ABSTRACT

The fact of starburst activity in a non-negligible fraction of galaxies implies a refuelling mechanism, since the star formation rates observed are sufficient to exhaust the gas in situ over relatively short timescales. A number of “gas bars” – elongated structures in which molecular gas is observed to flow in markedly non-circular orbits – has been observed in galaxies with circumnuclear star-forming activity. In this paper I discuss whether the observed properties of these flows fit the dynamical models which have been proposed for refuelling starbursts, concluding that they may well do so, but not according to some rather over-simplified previously postulated scenarios.

INTRODUCTION

Observations during the past five years of molecular gas in external galaxies on scales of order 100 pc, made possible by the commissioning of millimetre wave interferometers, have virtually put within our research the solution to the problem of how starbursts are refuelled. The need for refuelling is clear since in the majority of cases the observed star-formation rates, of order 0.1 to 1 M⊙ per year, would exhaust the molecular gas observed within 100 pc of the nucleus in times between 108 and 107 years. On the other hand the observed association of starbursts and also Seyferts with the presence of bars in spirals, implies a causal link which would be satisfied naturally by the property of a bar to brake the rotational motion of interstellar gas, and thus allow it to flow down the gravitational potential gradient towards the nucleus. These considerations have led the molecular observers to search for evidence of the phenomenology of molecular inflow, with some success as I describe below.

ABSTRACT

The kinematics of ionized gas in active (and normal) galaxies is reviewed. For clarity, discussion is divided first by emission region size, and then by galaxy type. Although a wide range of velocity fields are encountered on all scales, a number of recent developments are stressed : large scale outflows in Seyfert and Starburst galaxies (1–20 kpc); gravitationally dominated motion on intermediate scales in all galaxies (few × 102 pc); radial flows of uncertain direction on small scales in Seyferts (3 – 100 pc); and possible continuity of velocity field in Seyferts down to very small scales (≲0.1 pc, BLR).

INTRODUCTION

Ionized gas can be found on many scales in both active and normal galaxies — on large scales ∼ 1 – 20 kpc in the body and near environment of the host galaxy; on intermediate scales ∼ 0.1 – 1 kpc in the bulge dominated regions; on small scales ∼ 3 – 100 pc in the inner bulge cores; and on very small scales ∼ 0.01 – 1 pc within the Broad Line Region (BLR). My intention is to review observations which shed light on the kinematics of this ionized gas, recognizing that the velocity fields on one scale may be quite unrelated to those on another.

Studies of ionized gas kinematics focus, of course, on emission lines and their Doppler profiles. First order information comes from the profile center and width, while higher order information comes from profile asymmetry, kurtosis, and substructure, as well as the comparison of these for different emission lines.

ABSTRACT

Strong central starbursts occur most frequently in early-type disk galaxies, especially barred galaxies. We have taken U, B, R, I, and Ha images and long slit nuclear spectra of eight galaxies of type SO/SBO to Sa/SBa with bright central starbursts, selected on the basis of IRAS colors and fluxes. Four of the galaxies have circumnuclear ring-like distributions of hot stars and gas. Some or all of these four galaxies may be cases of barred galaxies with inner Lindblad resonances (ILRs), in which gas has been channelled into a ring at an ILR and compressed into star formation. Another galaxy is clearly interacting with a companion Seyfert galaxy. The strong interaction has apparently lead to the producion of the bright nuclear and near-nuclear emitting regions and the complex of fainter star-forming clumps and filaments surrounding them. Two of the remaining galaxies have compact nuclear starbursts. We conjecture that these galaxies may not have ILRs, so that gas perturbed by a bar potential or by a companion has accumulated at the nucleus instead of in a ring. The final galaxy appears to have a very bright off-center clump of star formation, conspicuous at all wavelengths. Although the possibility of a centered but dust-obscured burst is discussed, the apparent asymmetry may be real and in need of explanation.

INTRODUCTION

Strong central starbursts are occurring in a significant minority of S0 and early-type spiral galaxies. The IRAS survey data can be used reliably to find complete samples of these galaxies, since on-going bursts produce far infrared radiation that is warmer than that produced by disks.

