Central concentration of light

The galaxy classification system proposed by Morgan (1958,1959a), which is sometimes referred to as the Yerkes system, is a one-dimensional scheme based on central concentration of light. Morgan arranged galaxies in a sequence a–f–g–k, with objects of type a having the weakest central concentration of light and those of type k having the strongest central concentration. The Yerkes system also recognizes the intermediates types af, fg and gk. Abraham et al. (1996b) have shown that it is possible to measure a central concentration index C, determined from measurements of the intensity-weighted second-order moments of a galaxy image, which is closely related to the central concentration classification of the Yerkes system.

Galaxies with Morgan type a tend to have early-type (A) spectra, whereas galaxies of type k mostly exhibit late-type (K) integrated spectra. This linkage between morphology and spectral type shows that the dominant stellar population in centrally concentrated galaxies is old, whereas objects with a low central concentration of light tend to have a strong young population component. Probably this correlation between central concentration and integrated spectral type is largely due to the fact that regions of high gas density will usually collapse at earlier times than is the case for low density regions. This is so because the collapse time-scale τ∝(Gρ)−½. As we shall see in Chapter 11 a puzzling exception is provided by the dwarf spheroidal galaxies, most of which are dominated by an old stellar population, even though they are of low density.

Luminosity effects on morphology

From a study of galaxies in the Virgo cluster Holmberg (1958, p. 69) noted that giant galaxies have a higher surface brightness than dwarfs. When the prints of the Palomar Sky Survey first became available in the late 1950s it was immediately obvious that this large, and very uniform, database of galaxy images enabled one to segregate lowluminosity dwarfs from the much more numerous galaxies of average luminosity. Based on inspection of the prints of the Palomar Sky Survey, van den Bergh (1959, 1966) was able to compile catalogs of 243 DDO dwarf galaxies north of δ= −27°. The entries in these catalogs showed that the distribution of such dwarfs on the sky is broadly similar to that of nearby giant galaxies. This conclusion was confirmed for Virgo dwarfs by Reaves (1956, 1967). Furthermore, observations in the Local Group clearly show that dwarfs cluster around giants. Van den Bergh's data also showed that (1) the fraction of all galaxies classified as irregular increases dramatically with decreasing luminosity, and (2) the fraction of all spirals that are barred is much lower among giants than it is among dwarfs. In a subsequent study van den Bergh (1960a,b,c) was able to show that both the surface brightnesses of spiral galaxies and their morphologies are functions of luminosity. Supergiant spirals were found to have long and well-developed ‘grand design’ spiral arms, whereas low-luminosity spirals tend to have poorly developed ‘scraggily’ spiral arms.

Cores and power-law profiles

Lauer et al. (1995) have used the Planetary Camera of the HST to image the central regions of 57 early-type galaxies. They found that the radial surface brightness profiles of most of these fall into two distinct classes: (1) galaxies that have cores, and (2) galaxies that exhibit power-law profiles that continue down to radii near the resolution limit. Of the galaxies observed by Lauer et al. 15 have cores and 30 exhibit power-law profiles. Among the galaxies that have been classified as having either cores or power-law profiles 21 are contained in A Revised Shapley–Ames Catalog of Bright Galaxies (Sandage & Tammann 1981). Since the statistics of objects in this catalog are better understood than those of the entire sample, only the nine galaxies with cores and the 12 having power-law profiles that are in the Shapley–Ames Catalog will be considered below. The most striking feature of these data (which has already been commented on by Lauer et al. and others) is that the galaxies with cores tend to be more luminous than those with power-law profiles. For the Shapley–Ames sub-sample a Kolmogorov–Smirnov test rejects the hypothesis that the galaxies with bulges were drawn from the same luminosity distribution as those having power-law profiles at the 97% confidence level.

Most spiral galaxies are, to a good approximation, oblate spheroids that can be arranged on the sequence Sa–Sb–Sc. However, a significant minority exhibit bar-like structures and may be placed on the SBa–SBb–SBc tine of Hubble's ‘tuning fork’ diagram. Objects, with less pronounced bars, can be arranged on the intermediate sequence S(B)a–S(B)b–S(B)c between normal and barred spirals. Some disks appear to be globally oval (Kormendy 1982, p. 135). Such oval disks are of interest because, like bars, they represent non-axisymmetric distortions of the gravitational potential. An excellent review on the dynamics of barred galaxies has been given by Sellwood & Wilkinson (1993). Infrared imaging shows that some galaxies contain small IR bars (Frogel, Quillen & Pogge 1996). When such small inner bars are found in galaxies with large outer bars there is no correlation between the position angles of the large and small bars.

Bars are important to the dynamical evolution of galaxies (Kormendy 1982, 1993) because they can (a) lose angular momentum to cold disks and dark halos, and (b) gain angular momentum from rapidly rotating bulges. Furthermore, transfer of significant amounts of gas by bars to the nuclear regions of galaxies will increase the central galactic mass concentration, which in turn will make the galactic disk less prone to the development of bar-like distortions. In other words stellar bars can self-destruct (or transform themselves into lenses) by transporting too much gas to the nuclear regions of their parent galaxies.

