ABSTRACT

The balance of energy in AGNs and quasars is assessed. It is shown that there is no “energy budget” problem in AGNs and quasars and the covering factor of the broad line region in these objects is around 0.1-0.2 and this does not change with the luminosity.

INTRODUCTION

It has been claimed that there is an “energy budget problem” for some continuum energy distributions (CEDs) used to model the broad line region (BLR) in AGNs and quasars. In this paper the budget of energy in AGNs is assessed from UV and soft X-ray observations.

ENERGY BUDGET

Soft X-ray observations of AGNs were obtained from the ROSAT/PSPC archive (at the University of Leicester, England) and reduced in the standard way (see Gondhalekar et al. 1993 for details). A single power law was found to be adequate to represent the data and the power law index (Γ) is given in Table 1. The details of this analysis will be the subject of a separate publication. The soft X-ray spectra deviate significantly from the (best fit by eye) models of Laor and Netzer (1989) at energies higher than 0.5 keV.

RESULTS

Maffei 2, a nearby SBb pec galaxy behind the Galactic plane, is the site of a nuclear burst of star formation as evidenced by near infrared (Rickard and Harvey 1984, A. J., 89, 1520; Ho et al. 1989, Ap. J., 344, 135), Brackett line (Ho et al. 1990, A. J., 349, 57), and radio continuum emission (Seaquist, et al. 1976, Astr. Ap., 48, 413; Turner and Ho 1993, preprint). Its proximity (∼5 Mpc) and large angular extent (∼10′) make it an ideal candidate for high resolution neutral gas studies which are necessary for understanding the processes driving the nuclear starburst.

The integrated HI emission ∫ I(v)dv is shown in greyscale in Figure 1. These VLA observations combine C and D configurations and, with tapered uniform weighting, achieve 20″ resolution. The peak column densities of 3.7 × 1021 cm-2 can be seen to lie in two arms around the nucleus, coincident with enhanced 20 cm continuum emission (Fig. 1, left). This continuum emission traces the spiral arms of Maffei 2 as seen in the near infrared (Hurt, et al. 1993, A. J., 105, 121). Very little HI is present in the nucleus, although the region between the dashed lines is strongly affected by extinction/emission from foreground Galactic HI clouds. There is a strong N/S asymmetry in the HI distribution that reflects the observed peculiar morphology in the radio continuum near infrared.

The plasma state is sometimes referred to as the ‘fourth’ state of matter. As a solid is heated, it first goes through a transition in which bonds between adjacent molecules are loosened but not entirely broken, and the matter moves into the liquid state. As the matter is heated further, bonds holding adjacent particles close together are completely broken so that molecules can move more or less independently and the liquid becomes a gas. Further heating will lead to the dissociation of molecules into their constituent atoms. However, further heating may also lead to the ionization of the molecules or atoms of the gas, so that the gas then comprises neutral particles, ions and electrons. Although there is no sharp phase transition between the state of a simple neutral gas and the plasma state, the plasma state may nevertheless be regarded as part of the sequence solid-liquid-gas-plasma.

Since the plasma state includes free positive and negative charges, and since movements of these charges produce electrical currents, it is clear that the constituents of the plasma state will be influenced by electric and magnetic fields, and that the plasma can also produce electric and magnetic fields. Hence, in discussing the properties of a plasma, it is essential to regard the electromagnetic field as an integral part of the plasma system. This fact leads to a rich – indeed bewildering – array of properties of the plasma state.

ABSTRACT

Many bright elliptical galaxies are active in the sense of having compact radio cores of high brightness temperature (‘engines’) and/or a LINER–like optical emission spectrum. Nuclear activity is very common in the most luminous galaxies (brighter than absolute magnitude MB ∼ – 21) and essentially absent in those less luminous than MB ∼ -19.

ACTIVE NUCLEI IN EARLY–TYPE GALAXIES

The presence of LINER emission (Phillips et al. 1986) and the fact that many E and S0 galaxies have arcsecond–scale central radio sources suggest that some kind of active nucleus lurks at the centre of most galactic bulges brighter than MB ∼ -19 (H0= 100 km s-1 Mpc-1).

