This book includes contributions to the International Astrophysics Conference on Mass–Tranfer Induced Activity in Galaxies, held at the University of Kentucky, Lexington, on April 26–30, 1993. More than 140 participants from 17 countries attended, compared to 70 planned originally. We feel that such interest was fueled at least in part by the Conference being solely devoted to this fascinating subject.

This Conference was based on a number of review talks which prepared the audience for the follow-up discussion and contributed papers. We aimed at providing a balanced view of the field which by now has become well defined, reaching a certain degree of maturity. Some overlap between different review papers was planned and the careful reader will enjoy sometimes contradictory explanations of the same ‘facts’.

The main question concerning the origin of active galaxies is tantalizingly simple: how does nature remove all but ∼ 10-7 of the angular momentum (in AGNs) initially residing in the gas a few kpc from the center, accomplishing this on the orbital timescale? And how do the nuclear starbursts fit within this picture — as a passive by-product or a major player forcing the gas inwards and/or outwards?

We proceed from ‘inside out’ (in the footsteps of the Creator?) by addressing first the angular momentum transfer within the central parsec. Next, the kinematics and molecular, atomic and ionized gas distributions in the circumnuclear region are reviewed followed by large-scale gas properties in barred and normal disk galaxies and in ellipticals.

ABSTRACT

Rapid optical variations have been detected for the Seyfert 1 galaxy Akn 120 with time scales shorter than an hour. These variations are the most rapid which have been detected for any Seyfert galaxy, and, for the first time, clearly demonstrate that radio-quiet AGNs exhibit the phenomenon of microvariability. This result suggests that these variations are indepentent of the radio properties of these objects and are thus unlikely to be associated with any disturbance in a relativistic jet.

INTRODUCTION

Akn 120 was first identified as a Seyfert galaxy by Arakelian (1975) in a survey of high surface brightness galaxies. It has long been known to exhibit optical variations with time scales ranging from years to days (Miller 1979). In this paper, we report the first results of a program to determine if radio-quiet AGNs exhibit optical variations on time scales much shorter than a day.

OBSERVATIONS

The high time resolution optical observations of Akn 120 reported here were obtained with the 42-inch telescope at Lowell Observatory equipped with a direct CCD camera and an autoguider. The observations were made through a V filter with an RCA CCD. Repeated exposures of 60 seconds were obtained for the star field containing Akn 120 and several comparison stars. These standard stars, located on the same CCD frame as Akn 120 provide comparison stars for use in the data reduction process. The observations were reduced using the method described in Carini and Miller (1992).

INTRODUCTION

A wealth of information has been presented at this conference illustrating the basic theme that large-scale gas inflows can result in the release of large amounts of energy in galactic nuclei by starbursts and by non-thermal processes associated with black hole accretion. Particular attention has been paid to two situations in which such phenomena frequently occur: (1) bar-driven inflows in barred spiral galaxies can lead to gas accumulation and star formation in nuclear rings of ‘hot spots’, and (2) tidal interactions and mergers between galaxies can cause large amounts of gas to fall rapidly into their nuclei and produce luminous starburst and non-thermal activity. Although it is difficult to observe the gas inflows themselves directly, partly because they may be masked by more conspicuous outflows, inflows are commonly predicted by numerical simulations of barred and interacting galaxies, and the success with which these simulations match many of the observed properties of such systems leaves little doubt that inflows occur quite generally whenever a non-axisymmetric gravitational potential is present. Evidence that gas inflows from regions of galactic size are responsible for triggering the most energetic outbursts of activity in galactic nuclei is provided by the fact that in many of the most luminous systems, an amount of molecular gas comparable to the gas content of an entire large spiral galaxy is observed to be concentrated into a small nuclear region only a few hundred parsecs across.

OBSERVATIONS

The MBG survey (Montreal Blue Galaxy) is a spin-off project of the Montreal Cambridge Tololo (MCT) survey of southern subluminous blue stars (Demers et al. 1986). Using a subset of the plate material covering 7000 deg2, with b ≤ -40∘, we pick up all extended UV-bright objects. The analysis of our follow-up spectroscopy has shown that the bulk of our UV-bright candidates consists of H II galaxies or starbursts; the fraction of AGNs being somewhat less than 10% (Coziol et al. 1993). We expect to find ∼ 500 such galaxies, with magnitude B ≤ 15.5.

Recently, we undertook an imaging follow-up of our candidates to determine their morphology and search for clues to the origin of their activity. Our first sample consist of 11 MBG galaxies with strong H II-region like spectra suggesting an intense phase of star formation. Usually, these galaxies also possess relatively hot IRAS color typical of starburst galaxies (Sekiguchi 1987). Using our 1.6m telescope, located on mont Mégantic in Québec, Canada, we obtained CCD images, with BVRI filters. Details of the observations, reductions and analysis will be published elsewhere (Coziol, Barth, and Demers 1993). Following, is a summary of our first results.

We find mostly spiral galaxies with bright nuclear regions. There is a strong tendency to find early type galaxies. Further analysis of the surface photometry reveals that the bursts are located, preferentially, in the circumnuclear region prolonging far into the nucleus. This confirms the earlier spectral classification of our objects, using the excitation diagnostic diagram of [O III]λ5007/Hβ versus [N II]λ6584/Hα, which suggested that MBG objects are mostly starburst nuclei galaxies (see Salzer et al. 1989 for a definition).

NGC 3079 is a remarkable spiral galaxy which exhibits an unusual range of nuclear activity. Viewed edge-on, it harbors a reddened LINER (Low Ionization Nuclear Emission-line Region) that is kinematically complex, with several distinct components of Ha emission. It also contains a compact, flat spectrum nuclear radio source, and shows well-defined, kiloparsec-scale radio lobes of considerable complexity, as well as a smaller loop of Hα+[N II] emission extending approximately along the minor axis of the galaxy. The optical emission lines indicate that gas is outflowing from the nucleus in an energetic, bipolar outflow or “galactic superwind.” Armus et al. (1990) have suggested that this wind is driven by a powerful, central starburst. A number of different lines of evidence — its infrared brightness (measured by IRAS, the extended 10 μm emission, the spatially coincident, circumnuclear molecular gas, and the extremely luminous H2O maser — point to the presence of ongoing, vigorous star formation within the nucleus, in agreement with this idea. However, other authors (e.g., Irwin and Sofue 1992; Filippenko and Sargent 1992) argue that the nuclear activity originates from an AGN, perhaps supplemented by a powerful starburst.

We have obtained ROSAT PSPC observations of NGC 3079 and the adjacent spiral galaxy NGC 3073. Preliminary analysis indicates that X-ray emission from NGC 3079 consists of a point-like source, superposed on lower level emission which is extended by 2.5′ in approximately the same direction as the radio jet, Hα loop, and radio lobes (Figs. 1, 2). The extended emission is roughly coincident with two Hα filaments detected by Heckman et al. (1990), and may consist of either a shell-like or a filled structure.

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.