Yogi Berra once said that ‘you can observe a lot by just watching’. The truth of this aphorism struck me when the first prints of the Palomar Sky Survey started to arrive at the Universitäts Sternwarte in Göttingen, where I was a graduate student in 1955. Just looking at this marvellous atlas immediately showed a number of interesting things that had not been so obvious on the smaller, and less homogeneous, databases that had previously been available: (1) The most luminous galaxies in clusters are ‘pretty’ because they have long well-defined spiral arms, whereas ‘ugly’ spirals of lower luminosity tend to exhibit short patchy arms. (2) Intrinsically faint galaxies generally have lower surface brightnesses than do luminous ones. (3) Galaxies in rich clusters sometimes exhibit peculiarities, like fuzzy spiral arms, that are rare among isolated field galaxies.

In the present volume, which is based on a series of lectures given at the University of Victoria in early 1997, I have tried to provide an up-to-date summary of current ideas on the morphology [morphe = shape] and classification of galaxies. I am indebted to Roberto Abraham for suggesting that I write this review. I also thank Ralf Bender, Scott Tremaine and Stephen Zepf for discussions on the interpretation of the classification of elliptical galaxies, and Guy Worthey and Masafumi Noguchi for discussion of the abundance ratio of elements to iron in normal and barred spirals.

Introduction

Dwarf spheroidals are the most common type of galaxy in the Universe. The fact that they were not discovered until 1938 is entirely due to their feeble luminosity and low surface brightness. Of the 29 galaxies that are known to be located within 1.0 Mpc, approximately half are dwarf spheroidals (dSph). A listing of these Local Group dSph galaxies is given in Table 14. For the sake of completeness the dSph/dE galaxies NGC 147 and NGC 185, which are both brighter than Mv= −15.0, have been included in the table. Since most of the faintest known Local Group members are dwarf spheroidals it is almost certain that additional very faint dSph galaxies remain to be discovered in the Local Group. In particular it seems probable that more dSph companions to M31 will eventually be found. Only three such objects (And I, And II and And III) are presently known (van den Bergh 1972), whereas seven dSph companions (Sgr, UMi, Dra, Scl, Sex, Car, For) are known to be located within 150 kpc of the Galaxy – even though the Milky Way system is less luminous than the Andromeda nebula. It is, of course, possible that the small number of M31 dSph satellites is due to the fact that some dwarf companions to M31 were destroyed by tidal interactions with M32 and NGC 205. For reviews on dwarf spheroidal galaxies the reader is referred to Da Costa (1992), Gallagher & Wyse (1994) and Ferguson & Binggeli (1994).

A short history of stellar astrophysics

Since the Sun is a star it is probably correct to say that stellar astrophysics began with Newton's well-known explanation for the Keplerian laws of planetary motion. Although J. Goodricke observed the eclipsing binary variable Algol (β Persei) in 1782, it was not until 1803 that Sir William Herschel's observations of Castor proved that two stars revolve around each other owing to their mutual gravitational attraction.

The first measurements of stellar parallax were made by F. W. Bessel and F. G. W. Struve in 1838. F. Schlesinger revolutionized stellar distance determinations in 1903 when he introduced photographic parallaxes and thereby enabled astronomers to measure parallaxes to an accuracy of about 0.01 arc seconds. K. Schwarzschild initiated photographic photometry during the years 1904–8. Photoelectric photometry of stars began shortly after the photocell was invented in 1911.

J. Fraunhofer discovered Fraunhofer absorption lines in the solar spectrum in 1814 and subsequently observed similar lines in other stars. In 1860 Kirchhoff formulated the relationship between radiative absorption and emission of radiation which is known as Kirchhoff's law. The Doppler effect and Kirchhoff's law formed the conceptual basis of early studies of stellar atmospheres. The quantum theory of blackbody radiation was introduced by M. Planck in 1900. To a first approximation most stars radiate as blackbodies with superimposed absorption and emission lines. The modern theory of radiative transfer in stellar atmospheres was initiated in 1906 by K. Schwarzschild.

Remarkable progress in understanding stellar phenomena has occurred in recent decades. This textbook discusses in some detail those equations and physical processes that are of greatest relevance to stellar interiors and atmospheres and closely related astrophysics. Motivation for writing this book came from my own research interests and also from teaching graduate astrophysics courses, especially a course on stellar interiors at the University of Maryland. Although the text emphasizes physical principles, astronomical results and unresolved issues are also described.

Introductory material on the history of stellar astrophysics, astronomical observations, star formation and stellar evolution are given in Chapter 1, which also contains a discussion of spectroscopic binaries. Differences between single and binary star evolution have explained a number of interesting observations that are described further in later chapters.

Stellar interiors is one of the most fundamental subjects in astrophysics. Although complicated physical processes are decisive in explaining some predictions of stellar model calculations, the basic principles of stellar interiors do not require a comprehensive knowledge of them. Chapter 2 gives an introductory discussion of the physics and equations of stellar interiors. It also includes a short description of numerical methods.

Statistical physics provides the theoretical basis for much of stellar astrophysics. In Chapter 3 those aspects of statistical physics that are of greatest relevance are developed in some detail. Stellar opacities play a vital role in interpreting observations. Absorption processes are described in Chapter 4.