In this paper I will try to give the flavor of two different concepts of stellar population, namely, the population of cluster systems within galaxies, and the stellar populations within individual clusters. I hope to show, in concert with other papers in this volume, that a generic vision of Population I and II has been outdated for a long time. I believe that it is still proper to talk about populations within individual objects; in the case of galaxies, one might make a correspondence of populations with the important star forming events in the history of a galaxy.

If the Newtonian inverse-square law is an appropriate description of gravity, then disk galaxies, and the Milky Way in particular, contain large amounts of dark matter: in general, the observed shapes of rotation curves do not correspond to the gravitational potential of the observed stars and gas. The nature of this dark matter is still a topic of debate, the outcome of which will have important consequences for our ideas of how galaxies and larger structures in the universe formed.

In this review I will concentrate on three questions concerning the issue of dark matter in spiral galaxies. In the first place (and for the most part) I will discuss the analysis of rotation curves and the question whether the “maximum–disk hypothesis” is valid. Then I will discuss independent measurements of the disk mass distribution from the thickness of stellar and HI disks. Then follows a review of estimates of the flattening of the dark halos and a short discussion on the possible nature of the dark matter.

Studies of the mass–to–light ratio M/L as a function of radius in a galaxy usually focus on: a) the existence of dark matter in and around galaxies, and b) the presence of massive black holes in their nuclei. Here we concentrate on elliptical galaxies, and summarize some recent work in both areas. We refer to more extensive reviews for earlier developments (Schwarzschild, 1954; Trimble, 1987; Ashman, 1992; Kormendy, 1993). A third area of interest is the behavior of the stellar populations as a function of radius, and its effect on M/L (e.g., Peletier, 1989). We will not discuss this here.

An appropriate subtitle for this talk might be “Newton meets Einstein.” For many decades, the prime tool for studying the amount and distribution of matter in galaxy clusters was decidedly Newtonian, involving at first the measurements of the dynamics of the galaxies themselves and, for the past 15 years or so, the imputed dynamics of the hot, X-ray emitting intra-cluster gas. Einstein enters more recently with the introduction of gravitational lensing as a tool for studying cluster mass distributions. Rapid progress is being made in each of these areas, and there are now attempts to bring them together to give a consistent and more accurate picture of clusters.

The hot stellar component in elliptical galaxies offers clues to both stellar evolution and galaxy evolution. Current observations suggest that extreme horizontal branch (EHB) stars dominate the far-UV emission from galaxies with the strongest “UV upturns,” while post asymptotic giant branch (PAGB) stars are probably significant contributors for weaker galaxies. Spectra near the Lyman limit indicate that a rather narrow range of temperature (and hence EHB star mass) is required. However, other arguments suggest that most of the helium-burning stars in elliptical galaxies are in the red clump. The HB star mass distribution therefore appears to be strongly bimodal. Such bimodality is qualitatively reproduced by two radically different stellar population models, (those of Lee and Bressan et al.), both of which require that the galaxies be very old. However, the Galactic open cluster NGC 6791 also contains EHB stars and exhibits strong bimodality, indicating that old age may not necessarily be a requirement for the UV upturn phenomenon.

Integrated broad-band colors and metallic lines cannot discriminate clearly between the effects of age and Z in old stellar populations. Such data are more sensitive to Z than to age. The Hβ feature provides a way to break this degeneracy. New measurements indicate that the mean stellar ages of typical E galaxy nuclei are fairly young, ranging from 2 to ~12 Gyr. The outer parts of E galaxies are both older and more metal-poor than nuclei, consistent with the formation of E galaxies via mergers and starbursts. Age effects contribute strongly to the classic color-line strength sequence of E galaxies.

The study of stellar populations in external galaxies has so far concentrated on elliptical galaxies, spirals and a few dwarf galaxies, while S0 galaxies found only relatively little attention (with a few notable exceptions, e.g., Gregg, 1989; Bothun and Gregg, 1990). This is astonishing because S0-galaxies offer a unique possibility to study (a) old disk populations outside our own galaxy, (b) the properties of bulges in external galaxies without contamination by blue disk stars, and (c) the gradual change of the stellar populations of galaxies as a function of disk-to-bulge ratio. Last but not least, the comparison of the stellar populations of ellipticals, spirals and S0 galaxies allows to re-iterate on the question whether S0's are principally similar to spirals but have just stopped forming stars earlier, or, whether they form an class intermediate between ellipticals and spirals (e.g.Larson et al., 1980; van den Bergh, 1994).

