Annu. Rev. Astron. Astrophys. 1981. 19: 77-113
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5.4. Spiral Galaxies

5.4.1. COLOR AND ABSORPTION LINE STRENGTHS     Color indices are a much less useful metallicity indicator for spirals and irregulars than for ellipticals because of the overwhelming effect of the young stellar population. The color-aperture effect (de Vaucouleurs 1961, Tifft 1963, Griersmith 1980) indicates a significant reddening towards the nucleus, but this may more obviously be interpreted as an indication of the blue light contribution of young stars in the disk than as necessarily implying an abundance gradient. The spheroidal bulge dominates enough in the nuclear regions of early spirals (S0/a, Sa, Sab) to show a definite color-magnitude relation (Visvanathan & Griersmith 1977) analogous to ellipticals, with Delta(U - V) / DeltaMvs appeq 0.13 where Mvs is the absolute magnitude of the spheroidal component. This color-magnitude relation may well be a metallicity effect. Indeed, line-strength gradients have been found in the inner regions (leq 130 pc) of the bulge component of M31 (McClure & van den Bergh 1968, Oke & Schwarzschild 1975, Morton & Andereck 1976, Faber 1977), although the magnitude of the metallicity change is small (less than a factor of two) over such short distances. There is apparent continuity between the spheroidal component and the nucleus itself (Spinrad & Liebert 1975, Thuan & Oke 1976).

Line strengths in the inner disks of M31 and M81 (McClure 1969, Spinrad & Taylor 1971, Joly & Andrillat 1973, Joly 1973, 1974, Cohen 1978, 1979a) all indicate a decrease in metallicity away from the nucleus, with Cohen's calibration suggesting as much as a factor of four from the nucleus to 300 pc. The absolute mean metallicity of the stars in the nuclear region of M31 remains uncertain due to the calibration difficulties already discussed, but almost certainly lies between one and three times solar (O'Connell 1976, Cohen 1978, 1979a, Heckman 1980, Prichet & Campbell 1980).

Globular clusters in external spirals have been reviewed by Harris & Racine (1979) and we will not discuss them further except to note that Faber, Gaskell & Burstein (1980) find distinct spectral differences between the globular clusters of M31 and those of our own Galaxy.

5.4.2. HII REGIONS     Data for galaxies in which the abundances have been reasonably well determined are assembled in Table 4 and some of these are plotted in Figure 6 as a function of radius (normalized to the de Vaucouleurs 25 mag arcsec-2 isophote, face-on radius R0). The abundances quoted are those derived with electron temperature from weak lines (assuming zero temperature fluctuation), from HII region models, or by the semiempirical line-strength methods of Pagel et al. (1979, 1980), which give reasonable agreement with SNR abundances in M33, especially for N/H, see Figure 2. The considerable data of Jensen, Strom & Strom (1976) have not been included owing to their lower precision. All galaxies showing abundance gradients have lower abundances in their outer parts than in the center, but the best functional representation is not clear. Although some galaxies (e.g. NGC 300, NGC 1365) are reasonably represented by straight lines in Figure 6, others (e.g. M101) appear to show a steeper variation in their inner parts (M101 has the biggest range of any of the galaxies studied). Dufour et al. (1980) suggest that the linear relationship between log(O/H) and R may be more uniform between large galaxies than that defined by R/R0. The radial gradients extrapolate to nuclear gas abundances of order, or greater than, solar. It is not yet clear if there is a definite trend of steepness of gradient with morphological type. Although NGC 1365, M31, and M51 appear to show somewhat shallower gradients (~ 0.04 dex kpc-1) than the later Sc or Scd types (~ 0.1 dex kpc-1), the gradient is rather uncertain for M31 while in NGC 1365 and M51 the presence of noncircular gas motions induced by bar or companion could have influenced the chemical evolution and make general statements about the influence of morphological type unwise until more good data on earlier types is available. The importance of noncircular motions as a homogenizer of composition is demonstrated for the Magellanic Clouds (Pagel et al. 1978), and particularly for the late barred spiral NGC 1313 (Pagel, Edmund & Smith 1980), which shows a delightfully complex velocity field (Marcellin 1979, Blackman 1981). However, giant HII regions of high excitation, i.e. low abundance of oxygen, etc., are more usually seen in Irregulars, late barred spirals, and the outer parts of late-type normal spirals (Scd and Sd).