Wrobel and Heeschen (1991), however, argue that the central radio emission in many S0 galaxies may be associated with star formation rather than an active nucleus. Furthermore, the emission–line luminosity in E and S0 nuclei correlates more closely with the luminosity of the parent galaxy than with other indicators of activity such as radio emission (Sadler et al. 1989), suggesting that the dominant ionization mechanism may be linked to the underlying stellar population rather than to an active nucleus. Optical spectroscopy and arcsecond–scale radio maps, therefore, may not provide an unambiguous test for the presence of low–level ‘central engines’ in these galaxies.

ABSTRACT

Self-gravity is a key determinant of gas dynamics, especially in a galactic central region. We have investigated self-gravitating gas dynamics with 2-D PM and SPH methods. From simulations of a massive gas disk inside the first ILR, we found a rapid gas fueling accompanied by a forming gas bar which lead the bar potential. The background bar potential and resonances are not important for dynamics of the central self-gravitaing gas in the accreting stage.

GAS FUELING PROBLEM

Starburst regions are frequently located in the central regions of barred galaxies or interacting galaxies. A number of studies has been made on triggering mechanism of starbursts, that is, mechanism fueling a large amount of gas into the starburst region. Many people believe that oval distortion of a background potential caused by galactic encounters or a stellar bar can trigger the gas rapid fueling. However, a number of numerical simulations which does not take into account the self-gravity of gas have revealed that the distorted potential itself cannot supply a large amount of gas into a galactic center beyond ILRs, although gas accumulate to form an oval ring near ILRs (e.g. Matsuda and Isaka 1980; Schwarz 1985).

SELF-GRAVITY OF THE GAS

Fueling by Collapse of an Elongated Gas Ring

Fukunaga and Tosa (1991), and Wada and Habe (1992) reported that a very elongated gas ring leading a weak background bar potential is formed near ILRs provided that a pattern speed of bar is just below a maximum of ω – k/2.

ABSTRACT

Previous researchers have suggested that much of the cold interstellar gas in presentday elliptical galaxies is accreted from external sources. The strength of forbidden-line emission in elliptical galaxies provides a constraint on the enrichment history of the gas. Based on photoionization calculations, we conclude that the gas, if accreted, must originate in donor galaxies with metallicities > 0.5 Z⊙. This excludes primordial clouds and Magellanic Cloud-like objects as typical gas donors.

INTRODUCTION

Elliptical galaxies often contain modest quantities of interstellar gas that can be generated via normal mass loss by the galaxies' constituent stars, on timescales much shorter than a Hubble time (Faber and Gallagher 1976). Diffuse matter generated by such internal sources may be rapidly removed from the interstellar medium (ISM), however, if this material is heated to X-ray temperatures and expelled in a galactic wind, or compressed to form new stars in a cooling flow. In an alternative scenario, objects which feature significant cold interstellar gas may have acquired this matter by accretion from nearby galaxies or intergalactic clouds. Evidence in support of an external origin for the ISM in ellipticals includes a lack of correlation between interstellar and stellar masses (e.g., Knapp et al. 1985), and distinct kinematics for the gaseous and stellar components seen in some objects (e.g., Bertola et al. 1990).

ABUNDANCES AS A DISCRIMINATOR OF ISM ORIGIN

Stars in large elliptical galaxies are inferred to have average heavy element abundances ≳ 2 Z⊙, based on observational estimates and predictions from chemical evolution models (e.g., Bica et al. 1988).

ABSTRACT

On scales larger than 1 Mpc, Low Surface Brightness (LSB) galaxies are found in the same environment as the general population of disk galaxies. However, in a region of phase space defined by projected radius 0.5 Mpc and relative velocity = 500 km s-1, LSB galaxies are extremely isolated. In addition, the average distance to a nearby galaxy of comparable mass is 1.7 times farther for LSB galaxies than for conventional disks. Since it is this small scale environment which determines the frequency of tidal interactions, the data argue that LSBs have not experienced a mass transfer event in the last Hubble time. The lack of such interactions clearly give these disks a different star formation history than their high surface brightness brethren and further implies that mean galactic surface brightness is a function of small-scale environment. To add further complexity, we have also identified a particular class of large-scale length LSB galaxy that, although isolated, invariably hosts a Seyfert 1 nucleus.

