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

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3.3. The Galactic Halo

An abundance gradient also exists in the galactic halo, in that relatively metal-rich globular clusters with integrated spectral type G are usually confined to the "inner halo" within 10 kpc of the galactic center (Kinman 1959, Searle & Zinn 1978, Harris & Canterna 1979, Zinn 1980a; but see Cohen et al. 1980 on Pal 12, a metal-rich cluster at 14 kpc), whereas metal-weak clusters of type F are found at all galactocentric distances. The latter have [Fe/H] geq - 2.3 (Cohen 1979b), but in the case of the metal-rich clusters the estimated metallicities of two prototypes, M71 and 47 Tuc, have recently been drastically revised downwards from 0.0 and -0.4, respectively (Hesser, Hartwick & McClure 1977), to -1.2 (M71: Cohen 1980a, b; 47 Tuc: Pilachowski, Canterna & Wallerstein 1980), and with this revision of the calibrators carried through, the overall abundance gradient expressed as [Fe/H] virtually disappears (Zinn 1980b). These more recent estimates may well be slightly too low (cf. Zinn 1980b); e.g., for 47 Tuc, Mallia (1978) estimated [Fe/H] appeq -1.0, Dickens, Bell & Gustafsson (1979) suggested -0.8, and E.A. Mallia and B.E.J. Pagel (in preparation) find [Fe/H] = -0.9 from a detailed analysis of 11 red giants. Observations of RR Lyrae stars in Baade's window give <[Fe/H]> = -0.65 ± 0.15 for field stars in the inner halo (Butler, Carbon & Kraft 1976). However, there is no doubt that the earlier estimates are substantially too high, and that even these "metal-rich" globulars fall below the nearby galactic clusters in metallicity, so that the difference in color-magnitude diagrams commented on by Demarque & McClure (1977) is no longer so surprising (cf. Demarque & McClure 1980). Stars of a single cluster are usually found to have identical metallicities, with the marked exception of omega Cen where they range over an order of magnitude or more (Kraft 1979 and references therein, Rodgers et al. 1979, Cohen 1980a, Mallia & Pagel 1981). M22 may possibly also have some range of metallicities (Hesser & Harris 1979).

Rodgers, Harding & Sadler (1980) have found a population of intermediate-age A-type stars with near-solar abundances up to 8 kpc from the galactic plane which they suggest could be remnants of an encounter between the Galaxy and a third Magellanic-type object some 2 billion years ago. Gas in the galactic halo also exists and produces absorption lines detectable in the ultraviolet (Boksenberg et al. 1978, Savage & de Boer 1979, 1980, Ulrich et al. 1980). Visible interstellar absorption lines from gas far out in the disks or halos of other galaxies are also seen, e.g. Ca+ (but not Na) in NGC 3067 against the quasar 3C232 (Boksenberg & Sargent 1978) and in galaxies of the Klemola 31 group against PKS 2020-370 (Boksenberg et al. 1981); such gas is likely to cause many of the absorption-line systems observed in high redshift quasars in general (Boksenberg 1978, Sargent et al. 1979), but so far little is available in the way of precise quantitative abundance data.

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