Hubble Space Telescope observations of globular cluster systems along the Hubble sequence of spiral galaxies

We have studied the globular cluster systems of seven giant, edge-on spiral galaxies using Hubble Space Telescope imaging in V and I. The galaxy sample covers the Hubble types Sa to Sc, allowing us to study the variation of the properties of globular cluster systems along the Hubble sequence. The photometry reaches ~1.5 mag beyond the turn-over magnitude of the globular cluster luminosity function for each galaxy. Specific frequencies of globular clusters (S_N values) were evaluated by comparing the numbers of globular clusters found in our WFPC2 pointings with those in our Milky Way that would be detected in the same spatial region if placed at the distance of the target galaxies. Results from this method were found to be consistent with the more commonly used method of constructing radial distribution functions of globular clusters. The S_N values of spirals with B/T<~ 0.3 (i.e. spirals with a Hubble type later than about Sb) are consistent with a value of S_N= 0.55 +/- 0.25. We suggest that this population of globular clusters represents a `universal', old halo population that is present around each galaxy. Most galaxies in our sample have S<sub>N</sub> values that are consistent with a scenario in which globular cluster systems are made up of (i) the aforementioned halo population plus (ii) a population that is associated with bulges, which grows approximately linearly with the mass of the bulge. Such scenarios include the `merger scenario' for the formation of elliptical galaxies as well as the `multi-phase collapse' scenario, but it seems inconsistent with the `secular evolution' scenario of Pfenniger & Norman, in which bulges are formed from disc stars by means of the redistribution of angular momentum through bar instabilities and/or minor perturbations. However, there is one bulge-dominated spiral galaxy in our sample (NGC7814) with a low S_N value that is consistent with those of the latest-type spirals. This means that the `secular evolution' scenario can still be viable for some bulge-dominated spirals. Thus, our results suggest that the formation histories of galaxy bulges of early-type spirals can be significantly different from one galaxy to another.