INTRODUCTION

Most conferences on topics in extragalactic astronomy are an entertaining mixture of apparent observational data which gives rise to theoretical conjecture followed by rampant folklore, wishful thinking and/or just plain rejection of the data as being relevant. This allows most theories to remain relatively unconstrained. For instance, the role that environment plays in the evolution of galaxies remains a contentious issue. To be sure, the present arrangement of galaxies into clusters, low density but large scale walls, or shells surrounding large scale voids means that a wide range of environments do exist.

ABSTRACT

We examine the question of whether the molecular mass fractions of > 50% in the inner 100–200 pc of nearby starburst galaxies are real. If so, this result would imply that molecular gas has a significant impact on the dynamics of the nuclear regions of these galaxies.

INTRODUCTION

It has been known for some time that the centers of spiral galaxies are often rich repositories of molecular gas (Young and Scoville 1991), often much richer than the spiral disks. Molecular gas masses are sufficient to fuel the vigorous star formation seen in galactic nuclei, even in “starbursts”. Recent interferometric maps have revealed that nuclear gas is distributed in coherent, nonaxisymmetric structures which are often described as “bar-like”, structures which are generally seen reflected in the starburst as well.

An example of a gas-rich nucleus with a high star formation (SF) rate is the center of the nearby spiral galaxy, IC 342. IC 342 has a barlike CO distribution in the nucleus (Lo et al. 1984), which at high resolution resolves into two very open arms of gas that continue to within 50 pc of the nucleus (Ishizuki et al. 1990; Turner and Hurt 1992). There are also spiral arms observed in Hα (J. S. Young, private communication).

ABSTRACT

Galaxy-galaxy collisions induce nuclear and extranuclear starbursts. The sudden reduction of angular momentum of the interstellar medium due to the gravitational impact of the encounter leads to the subsequent infall to the central regions of a large fraction of the overall interstellar gas. Starburst galaxies with bolometric luminosities ≥ 1011 L⊙ have converted most of the H I into H2 reaching extreme nuclear densities of molecular gas. We also discuss extranuclear starbursts in relation to the formation of dwarf galaxies in mergers. As a consequence of tidal interactions a fraction of the less gravitationally bound atomic hydrogen that populates the outskirsts of the pre-encounter disk galaxies may escape into intergalactic space. We find that the ejected gas may assemble again and collapse, leading to the formation of intergalactic starbursts, namely, tidal dwarf galaxies.

“STARBURST GALAXIES”

“Starburst” denotes star formation at higher rates than in normally, self-regulated processes. They are non-equilibrium episodes that last only a small fraction of the total life-time of the host stellar systems. “Starburst galaxies” are stellar systems where the overall energy output is dominated by recently formed stars. In the context of this definition we must distinguish the “extragalactic H II regions” (Searle and Sargent, 1972) from the “nuclear starburst galaxies” (Weedman et al. 1981). The first are small, irregular, and dust-poor galaxies where the starburst is encompassing most of the visible galaxy; the second are massive luminous galaxies where the most violent starburst takes place embedded in dust in the central regions.

RESULTS AND DISCUSSION

Mkn 298 is known as a morphologically peculiar system (Fig. 1), showing a chain (c, d, e) of small compact blue emitting regions (Stockton 1972) aligned on its eastern side up to ∼ 80 arcsec from the main body of the galaxy. These regions are aligned, with a tail (b) resembling a spiral arm located on the eastern side of the galaxy (a). On the western side a very faint trace of spiral arm is also visible, d and e are characterized by an emission line spectrum, while spectra of c do not show any trace of either emission or absorption (Stockton 1972; Metik and Pronik 1982).

The contour maps of the extended emission lines Hα, Hβ and [OIII]λλ4959, 5007 have been used to isolate six different emitting regions in a and b. Each strip on the 2D-spectrum has then been mashed into a ID-spectrum and the diagnostic line ratios proposed by Veilleux and Osterbrock (1987) have been plotted in the diagnostic diagrams giving [OIII]λ5007/Hβ versus [SII]λλ6716+6731/Hα, [N II]λ6583/Hα, [OI]λ6300/Hα. The effect of reddening has been evaluated from the Hα/Hβ ratio using the Whitford (1958) reddening curve as parameterized by Miller and Mat hews (1972) and adopting the intrinsic ratio 2.81.

