the line of sight is i = (42±5)°. However, if the bright spiral structures are real, and represent shocked region in the disk, we expect that the disk is strongly non-Keplerian and therefore the mass of the black hole must be higher than above estimation.

In the present contribution, we provide a complete description of the velocity field of the ionized disk and compute the shape of typical line profiles expected from various parts of the disk. Our analysis is based on the solution of a non-axisymmetric disk which includes two armed spiral density waves. We find a very good agreement between the theoretical and observed line profiles as regards to the Doppler shifts, line widths and the intensity ratios and estimate the mass of the black hole to be (4 ± 0.2) × 109M⊙. Details of this work will be published elsewhere (Chakrabarti, 1995).

In a binary system with a thin accretion disk, the binary companion can induce two armed spiral shocks in the disk (e.g., Matsuda et al. 1987, Spruit 1987, Chakrabarti & Matsuda, 1992). In the case of active galaxies, a passing companion (or a globular cluster or a dwarf galaxy) which is more massive than the disk can induce the same effect.

We present results from a recent study of the inner 1 kpc zone of the weakly barred spiral galaxy NGC 4321 (M100), which reveals a circumnuclear starburst, organized in a pair of tightly-wound spiral arms. Combining high-resolution imaging in the optical and near-IR with a detailed dynamical model, we show that the morphology of this region can be explained by a resonance interaction between the disk and the stellar bar and its effects on the gas.

Radio galaxies are unique cosmological probes. As with radio-loud quasars, the presence of luminous radio continuum and optical line emission enable radio galaxies to be observed and recognized at large distances, up to z = 4.2. However, unlike the situation for most quasars, their optical emission can be spatially resolved from the ground and studied in detail.

Progress in detecting distant radio galaxies has been rapid in recent years due to the use of CCDs and the exploitation of new selection criteria. Now, more than 60 radio galaxies are known with z > 2. More than half of these have been found by our group by concentrating on radio sources with the steepest spectra, most of these in a “Key Programme” of the European Southern Observatory. Although several people contributed to this Key Programme, most of the work was done by Huub Röttgering, who presented his Ph.D thesis in January and Rob van Ojik, who succeededhim. Redshifts of 1.5 to 4 correspond to a time when the Universe was 10% -20% of its present age. This was a crucial period in history when galaxy formation must have been rampant. It corresponds to the AGN era, a two-billion year “delta function” in the population evolution of luminous quasars and radio galaxies, when their space-density rose to a value several hundred times larger than the present density before the species mysteriously and suddenly became almost extinct.

I briefly discuss the flow of gas in and around bars and the amount of inflow it entails. I then focus on two ways of bringing gas very near the center of a barred galaxy. One relies on the absence of an inner Lindblad resonance and the other on the existence of bars within bars.

Unification of Active Galactic Nuclei is obviously multidimensional. Dimensions include the mass and angular momentum of the postulated central black hole, the mass accretion rate through the accretion disk surrounding the black hole, and the orientation of this central engine plus associated dust with respect to the line of sight (e.g., Blandford 1990). As for orientation related unification, considerable progress has been made over the past decade. This review will deal with this geometric unification.

Mechanisms in AGN include:

1. 1. relativistic beaming in radio jets (as manifested by rapid variability from radio to ϒ-rays, superluminal motion, synchrotron self-Compton

2. 2. emission by an optically thick, non-spherical body (e.g., hot accretion disk);

3. 3. external collimation and shadowing by an anisotropic dust distribution (as manifested by the existence of hidden continua, observed through polarization or photon deficit in extended emission line regions, and hidden broad line regions, observed through polarization or in the less opaque near infrared part of the spectrum).

These anisotropic mechanisms introduce selection biases, they lead to incorrect inferred luminosities and make us overestimate the AGN diversity. They furthermore imply unification at some level.

In this contribution I shall first briefly review the arguments suggesting that most galaxies pass through a quasar or AGN phase early in their lifetime, perhaps concurrent with bulge formation. Then, as a footnote to the paper by van der Marel, I will discuss how relic black holes in quiescent galaxies could reveal themselves by the occasional flares resulting from tidal disruption of stars. In conclusion, I shall briefly discuss the phenomena associated with binary black holes, arguing that these are the likely outcome of galaxy mergers.