2001A&A...366...68P

We have measured Mg₂, Fe 5270 and Fe 5335 spectrophotometric indices (LICK system) in the bulge of 89 galaxies, mostly spirals from the Heraudeau (1996, Ph. D. Thesis, Univ. of Lyon) sample. The indices are reduced to a null velocity dispersion and normalized to an aperture of 0.2 h^–1kpc. The mean errors are 0.009 mag on Mg₂, and 0.3Å on the iron indices. These measurements almost double the amount of similar data already available on spiral galaxies. Our data confirm the existence of the relation between Mg₂ and σ₀, the central stellar velocity dispersion; we find an even tighter relation between Mg₂ and V^m_rot, the maximum rotational velocity of the galaxy, deduced from HI observations. For the most massive bulges, these correlations may be interpreted as a mass-metallicity relation. However, the presence of young stellar populations, traced by the detection of [OIII] λ 5007Å emission, provides clear evidence that age effects do play a role. Since the contribution of the young population is anti-correlated to the mass of the galaxy, it continues the Mg₂ vs. σ₀ relation toward the low-σ₀ region and globally increases its slope. We also present evidence for a new positive correlation between Fe indices and σ₀, and for a significant correlation between the line-strength indices and the total or disk luminosity. We propose to model the whole sequence of bulges within the following framework: bulges are composed of a primary population formed prior to the disk, during the initial collapse, and of a secondary population formed during its evolution. The whole family of bulges can be classified into three classes: (A) the bulges dominated by young populations are generally small, have ionized gas, low velocity dispersion and low line strengths; (B) the bulges dominated by the primary population lie along the mass-metallicity sequence defined for elliptical galaxies; and (C) the bulges where the secondary population is significant are less Mg-over-abundant than (B)-type bulges and deviate from the Mg₂ vs. σ₀ relation of elliptical galaxies.