ABSTRACT

Many elliptical galaxies contain cold gas of external origin, which often forms a warped disk or ring. Recent observations of the atomic and the molecular gas in nearby ellipticals have shed considerable light on the properties of this accreted material. Resulting progress in understanding the details of the settling process are discussed, with attention to questions such as: Are any of the observed gas disks settled? How do the settling times and the observed properties of the gas disks depend on the structure and shape of the galaxy in which it resides? How much mass is delivered to the nucleus?

INTRODUCTION

Many elliptical galaxies contain significant amounts of cold gas. This is often detectable by its Hα emission which sometimes extends to 3–5 kpc from the center and appears as a warped disk (Demoulin–Ulrich, Butcher and Boksenberg 1984; Trinchieri and di Serego Alighieri 1991; Shields 1991; Macchetto and Sparks 1992; Buson et al. 1993). When seen nearly edge–on, such disks are seen as a dust lane across the image of the galaxy (Bertola and Galletta 1978; Hawarden et al. 1981; Ebneter and Balick 1985; Sadler and Gerhard 1985; Bertola 1987). These disks contain 103–105 M⊙ of ionized gas (Phillips etal 1986), 104–106 M⊙ of dust (Forbes 1991), and 106–108 M⊙ of neutral gas (Lees et al. 1991; Bregman, Hogg, and Roberts 1992). Large amounts of neutral hydrogen extending to tens of kpc have been found in only a half–dozen elliptical galaxies (van Gorkom 1992).

ABSTRACT

From the preliminary analysis of a sample of ≃ 600 galaxies with bj ≤ 19.4, and spanning a redshift interval up to z ≃ 0.3, we deduce that star formation per unit luminosity, as indicated by the OII 3727 line equivalent width, is a function of galaxy luminosity (decreasing at increasing luminosities), redshift (increasing at increasing redshift, or, physically, look–back time) and environment.

INTRODUCTION

At the ESO 3.6m telescope at La Silla, we are currently performing a redshift survey of galaxies with bj ≤ 19.4, in a rectangular area ≃ 22° × 1° (plus a nearby area of ≃ 5° × 1°) in a region around the South Galactic Pole. Up to now we have acccumulated spectra for ≃ 2,000 galaxies over ≃ 70% of the area.

The distribution in distance of the survey galaxies exibits significative peaks above the expectation at D ≃ 180h-1 Mpc and D ≃ 300h-1 Mpc. These peaks correspond to large scale structures extending over a significant fraction of the strip.

A large fraction of galaxies (≃ 40%) shows the presence of one or more emission lines (OII λ3727, Hβ, OIII λλ4959,5007). These objects can be either spiral galaxies, where lines originate mostly from HII regions in the disks, or galaxies undergoing a significant burst of star formation. The observed peaks in the galaxy distribution are much less pronounced when only emission line galaxies are considered. This suggests that either spiral galaxies are less frequent in the densest regions, thus confirming a large scale validity of the well known morphology–density relation, or starburst phenomena in galaxies occur preferentially in low density environments, or both.

ABSTRACT

The nuclear star formation rates have been measured for a complete sample of 156 spiral galaxies. The nuclear star formation rates of barred and unbarred galaxies are compared separately for early and late Hubble types. Significant differences are observed and critically evaluated within the context of a possible causal relationship between stellar bars and nuclear starbursts.

INTRODUCTION

Considerable interest surrounds the possible association between nuclear activity and galaxy morphology (Sersic and Pastoriza 1967; Heckman 1978; Simkin, Su, and Schwarz 1980). Indeed, the unambiguous association with a morphological feature, such as a stellar bar for example, may provide an important clue as to the origin of the activity. A number of studies have demonstrated the frequency of nuclear star formation to be enhanced in the nuclei of barred spiral galaxies. Heckman (1980) demonstrated a higher incidence of HII regions in the nuclei of barred spirals and Balzano (1983) noted an excess of barred spirals in an optically selected sample of starburst galaxies. The results discussed in this contribution, however, I believe are the first to demonstrate that the high mass star formation rate is enhanced in the nuclei of barred spirals when compared to unbarred spirals.