ABSTRACT

Here we summarize the main results derived from optical fiber observations (bidimensional spectroscopy) related to the kinematics of the circumnuclear region of NGC 5728. We present additional arguments supporting that the ‘true’ nucleus of this galaxy is displaced in velocity and space from the optical nucleus. In view of this result, the region of double-peaked emission line profiles is connected to the nucleus, and the red component probably represents outflowing gas.

INTRODUCTION

NGC 5728 is a spiral barred galaxy classified as Seyfert 2 (Véron-Cetty et al. 1982; Phillips et al. 1983), which shows some peculiar features in its circumnuclear environment. Here we can point out the presence of a region showing double-peaked emission line profiles in the vicinity of its nucleus, the asymmetric position (with respect to the optical nucleus) of a ring of blue stars and ionized gas (Rubin 1981; Schommer et al. 1988; Wagner and Appenzseller 1988), and the possible presence of a weak BLR misaligned with respect to the NEL maximum (Pecontal et al. 1990).

We have observed the circumnuclear region of NGC 5728 with a new observational arrangement based on the use of optical fibers (Arribas et al. 1991), which is similar to the one presented by Shapovalova (see Afanasiev and Shapovalova, these proceedings). The observations were performed on May 8, 1989, using the 4.2m WHT sited at the ORM. The detailed analysis and results on NGC 5728 can be found in Arribas and Mediavilla (1993).

ABSTRACT

The circularization and accretion of the debris created by tidally disrupting a star passing near a supermassive black hole depends on the transverse structure of the debris stream. The transverse structure is modified by crossing points in the stream where the orbits are focussed across the stream center or through the orbital plane, the velocity shear across the stream, self-gravity, recombination, shocks, and shear viscosity. Stream-stream collisions may have a weak effect on the orbits because of Lense-Thirring precession, mismatched geometric cross sections, and the kinematics of collisions.

INTRODUCTION

Tidal disruption of stars passing near supermassive black holes (M ∼ 106 M⊙) provides a mechanism for fueling low-luminosity active galactic nuclei (AGN). It is ineffective for more massive AGN (M ≳ 108 M⊙) because the tidal gravity of the black hole is too small to destroy a star before it passes through the event horizon. Other processes in dense central star clusters such as star-star collisions may provide a steady accretion rate (Hills 1975, 1978; Frank 1978; Young, Shields, and Wheeler 1977), but the disruption of a star may lead to an observable flare in the luminosity of the AGN. Lacy, Townes, and Hollenbach (1982) first understood the kinematics of disruption, and the current picture of tidal disruption is reviewed in Rees (1988) and Phinney (1989). For a star of mass M* and radius R* passing at pericentric distance Rp from a black hole of mass M, the strength of the encounter can be parametrized by the square root of the ratio of the surface gravity to the tidal gravity η = (M*R3p/MR3*)½ (Press and Teukolsky 1977).

This book is based on a series of lectures that has been given at Stanford University, for longer than I care to remember, to graduate students from several departments: Aeronautics and Astronautics, Applied Physics, Electrical Engineering, Mechanical Engineering, and Physics. The course has also formed part of the Astronomy Course Program and of the Space Science Program.

The course has changed over the years, beginning as a three-quarter sequence emphasizing laboratory and geophysical plasmas, and evolving into a two-quarter sequence emphasizing solar and other astrophysical applications. Selected material has also been offered as a one-quarter course. The course has been much improved by input from many students (in fact, the first set of lecture notes was produced by students in the class) and from a sequence of dedicated teaching assistants, notably, in recent years, Dr Anton Bergmann, Ms Lisa Porter and Dr Yuri Taranenko.

For invaluable assistance in the preparation of this text, I am indebted to Mrs Louise Meyers-Norney, who entered the text, to Dr James and Mrs Maria Klimchuk, who entered the equations, and to Dr Taeil Bai and Mr David Faust, who helped prepare the figures. Thanks are due also to Dr George Field, Dr Robert Helliwell, Dr Eric Priest and Dr Gerard Van Hoven, who kindly reviewed some of the chapters, and to Dr Simon Mitton and Ms Fiona Thomson of Cambridge University Press for their generous support.