Constraining the star formation histories of spiral bulges

Stellar populations in spiral bulges are investigated using the Lick system of spectral indices. Long-slit spectroscopic observations of line strength s and kinematics made along the minor axes of four spiral bulges are report ed. Comparisons are made between central line strengths in spiral bulges an d those in other morphological types [elliptical, spheroidal (Sph) and S0]. The bulges investigated are found to have central line strengths comparabl e to those of single stellar populations of approximately solar abundance o r above. Negative radial gradients are observed in line strengths, similar to those exhibited by elliptical galaxies. The bulge data are also consiste nt with correlations between Mg-2, Mg-2 gradient and central velocity dispe rsion observed in elliptical galaxies. In contrast to elliptical galaxies, central line strengths lie within the loci defining the range of < Fe > and Mg-2 achieved by Worthey's solar abundance ratio, single stellar populatio ns (SSPs). The implication of solar abundance ratios indicates significant differences in the star formation histories of spiral bulges and elliptical galaxies. A 'single zone with infall' model of galactic chemical evolution , using Worthey's SSPs, is used to constrain the possible star formation hi stories of our sample. We show that the < Fe >, Mg-2 and H beta line streng ths observed in these bulges cannot be reproduced using primordial collapse models of formation but can be reproduced by models with extended infall o f gas and star formation (2-17 Gyr) in the region modelled. One galaxy (NGC 5689) shows a central population with a luminosity-weighted average age of similar to 5 Gyr, supporting the idea of extended star formation. Kinemati c substructure, possibly associated with a central spike in metallicity, is observed at the centre of the Sa galaxy NGC 3623.