This book is concerned with the structure and evolution of the stars, that is the life history of the stars. Its aim is to show how observations of the properties of stars and knowledge from many branches of physics have been combined, with the aid of the necessary mathematical techniques, to give us what we believe is a good understanding of the basis of this subject.

Because the stars are so remote from the Earth it may seem surprising that we can learn anything about their physical dimensions. To hope to be able to describe their internal structure and, still more, their evolution appears extremely optimistic. The mass and radius of a few stars can be measured directly, but for most stars the only source of information is in the light that we receive from them. This gives us some idea about the temperature and chemical composition of the surface layers of the star and about the total light output (luminosity) of those stars whose distance from the Earth is known. It also indicates that some stars are rotating rapidly or have strong magnetic fields and that others are losing mass from their surfaces. No direct information is obtained about physical conditions in the interiors of the stars, with the exceptions (discussed in Chapters 4 and 6) that the neutrinos emitted in the solar centre can be detected on Earth and that vibrations of the solar surface can provide information about the interior by techniques similar to seismology.

Introduction

In Chapter 6 I have given an account of calculations of stellar evolution away from the main sequence. These calculations have not followed a star through its entire life history except approximately in the case of those low mass stars which do not burn their hydrogen and helium before their central temperatures cease to rise and the star as a whole then cools down and eventually ceases to be luminous. For more massive stars there are several evolutionary stages after those which have been discussed in Chapter 6.

It is difficult to calculate the evolution of a star all the way from its initial main sequence state to the end of its life history. There are many reasons why this should be so. One important difficulty is that in all calculations errors tend to accumulate. The numerical processes used in solving differential equations can never be completely accurate and over a long period of integration these mathematical errors tend to pile up. In addition, the mathematical expressions for the physical laws are only approximate. In many cases the physical processes which occur in stars cannot be observed directly in the laboratory and in the case of stellar convection there is neither a good theory nor a good experiment. The uncertainties in the internal structure of a star and particularly in its observed properties may be small when it is on or near to the main sequence, but they might lead to the prediction of an incorrect physical process at a later stage.

ABSTRACT

Simulations which explore mergers like those thought responsible for the shells around many elliptical galaxies find little correlation between the distribution of stars and gas in remnants. Mergers of small companion disks consisting of both gas and stars with non-spherical primary potentials produce shell galaxies with gaseous nuclear rings and clumps.

INTRODUCTION

Models which follow the infall of less-massive companion galaxies show that shell galaxies can be formed by accretion. However, it is probable that the sources of material also contain significant amounts of gas. We investigate encounters that produce shells by modeling interactions between non-spherical primary galaxies and companions containing both stars and gas with a three-dimensional code (TREESPH: Hernquist and Katz 1989). Primaries are modeled with rigid elliptical potentials of the form presented by Hernquist (1990) with scale-length a = 1. Physical time t′ is related to the calculation time unit by t′ ≈ 4.3 × 106t. The companion is a rotationally supported disk in which particles are distributed according to an exponential surface density profile. Stars have a total mass 1/10 and gas 1/100 that of the primary. In most interactions, the companion potential is disrupted at a small distance from the primary after which the particles evolve in the solitary primary gravitational field.

ABSTRACT

Formation of first AGN probably follows closely the formation of their host galaxies. Both processes may involve similar astrophysical events or processes, with comparable energetics. Young AGN could have had a profound effect on the host galaxies. High–Z AGN may be used as markers of galaxy formation sites, or probes of early large-scale structure. Some may well be examples of galaxies in the early stages of formation, although the nature of their dominant energy sources and their evolutionary status remain ambiguous. Future observations at IR through sub-mm wavelengths lead to a discovery of possible obscured protogalaxies and nascent AGN, and probe their formation at very high z.

INTRODUCTION: FORMATION OF GALAXIES AND AGN

Probably the two key problems in extragalactic astronomy and cosmology today are the understanding of formation and evolution of galaxies and large-scale structure, and the understanding of formation and evolution of AGN. These problems may be fundamentally connected: it is now generally understood that the same kind of astrophysical processes, dissipative merging and infall, may be central to both formation of galaxies and formation of AGN, and subsequently to be contributing both to the simulation of star forming activity in galaxies and to the feeding of their central engines. AGN may also exert a considerable feedback to their host galaxies, and perhaps even determine some of their global properties.