THE SAMPLE

A complete sample of 227 nearby (15 ≤ D)[Mpc] ≤ 40), infrared luminous, L(40 – 120 μm) ≥ 3 × 109 L⊙ spiral galaxies were selected upon comparison of the Infrared Astronomical Satellite (IRAS) Point Source Catalog with the Nearby Galaxies Catalog (Tully 1988).

ABSTRACT

Low-ionization broad absorption line quasars, also known as Mg II BAL QSOs, are intriguing objects that might represent an evolutionary link between the infrared-luminous aftermaths of galaxy-galaxy collisions and the normal, unshrouded quasars thought by some to result from such collisions. This contribution briefly summarizes Mg II BAL QSO phenomenology, sketches the results of a more detailed analysis of Mg II BAL QSO spectra, and estimates the prodigious kinetic energy outputs of these objects. It appears likely that the kinetic luminosities of these quasars exceed 10% of their radiative luminosities.

INTRODUCTION TO MG II BAL QSOS

Broad absorption-line quasars (BAL QSOs), comprising ∼ 10% of all opticallyselected radio-quiet quasars, have puzzled astronomers for over two decades. Broad, blueshifted absorption features in UV resonance lines like CIV, N V, Si IV, and Lyα show that these quasars are ejecting 104 K material at velocities of up to 0.1c. In the spirit of this conference, we could call the BAL QSO phenomenon “Activity-Induced Mass Transfer in Galaxies”.

We still do not understand how these quasars accelerate the absorbing material to such high velocities. Even though BAL outflows could be telling us something important about the innards of active galactic nuclei, BAL QSOs have been somewhat neglected in recent years, partly because we lack a basic understanding of the acceleration mechanism. The field has progressed more by ruling out possible acceleration mechanisms than by identifying schemes that might work. Weymann, Turnshek, and Christiansen (1985) and Begelman, de Kool, and Sikora (1991) provide particularly insightful analyses of the problems involved in accelerating BAL material.

ABSTRACT

We present a study of the O/H gradients derived from large numbers of HII regions in a sample of barred galaxies. It is shown that the slopes of the O/H gradients in galaxies with a barred structure are related to the bar strength: the stronger is the bar, the flatter the abundance gradient becomes. This result is in agreement with a scenario of mixing of the interstellar gas by radial flows across the disks of barred galaxies. Thus we concluded that a bar acts as a homogenizer on the chemical composition in a spiral galaxy.

INTRODUCTION: RADIAL FLOWS

Observations and numerical simulations have shown that strong radial flows are present in barred spiral galaxies (e.g. Sellwood and Wilkinson 1993). Although the exact origin of such flows is unclear, their amplitudes seem to be related to the bar properties and to the presence of radiative shocks induced when the gas passes through the bar. A possible important consequence of radial flows in a spiral galaxy is a radial redistribution of the chemical composition across the disk. Such mechanism was suggested to explain why the O/H gradients in few barred galaxies (e.g. NGC 1365) are shallower than gradients observed in normal spirals.

Review works by Vila-Costas and Edmunds (1992, VE) and Zaritsky et al. (1993, ZKH) have confirmed this difference. However, their large samples of galaxies included very few barred spirals where the O/H gradient is derived from a large number of HII regions.

ABSTRACT

A nearby interacting galaxy NGC 3627 was observed in the CO (1–0) transition and in H I using aperture synthesis technique. The combined CO and H I data indicated that, the gravitational torque experienced by NGC 3627 during its close encounter with NGC 3628 triggered a sequence of dynamical processes, including the formation of prominent spiral structures, the central concentration of both the stellar and gaseous mass, the formation of two widely separated and outwardly located Inner Lindblad Resonances, and the formation of a gaseous bar inside the inner resonance. These processes in coordination allow the continuous and efficient radial mass accretion across the entire galactic disk. The observational result in the current work provides a detailed picture of a nearby interacting galaxy which is very likely in the process of evolving into a nuclear active galaxy. It also suggests one of the possible mechanisms for the formation of successive instabilities in the post-interacting galaxies, which facilitates the central channeling of interstellar medium to fuel nuclear activities.

