Structure and dynamics of galaxies with a low surface-brightness disc -
  II. Stellar populations of bulges
  The radial profiles of the Hb, Mg, and Fe line-strength indices are presented
for a sample of eight spiral galaxies with a low surface-brightness stellar
disc and a bulge. The correlations between the central values of the
line-strength indices and velocity dispersion are consistent to those known for
early-type galaxies and bulges of high surface-brightness galaxies. The age,
metallicity, and alpha/Fe enhancement of the stellar populations in the
bulge-dominated region are obtained using stellar population models with
variable element abundance ratios. Almost all the sample bulges are
characterized by a young stellar population, on-going star formation, and a
solar alpha/Fe enhancement. Their metallicity spans from high to sub-solar
values. No significant gradient in age and alpha/Fe enhancement is measured,
whereas only in a few cases a negative metallicity gradient is found. These
properties suggest that a pure dissipative collapse is not able to explain
formation of all the sample bulges and that other phenomena, like mergers or
acquisition events, need to be invoked. Such a picture is also supported by the
lack of a correlation between the central value and gradient of the metallicity
in bulges with very low metallicity. The stellar populations of the bulges
hosted by low surface-brightness discs share many properties with those of high
surface-brightness galaxies. Therefore, they are likely to have common
formation scenarios and evolution histories. A strong interplay between bulges
and discs is ruled out by the fact that in spite of being hosted by discs with
extremely different properties, the bulges of low and high surface-brightness
discs are remarkably similar.
