The formation of galaxy bulges: Spectrophotometric constraints

We have measured Mg-2, Fe 5270 and Fe 5335 spectrophotometric indices (LICK system) in the bulge of 89 galaxies, mostly spirals from the Heraudeau (1996) sample. The indices are reduced to a null velocity dispersion and normalized to an aperture of 0.2 h(-1) kpc. The mean errors are 0.009 mag on Mg-2, and 0.3 A 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 Mg-2 and sigma (O), the central stellar velocity dispersion; we find an even tighter relation between Mg-2 and V-rot(m), 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] lambda 5007 Angstrom 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 Mg-2 vs. sigma (O) relation toward the low-sigma (O) region and globally increases its slope. We also present evidence for a new positive correlation between Fe indices and sigma (O,) 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 Mg-2 vs. sigma (O) relation of elliptical galaxies.