The Fundamental plane provides a sensitive tool to measure the change in the M/L ratio of early type galaxies with redshift. The evolution of the M/L ratio is a function of the star formation history. It depends on the IMF, the formation redshift, and cosmology. Some model examples are shown, and a first result on the cluster Abell 665 at z=0.18 is given. The measurements confirm the cosmological surface brightness dimming, and imply an evolution of the (red) L/M ratio ∝ (1 + z)1,8±0,7. More data are needed to extend this result to higher redshifts, and to test the underlying assumptions.

Perhaps the single most important component missing from Baade's famous stellar-populations paper is the cold gas, in both its atomic and molecular forms. This review of the effects of late mergers concentrates on the role that this gas plays in forming new stellar populations and defining the characteristics of merger remnants and of some early-type galaxies.

While early work successfully modelled the stellar population of elliptical galaxies as old, single-age, metal-rich systems, there is evidence that a small amount of recent (i.e. a few Gyrs ago) star formation has occurred in some nearby ellipticals (O'Connell, 1980; Rose, 1985; Pickles, 1985; Bica, 1988). Elliptical galaxies reside in environments ranging from the dense cores of rich clusters to isolated field regions. It has long been suggested that the global properties of ellipticals depend on environment. For example, McClure and Van den Bergh (1968) noted apparent differences in the 380-450nm colours between a set of cluster ellipticals and three from the field. Recent work has tended to support the view that the properties of ellipticals depend on environment (e.g. de Carvalho and Djorgovski, 1992). The most likely cause of these differences is the presence of a small amount of recent star formation in the field ellipticals. This environmental dependency may have important consequences for the use of ellipticals as distance indicators.

I review two observational programs which, together, promise to unravel the detailed astrophysical evolution of normal field galaxies over the last 5-7 Gyr. Systematic ground-based spectroscopy of faint galaxies have revealed an increasing faint end slope for the luminosity function with redshift. The trend is strongest for galaxies undergoing intense star-formation. Deep images taken with the repaired HST can be used to count galaxies as a function of morphological type. Regular “Hubble sequence” galaxies follow the no-evolution prediction, but irregular/peculiar sources have a steeper count slope and provide the excess population. Although the overlap between the spectral and HST samples is currently small, plans to merge similar datasets should reveal the physical explanation for the demise of star formation in faint blue galaxies since z ≃0.5-l.

It is now possible to probe stellar populations at large lookback times. The Butcher-Oemler Effect in distant clusters is unambiguous evidence for unexpectedly recent evolution of disk galaxies in dense environments, probably through starbursts induced by interactions. Recent applications of the “red envelope” test at z ≲ 1 indicate that some luminous galaxies formed ~ 10-13 Gyr ago.

We present deep galaxy number counts and colours of K – band selected galaxy surveys. We argue that primeval galaxies are present within the survey data, but have remained unidentified. There are few objects with the colours of an L∗ elliptical galaxy at a redshift of z ≈ 1, in contradiction to standard luminosity evolution models. We present K – band photometry of the objects in a spectroscopic redshift survey selected at 21 < B < 22.5. The absolute K magnitudes of the galaxies are consistent with the no-evolution or pure luminosity evolution models. The excess faint blue galaxies seen in the B – band number counts at intermediate magnitudes are a result of a low normalization, and do not dominate the population until B ≈ 25. Extreme merging or excess dwarf models are not needed at z < 1.

Unfortunately Dr. White was not able to finish a written version of his presentation at the Symposium in time for inclusion in these proceedings. He refers the interested readers to his lecture notes at the August 1993 Les Houches summer school (White, 1995).

In recent years a view spread widely according to which spheroidal stellar systems are not as old as argued by Baade forty years ago and for a long time given for granted. In this – more modern view – most galactic bulges may form late, e.g. as a spontaneous corruption of disks, and most ellipticals may seemingly be late comers, perhaps the result of merging spirals.