ABSTRACT

We model a range of flyby galaxy interactions in order to investigate the formation of starbursts in galaxies with induced stellar bars and/or mass loss or accretion. The models indicate that mass transfer is important in triggering radial gas flows and starburst activity if counter-rotating accretion occurs; i.e., accretion from a prograde disk onto a retrograde disk. Such accretion proves effective in shedding rotational angular momentum of the ISM, resulting in the radial gas flow and subsequent nuclear ISM concentration, while leaving behind a relatively unperturbed stellar disk. However, bar formation proves more important under a wider range of interaction scenarios than does mass transfer, and thus bar formation is the dominant process in triggering nuclear activity in interacting systems as a whole.

INTRODUCTION

The link between galaxy interactions and elevated star formation rates has been demonstrated through observations of such star formation tracers as optical emission lines (e.g., Kennicutt et al 1987; Bushouse 1987), strong far infrared emission (Lonsdale, Persson, and Matthews 1984), and radio continuum emission (Hummel 1981). However, while this large body of evidence indicates that interactions can cause starbursts, it is not at all clear that they must do so. A large fraction of interacting galaxies show little or no increased star formation (Kennicutt et al 1987; Bushouse 1987), suggesting that the triggering mechanism for these starbursts must involve some complicated function of interaction geometry and galaxy properties.

ABSTRACT

We have detected a diffuse, continuum knot which may be a reminant nucleus, in the inner regions of Markarian 315. This knot is associated with a complex, ring-like structure in both the continuum and ionized gas emission. The occurance of this feature in combination with an extensive, ionized streamer, or tidal tail, and highly non-circular kinematics in the ionized gas, suggests that this galaxy has suffered a disruptive, tidal interaction whose influence extends well into the inner one kiloparsec region.

INTRODUCTION

Markarian 315 (Markarian and Lipovetskii 1971) (also IIZwl87), is a moderately luminous Seyfert 1.5 galaxy (Koski 1978). It has a redshift of 11,820 km s-1 relative to the galactic center and Mv= –21.6 (Sargent 1970). For this discussion, we adopt a scale of 0.57 h-l kpc arcsec-1.

Radio frequency images of Markarian 315 show that it is a steep spectrum source with a diffuse morphology and a total extent of 2.9 h-l kpc (Wilson and Willis 1980; Ulvestad, et al. 1981). This extended structure is the largest in the sample of Seyfert galaxies studied by these authors. (These typically ranged between 0.4 and 1.0 h-l kpc). It is, however, consistent with an extended starburst in the galaxy and the IRAS fluxes are also consistent with this interpretation (MacKenty 1989).

MacKenty (1986) discovered an extraordinary, 80 kpc, streamer of ionized gas emerging from near the nucleus, extending in a straight line for 60 kpc then bending back in a hook. He suggested two possible origins for this feature: a tidal interaction or a dormant radio jet.

ABSTRACT

We examine the distribution and kinematics of atomic and molecular gas mapped in a number of galaxies suspected to be in the process of merging. In most cases, the nuclear region of the merger has a high concentration of molecular gas, and a deficiency of atomic gas as compared with larger radii. Thus the total surface mass density of gas often has a minimum at an intermediate radius. In cases where the gas rotation curve is measured, the transition from regions dominated by molecular gas to those of atomic gas corresponds to abrupt changes in rotation characteristics. We propose that the merger is efficiently converting ISM from atomic into molecular form in central region of these galaxies, and that the dense clouds are experiencing radial accretion at a higher rate than diffuse gas.

