The Luminosity-Metallicity Relation for Bulges of Spiral Galaxies

Optical integrated spectra of bulges have been obtained for 28 spiral galaxies. By applying an appropriate aperture size for each galaxy, the unavoidable contamination of disk starlight has been carefully minimized and set to $\sim$ 1/7 of the total stellar light. The sample covers a wide range in bulge luminosity and morphology. The Mg$_2$ index shows a tight correlation with the bulge intrinsic luminosity, similar - and extended at fainter magnitudes - to the relationship known for ellipticals. Other features such as CaIIK, CN, G-band, and TiO show similar trend with the bulge luminosity. On the contrary, the Fe5270 and Fe5335 iron lines show a constant equivalent width - within some dispersion - for all bulges. A simple application of a stellar population synthesis model suggests that magnesium should be most enhanced with respect to iron in the brightest bulges. Concerning the structural parameters, bulges do occupy the same region in the fundamental plane as ellipticals. In conclusion, the present study favors the idea that the bulk of stars in bulges have formed with only moderate or negligible influence of the disk material, likely by very similar processes as those driving the formation of elliptical galaxies.