4.5. Metallicity vs. Surface Brightness
The uniformity of abundance gradients as a function of scale length suggests a close correlation between metallicity and disk surface brightness. Indeed, McCall (1982) and Edmunds & Pagel (1984) noted a remarkably tight correlation between O/H and disk surface brightness for late-type spirals. This has provided part of the motivation for models of self-regulated star formation, in which the radiation and mechanical energy produced by stars feeds back into the surrounding ISM and acts to inhibit further star formation. Models of this kind have been explored by Phillips & Edmunds (1991) and Ryder (1995), and appear to do a good job of reproducing the trends of both star formation rate and O/H with surface brightness. One caveat is that the interaction of the stellar energy output with the ISM is still poorly understood. Viscous disk models also provide a possible mechanism to tie the abundances to the underlying surface density distribution.
Edmunds & Pagel (1984) also noted that early-type spirals do not follow the same O/H-surface brightness correlation as the late types. Garnett et al. (1997a) put this on a more quantitative basis. Figure 13 displays the characteristic metallicity at two fixed values of disk surface brightness for a sample of spirals having either I- or R-band surface photometry; these bandpasses presumably sample the light from the old disk population better than B. The figure shows that metallicity-luminosity correlation appears to hold at all values of surface brightness across spiral disks. This result argues for two modes of enrichment in disk galaxies: a local mode, in which the metallicity is connected to the local mass density, and a global mode, in which an entire galaxy is enriched in a manner dependent on its total mass. One can imagine a global enrichment event which raises the metallicity of a galaxy to some level which depends on total mass, followed by sequential local enrichment which follows the mass density distribution. One caveat is that M/L, and thus the mass surface density at a given surface brightness, may vary systematically along the luminosity sequence in Figure 13. A more comprehensive study of mass surface density and gas fraction along this sequence should prove enlightening.
Figure 13. O/H at fixed value of galaxy surface brightness vs. MB. Top: abundances at 22 mags arcsec-2. Bottom: abundances at 24 mag arcsec-2. More luminous spirals have higher abundances at a fixed surface brightness. Open symbols are from Garnett et al. (1997a); filled symbols represent additional data obtained from the literature.