Table 4. Abundances in HII regions of spiral galaxies

        12 + log (O/H)  
Galaxy       Max b Min b  
NGC,etc. Type logMa / Modot Ref.d (near centre) (outer) Ref.d

NGC 1365 SBbc(Sy) 11.8 1 geq9.1 8.7 1,9
NGC 1566 Sbc(Sy) 11.5: 2 9.6 8.8 10
NGC 5457 (M101) SABcd I 11.4 3 9.5: 8.1 11,12,1,9
NGC 6946 SABcd I 11.3 4 9.1 8.5 13
NGC 0224 (M31) Sb I-II 11.3 3 9.1 8.5 c 14
NGC 5194 (M51) Sbcp I 11.3 3 9.6: 9.0 15
        (9.6:   16
Galaxy Sbc >11.1 5 (9.0 8.3 17
        ( 8.0 18
NGC 2997 SABc 11.0: 2 9.1 8.7 9
NGC 5236 (M83) SABcd I-II 10.9 3 9.7: 9.1 15
NGC 1313 SBd 10.3 6 8.25 8.25 19
NGC 0300 Scd 10.3 7 8.8 8.2 1,9
NGC 0598 (M33) Scd II-III 10.2 3 9.2 8.1 11,1,9,20
NGC 7793 Sd 9.5 8 9.0 8.4 9

a H0 = 50 km s-1 Mpc-1 assumed in mass estimates.
b The Min and Max of columns 5 and 6 refer only to H II regions so far measured, and do not imply absolute limits for the galaxies.
c Large scatter.
d References:

1. Pagel et al. 1979
2. From BGC blue magnitude with mean mass-luminosity relation of type from Faber & Gallagher (1979).
3. Faber & Gallagher 1979
4. Rogstad, Shostak & Rots 1973
5. From V0 = 220 km s-1 assuming spherical distribution. Total mass may be much greater (see reference 3).
6. Carranza & Aguero 1974
7. de Vaucouleurs & Page 1962
8. Aguero 1979
9. M.G. Edmunds and B.E.J. Pagel (in preparation)
10. From ([O II] + [O III]) / Hbeta measured by Hawley & Phillips (1980).
11. Smith 1975
12. Shields & Searle 1978
13. McCall, Rybski & Shields 1980
14. From [O III]/[N II] measured by Rubin, Kumar & Ford (1972)
15. Dufour et al. 1980
16. Shaver, McGee & Pottasch 1979
17. Mezger et al. 1979
18. Peimbert, Torres-Peimbert & Rayo 1978
19. Pagel, Edmunds & Smith 1980
20. Stasinska et al. 1981

Figure 6

Figure 6. Oxygen gradients in Spiral galaxies. References for abundance are given in Table 4. Abscissa is distance of HII region from center, normalized to the de Vaucoulcurs radius.

There is no obvious dependence of either gradient steepness or absolute abundance at a given radius as a function of mass of the spiral (see Figure 7). The gaseous abundances in nuclei of spirals remain somewhat uncertain. Although higher excitation in the nuclear regions than in the surrounding spiral arms might indicate somewhat decreased abundances (e.g. M83, Dufour et al. 1980; NGC 1365, Pagel et al. 1979), the interference of high excitation caused by a nonthermal nuclear source has been demonstrated for the latter case and HII regions near to the nucleus show the higher abundances expected from extrapolation of the disk gradient (M.G. Edmunds and B.E.J. Pagel, in preparation). Thus, care must be exercised in the derivation of abundances from nuclear spectra; the problem is an important one. For example, Heckman (1980) claims evidence from nuclear spectra that metal abundance and absolute luminosity correlate for late-type galaxies. Tests to distinguish conventional HII region spectra from those due to other forms of excitation have been devised by Heckman and by Baldwin, Phillips & Terlevich (1981).

Figure 7

Figure 7. Oxygen (or metal) abundance as a function of mass. The mean elliptical relation was derived from Visvanathan & Sandage's (1977) color-magnitude relation, transformed to (U - V) as in Sandage & Visvanathan (1978a) and calibrated from Aaronson et al. (1978), the masses being implied from Mv assuming M/Lv = -6.5 (Schechter & Gunn 1979). The masses of the dwarf spheroidals are calculated in the same manner and probably represent upper limits; metallicities are from Table 3. For the ellipticals, S0, and spheroids the log(O/H) values plotted are simply those implied by the [Fe/H] values and the solar oxygen abundance; it is possible (see Section 2.4) that these should be increased in metal-poor systems by an amount [O/Fe] leq 0.6. The metallicity of NGC 3115 is taken from Figure 5, and its mass deduced by extrapolating a flat rotation curve (Rubin, Peterson & Ford 1980) to the de Vaucouleurs radius. Masses and oxygen abundances in Irregular and blue compact galaxies are from Lequeux et al. (1979), Kinman & Davidson (1981), and Talent (1980).

There is little evidence (see Figure 3) for marked N/O gradients across spirals, except that N/O may increase somewhat in the inner regions of M101 (a factor of 2 overall).

5.5. Quasars and Active Galaxies

Although interpretation of the spectra of quasars and active galaxies is necessarily uncertain, there is little evidence of any major departures from either an approximately solar overall metal abundance or from approximately solar relative atomic abundances, including helium (Davidson & Netzer 1979). The emission spectrum from the extreme radio galaxy Cyg A, for example, shows a solar composition (Osterbrock & Miller 1975). The only major anomaly so far found is the quasar Q0353-383 (Osmer 1980), which shows evidence of a marked carbon deficiency.

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