INTRODUCTION

In the recent years, it has become increasingly apparent that the interaction among galaxies plays an important role in the evolution of a disk galaxy (Barnes and Hernquist 1992 and the references therein). It is well known that the gravitational tide excited during the galaxy interaction could induce significant inflow of gas to fuel nuclear activities. But since very few interacting galaxies have so far been mapped with high resolution, a detailed picture of the type of instabilities that occur after galaxy interaction, and of the mass accretion process in the inner kiloparsec region of the galaxy is still lacking.

ABSTRACT

Major features of star complexes as basic “building blocks” of disk galaxies are presented and their role in the galactic formation and evolution is discussed.

Star complexes are the largest aggregates of individual stars, stellar associations and clusters, together with interstellar gas clouds. They have been recently recognized as universal and ubiquitous “building blocks” of disk galaxies and the contemporary star formation is concentrated within the complexes (Efremov 1979, 1988, 1989).

A typical complex is a kpc-size system containing about 107 M⊙, mostly in HI and H2 clouds. Young star clusters and OB-associations are strongly confined within the star complex and usually fill in only a small part of its volume. Star formation lasts 50–70 Myr in a common complex, and it is probable that after 100 Myr the complexes are disrupted. Super-associations may be considered as a rare kind of complexes with violent star formation over the whole complex: there are more than 200 star complexes in M31 and only one super-association.

The well known Gould's Belt, the Local system, is the only star complex whose structure and dynamics we are able to study. It contains 8 small clusters, 3 OB associations, at least 3 Cepheides, a dozen other late supergiants, and surely a lot of main sequence stars. The oldest stars and clusters in the Local system complex are about 60 Myr old. Tayler et al. (1987) estimated that the total stellar, HI, and molecular masses of the complex are 0.5, 1.0, and 0.4 × 106 M⊙, respectively. Other complexes in the Galaxy are probably more massive, especially those concentrated within the Car – Sgr arm (Alfaro et al. 1992).

ABSTRACT

We report the results of ROSAT PSPC observations of two merging galaxy systems. The nearest classic example, the Antennae, shows X-ray features alt the two galactic nuclei, together with features interpreted as large H II regions, all embedded in apparently diffuse emission. Another feature with no radio or optical counterparts may be a hot outflow. NGC 2623, a more infrared-luminous system, is at a later merger stage. Although less X-ray lumiiious than the Anteniiae, it has a hotter nuclear feature, and a larger outflow. Thesfc observations are part of a chronological sequence of merging galaxies, carefully chosen in order to study the evolution of their X-ray properties through the merging process. These early results suggest that, as a rrierger progresses, the X-ray emission may evolve more rapidly than the infrared, and that massive hot outflows are generated.

INTRODUCTION AND OBSERVATIONS

Although merging galaxies have been the subject of intensive multiwavelength studies, particularly over the past decade, studies in the X-ray band have been severely restricted by the limited spatial resolution and sensitivity of instrumentation available. The best study prior to the launch of ROSAT was the Einstein IPC observation of the Antennae, which produced interesting but inconclusive results.

We are studying the X-ray development of merging galaxies at different stages of coalescence. ROSAT PSPC results for two such systems are presented here.

The Antennae, NGC 4038/9 (Arp 244), has much multi-wavelength information available. It is one of the closest merging systems (hence both large and bright) and its classic two-tailed form makes the presence of an interaction unambiguous.

ABSTRACT

A 2D hydrodynamic Eulerian code is applied to study the effect of induced star formation in galactic cooling flows. Our main findings are: 1) Star formation activity is induced by ram pressure when the mass loss is dominated by late type giants. 2) Star formation activity is insensitive to the environment when the mass loss is dominated by early type giants. The present calculation supports the claim that the Butcher-Oemler effect is caused by star formation activity and provides a natural explanation for its source.