INTRODUCTION

Since the early work of Toomre and Toomre (1972), much progress has been made in understanding dynamical processes of merger galaxies (cf. Barnes and Hernquist 1993). One result that appears consistently in theoretical studies and numerical simulations alike is that the end product of the merger is an early type galaxy. Increasingly, this scenario has acquired observational support as well. For example, the K-band light profiles of many Arp galaxies, mostly advanced mergers, show r1/4 law typical of ellipticals (e.g., Wright et al. 1990; Stanfrd and Bushouse 1991). Yet, the process in which the merging disks shed their abundant gas mass remains unclear, and numerical simulations are far from adequately resolving this problem given the enormous dynamical range required to mimic the changes in the ISM.

ABSTRACT

I review recent work on galaxy formation and relate it to questions concerning the formation and fuelling of active galactic nuclei. The theory of galaxy formation has developed dramatically in recent years as a result of new analytic methods coupled with substantial programs of direct numerical simulation. Many aspects of how galaxies might form in a universe where structure grows by hierarchical clustering are understood quite well. Others, particularly those that are closely linked to the star formation process, remain highly uncertain. Nevertheless, it is now possible to calculate formation and interaction rates for galaxies with some confidence in a wide variety of cosmogonies. It seems likely that nuclear activity, either starburst or AGN, is an inevitable consequence of the violent, asymmetric, and time-dependent processes which occur during the assembly of galaxies.

INTRODUCTION

The idea that quasars might be related to galaxy formation followed quickly after the first measurements of QSO redshifts but was somewhat neglected after the near-universal acceptance of the argument that QSO luminosities are more easily explained by accretion onto a supermassive black hole than by starlight. In such a model black hole formation and fuelling are major issues which must be addressed before QSO's and galaxy formation can be linked. There has always been some dissent from this model, most notably in recent years from R. Terlevich and his collaborators (e.g. Terlevich and Boyle 1993), but recent discussions of quasar formation have tended to emphasize how late the onset of quasar activity may be in comparison with the initial collapse of a protogalaxy (see, for example, Turner 1991).

ABSTRACT

It is generally believed that galaxy interactions induce bursts of star formation. We observed a sample of galaxies undergoing different types of interactions in the expectation that the location and nature of the induced star formation could be related to the dynamics of the interaction. We found instead that in almost all galaxies the star formation is concentrated in the nucleus or nuclei, sometimes to a remarkable degree. It appears that extra-nuclear star formation is either difficult to trigger or so short-lived as to be rarely observed. We discuss in detail two galaxies: NGC 5253, site of the most concentrated star formation region yet known, and Arp 30, the exception where star formation is broadly distributed and is seen in both nuclei and in clumps in the bridge connecting them.

INTRODUCTION

Studies of the global properties of large samples of galaxies have shown that interactions between galaxies correlate with bursts of star formation and have lead to the generally accepted belief that interactions can trigger such bursts. Attempts to correlate the local star formation properties of individual galaxies and the interactions they have undergone have been much less successful. We therefore undertook a multiwavelength study of star formation in a sample of interacting or post-interaction galaxies, in the hope of being able to relate the location and type of star formation to the interaction history. The observations include infrared spectra, radio continuum maps, and images in continuum bands, the Wolf-Rayet feature and Hα.

Introductory remarks

In previous chapters, we have considered microscopic instabilities, such as the two-stream instability, and MHD instabilities. The third major category of instability is that of ‘resistive instabilities.’ They differ from two-stream instabilities in that they may be treated by fluid equations, and they differ from MHD instabilities in that they are due essentially to the fact that the resistivity is nonzero. A nonzero resistivity allows the magnetic-field lines to move independently of the plasma, so that the ‘frozen flux theorem’ is not applicable in this context. A crucial consequence of this effect is that ‘magnetic reconnection,’ shown schematically in Fig. 17.1, is allowed. In addition, field lines can ‘vanish.’ Consider, for instance, the azimuthal magnetic field set up in a linear pinch. If the resistivity is nonzero, the current will dissipate. As a result, the circular field lines will shrink in diameter and the innermost loops will shrink to a point and disappear (see Fig. 17.2). If there is no external energy supply, the decrease in magnetic energy is converted into joule heating.

