The Evidence for a Gradient of H II Region Electron Temperatures with Galactic Radius

Abstract

In the past several years a substantial effort has been devoted to the determination of radial abundance and temperature gradients in spiral galaxies. Searle (1971), Shields (1974), Rubin, et al. (1973), Smith (1975), Pagel, et al. (1979) and others have established beyond a reasonable doubt that the relative metal abundances (e.g., O, N, Ne and S) systematically increase from the edge to the nucleus of several spiral galaxies (e.g., M31, M33, M51, M101, NGC 300), see Figure 1. The electron temperatures deduced from the optical [0 III] or [N II] lines show an accompanying decrease as the abundances increase (Smith 1975, Pagel et al. 1979). Similar abundance gradients have been derived for our galaxy based on optical studies of planetary nebulae, (D’0dorico, Peimbert. and Sabbadin, 1976; Torres-Peimbert and Peimbert, 1977; see Figure 2); of H II regions (Hawley, 1978; Peimbert, Torres-Peimbert, and Rayo, 1978; see Figure 3), and of the kinematic and photometric properties of F, G and K stars (Mayor, 1976). In external spirals and in our own galaxy 0, Ne and S appear to have about the same gradient but N is a factor of two or more steeper.