INTRODUCTION

The intra-cluster gas (ICG) is a hot rarefied gas that experiences negligible cooling during a Hubble time. This fact changes as the gas flows into the inner parts of elliptical galaxies and encounters the hot inter stellar medium (ISM). The cooling time in the ISM dominated region may become 6 to 7 orders of magnitude smaller then the Hubble time (cf. Portnoy et al. 1993, hereafter PPS). Regions in which cooling times are very short are also found in the centers of many cluster of galaxies (for a review cf. Sarazin 1988 and references therein). These high density and low temperatures regions are strong sources of thermal X-ray radiation, and are known as cooling flows.

The ISM Removal Problem

In contrast to cluster cooling flows that were discovered observationally (cf. Sarazin 1988), galactic cooling flows (in normal ellipticals in contrast to CDs) were discovered numerically in simulations performed by Shaviv and Salpeter (1982, here-after ShS). The main motivation for their numerical simulation was the introduction of the cooling process of the gas into the simulations. As a consequence ShS frequently encountered a cooling instability.

ABSTRACT

During the stage of galaxy formation, or because of subsequent accretion events, cold, rotationally supported matter carrying substantial angular momentum is deposited and organized in disks/rings at the outer parts of many types of galaxies. Examples are warped HI disks in spirals, disks of gas and dust in ellipticals, and polar rings around SOs/ellipticals. The observed nuclear activity in many galaxies has its origin in mass transfer from large radii into the centers of galaxy potential wells. We describe two stages of such a mass transfer process and present related numerical multidimensional hydrodynamical simulations. In the first stage, gas still carrying some angular momentum flows toward the nuclear region as a moderately inclined outer disk is attempting to settle toward an energetically “preferred orientation.” In the second stage, a nuclear accretion disk, influenced by the gravitational potential well of a central massive black hole and possibly by a coherent weak magnetic field, suffers one of several known dynamical nonaxisymmetric instabilities that drives gas deeper into the potential well and regulates its accretion onto the black hole.

INTRODUCTION

Although gas is predominantly found in spiral galaxies distributed in rotationally supported large-scale disks (∼ 10 kpc), recent observations reveal the existence of counter-rotating gaseous disks in the central regions of many elliptical and SO galaxies (Franx and Illingworth 1988; Bertola et al. 1990; Bertola, Buson, and Zeilinger 1992). The accepted interpretation of such observations is that gas in ellipticals and SOs must be of external origin.

ABSTRACT

We have made the first neutral hydrogen maps of the galaxy pair, NGC 5775, and NGC 5774, as well as a number of radio continuum maps from several independent observing runs at 6 and 20 cm. Although the galaxies are not strongly distorted, optically, we have discovered clear evidence for an interaction in the form of two connecting H I bridges through which gas is travelling from NGC 5774 to NGC 5775. Along the southern bridge, we have also detected non-thermal radio continuum emission, suggesting (but not requiring) that star formation may also be occuring between the galaxies. In addition, we have discovered H I arcs and extensions away from the plane of the IR-bright, edge-on galaxy, NGC 5775, and can confirm previous detections of arcs and plumes in the radio continuum. In this respect, NGC 5775 appears to be very similar to NGC 891.

INTRODUCTION

NGC 5775 is an edge-on, spiral galaxy with a face-on companion, NGC 5774, 4.3′ to the NW. It belongs to a small group (Group #148; Geller and Huchra 1983) which, in addition, includes IC 1070, only 3.9′ to the SW of NGC 5775. NGC 5775 is the dominant galaxy in the group, it is an IR bright galaxy (Soifer et al. 1987), and has a nuclear HII region spectrum (Giuricin et al. 1990). NGC 5774 is a barred “low surface brightness” (LSB) galaxy (Romanishin et al. 1983); however, its central surface brightness is more than a factor of 5 brighter than the brightest LSB galaxies studied by G. Bothun (these proceedings). Neither galaxy appears strongly tidally disturbed on the POSS prints.