Annu. Rev. Astron. Astrophys. 1981. 19: 77-113 Copyright © 1981 by Annual Reviews. All rights reserved

4.3. Integrated Light from Stellar Populations

The absence of HII regions from elliptical or SO galaxies, except in some of their nuclei where excitation mechanisms may be uncertain, means that abundance determinations must be based on color or absorption-line-strength measurements. Since individual stars can be resolved only for the nearest extragalactic systems, these measurements are inevitably of an integrated stellar population, so that theoretical population models and/or some form of empirical calibration against galactic globular clusters must be employed in their interpretation. Integrated colors and line strengths have also been measured in and around the nuclei of spiral galaxies (particularly M31), although interpretation becomes even more difficult in the presence of young blue stars.

The variation of color indices with metallicity results from a combination of effects: (a) the U band is strongly influenced by metallic line blanketing in the stellar atmospheres; (b) as the metallicity of the stars increases, so does the envelope opacity, causing a drop in their effective temperatures and a shift of flux towards the infrared; (c) the relative number of stars on the blue horizontal branch decreases as metallicity increases. All three effects cause a reddening of color with increasing metallicity. The (U - V) index is far more sensitive to metallicity changes than is (B - V), but the U band is also particularly sensitive to the presence of young blue main-sequence stars and theoretical population models have difficulty in accurately reproducing the ultraviolet fluxes observed. Indices including an infrared or red color such as (V - K) or (U - R) benefit from effect (b) above. The infrared colors are more sensitive to the presence of a dwarf-enriched population, but such a population seems to be observationally excluded (see section on luminosity indicators below) so this is probably not important.

Line strengths have been measured mainly by intermediate-band photometry on various systems, particularly those of Spinrad & Taylor (1969), DDO (McClure & van den Bergh 1968), and Faber (1973), important features being CN around 4200, the sodium D lines, the magnesium b triplet and MgH band head around 5175, various groups of iron lines, and the infrared luminosity-sensitive features discussed later. The presence of young blue stars can be detected by the addition of a hydrogen line (Heckman 1980) or by noting discrepancies between line strengths and colors (Burstein 1979). Even when uncalibrated, spatial variations of line groups can provide evidence that at least something is varying between or across galaxies.

4.3.1. CALIBRATION OF COLORS AND LINE STRENGTHS     For calibration an initial mass spectrum of the form N(m)dm m^{-(1 + x)} dm is assumed where N(m)dm is the number of stars formed between masses m and m + dm;x = 1.35 would correspond to the usual Salpeter function. A single age and metallicity are also usually assumed for the population, both rather dubious assumptions, using theoretical stellar evolutionary tracks to predict the fraction and parameters of the giant stars present. Color calibrations made in this way have been given by Strom et al. (1976) suggesting (U - V) / logZ 0.85 and (U - R) / logZ 1.18 (Strom & Strom 1978), by Tinsley (1978, revising Larson & Tinsley 1974) suggesting (B - V) / logZ 0.42, and particularly by Aaronson et al. (1978) who tabulate model (U - V), (V - K), and (J - K) values for several metal abundances. For abundances near solar, Aaronson et al.'s results suggest (U - V) / logZ 1.13 and (V - K) / logZ 0.62. These calibrations have been revised somewhat by Frogel et al. (1980), but without altering the overall trends. Although these calibrations give a reasonable estimate of relative abundance changes, the fixing of an absolute scale is difficult. The major problem [which also applies for semi-empirical calibration of indices, like that of (U - R) by Hartwick (1980), which rely on globular cluster colors] is the uncertainty in the true metal abundance of the few so-called "metal-rich" globular clusters (cf. Section 3.3 above), since most of the galaxies are more metal-rich than the majority of globulars. These revisions, even if we assume a fairly conservative -0.8 for [Fe/H] in both M71 and 47 Tuc, greatly affect the metal-rich end of the color calibration, with the galaxies lying on an extrapolation of color-metallicity relations for globulars. Reliable metallicity determinations in metal-rich globular clusters are thus of prime importance.

Since the early work of Spinrad & Taylor (1969), theoretical line-strength calibrations have been attempted by Faber (1973), Larson & Tinsley (1974), Mould (1978) for the Mg 5175 index, and Cohen (1979a) for several groups of lines.

4.3.2. POPULATION MODELS AND LUMINOSITY INDICATORS     Population modeling involves the fitting of colors and line strengths by mixtures of stars of various types, ages, and metallicity, usually with imposed astrophysically reasonable constraints (such as smooth variation of the main-sequence mass function) on the model. It has been attempted with varying degrees of sophistication for both ellipticals and the nuclear regions of spirals (Joly 1973, 1974, Turnrose 1976, Williams 1976, Prichet 1977, Prichet & Campbell 1980, O'Connell 1976, 1980, Heckman 1980). Separation of age and metallicity effects is not simple, and the assumption of a uniformly old population for elliptical galaxies is seriously questioned by O'Connell's claim that star formation in M32 continued up until 5 × 10⁹ yr ago. But at least the mass function can be constrained by luminosity-sensitive line strengths. The early suggestions (Spinrad & Taylor 1971, Joly 1973) were of a dwarf-enriched population in the nuclei of spiral galaxies, i.e. x 3.5. Measurement of infrared features sensitive to the luminosity of stars, i.e. H₂O 2.1µ, CO 2.3µ (Baldwin et al. 1973, Frogel et al. 1975, 1978, Aaronson, Frogel & Persson 1978), the NaI doublet near 8185 and CaII triplet near 8500 (Cohen 1978), the Wing-Ford band 9910 (Whitford 1977, Cohen 1978) identified as due to Fe H (Wing, Cohen & Brault 1977), all indicate that in elliptical galaxies and the nuclei of spirals the late-type giant stars are the dominant contributors to the observed light, and that x 2. Cohen (1978) also investigated infrared TiO bands, but concluded that they were not sufficiently sensitive to luminosity to be a useful population discriminant.