Current sheet configuration

The resistive instability that is of special importance in astrophysics is the ‘tearing-mode’ instability that can develop in a current sheet configuration. We shall find that this instability leads to development, in a field configuration of the type shown in Fig. 17.3(a), of ‘magnetic islands,’ as shown in Fig. 17.3(b).

ABSTRACT

The Galactic Center shows evidence for the presence of three important AGN ingredients: a black hole (M* ∼ 106 M⊙), an accretion disk (10-8.5 – 10-7 M⊙ yr-1) and a powerful jet (jet power ≥ 10% disk luminosity). However, the degree of activity is very low and can barely account for the energetics of the central region.

INTRODUCTION

The dynamical center of the Galaxy is the radio point source Sgr A*, which is also the center of the central star cluster (Eckart et al. 1993). Investigations of the enclosed mass in the central region show that there is evidence for a mass concentration of the order of 106 M⊙ within the central arcsecond (Genzel and Townes 1987). There is good reason to assume that this “dark mass” indeed is the mass of a massive black hole (BH) powering Sgr A*. The total spectrum of this source from radio to NIR was compiled by Zylka et al. (1992). There is a flat radio spectrum up to 7 mm, and a steeply rising submm spectrum, which Zylka et al. interpret as thermal emission from a dust torus surrounding the BH. In the FIR one finds a spectral break at 30 μm indicated by upper limits and a third spectral component rising in the NIR, which has been interpreted as emission from an accretion disk around the BH.

HERTZSPRUNG-RUSSELL DIAGRAM FOR THE SGR A* DISK

Because of strong obscuration in the galactic plane we probably will never be able to measure exactly the optical and UV part of Sgr A*, which is needed to discriminate between different disk models.

ABSTRACT

The effect of an active nucleus on its host galaxy (Shanbhag 1991; Shanbhag and Kembhavi 1988; Begelman 1985) can be large. In particular, heating by the radiation from the nucleus strongly affects the hydrodynamic evolution of gas in the interstellar medium (ISM) of the host galaxy. Enhanced star formation activity on a galaxy-wide scale can be induced. The proximity of NGC 1068 makes it an interesting candidate to study such interaction. The model explains extended X-ray emission and some properties of the diffuse ionized medium (DIM) observed in the galaxy.

INTRODUCTION

NGC 1068 is a nearby Sy 1 galaxy disguised as Sy 2 with its “buried” nucleus obscured from direct view (Antonucci and Miller 1985). The unobscured solid angle as seen from the nucleus, is inferred to be ∼ π (Krolik and Begelman 1986). The emission from the nucleus is modeled by bipolar conical outflows of opening angle ≃ 82° with the symmetry axis inclined to the plane of the galaxy by ≃ 35° (Cecil et al. 1990). Its inferred intrinsic luminosity (Sokolowski et al. 1991) is ≃ 7 x 1043 erg s-1. Many observations (Bergeron et al. 1989; Cecil et al. 1990) show existence of activity in the direction of the cone open angle; implying a connection with the active nucleus. The motivation behind this paper is to understand the nature of such interaction.

ABSTRACT

The infrared luminous galaxy MK 231 appears to exhibit characteristics similar to those of active galactic nuclei — it has been classified as a Seyfert 1 system. However, it has been shown to contain 3 ×x 1010 M⊙ of gas via CO observations. If the CO is confined to the region shielded by dust as in galactic molecular clouds, the molecular gas occupies a much smaller volume than previously thought. The ultimate questions are which characteristic is primarily responsible for most of the luminosity and whether AGN and starbursts are interdependent or coincidental.

THE SUPERGIANT MOLECULAR CLOUD

Scoville et al. (1989) and Radford et al. (1991) have discussed the physical conditions implied by the molecular observations of MK 231. It is inferred from CO observations, that 3 × 1010 M⊙ of molecular gas (H2) resides within a volume of radius RCo < 3 kpc. However, recent infrared observations reveal that the emission from dust at λ ≃ 10 μm arises within a volume of size RIR < 400 pc (Keto et al. 1992). This is an important complementary result to the CO observations because studies within our galaxy show clearly that the molecular gas is largely confined to the region shielded by dust extinction (Young et al. 1982).