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

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

Authors:
Paul Goudfrooij (1),
Jay Strader (2,1),
Laura Brenneman (3,4),
Markus Kissler-Patig (5),
Dante Minniti (6),
Edwin Huizinga (1) ((1) STScI, (2) UCO/Lick Observatory, (3) Williams College, (4) LHEA/GSFC, (5) ESO, (6) Univ. Catolica)

Comments: LaTeX, 14 pages, 5 figures; accepted for publication in MNRAS

We have studied the globular cluster (GC) systems of 7 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 GC systems along the Hubble sequence. The photometry reaches ~1.5
mag beyond the turn-over magnitude of the GC luminosity function for each
galaxy. Specific frequencies (S_N values) of GCs were evaluated by comparing
the numbers of GCs found in our WFPC2 pointings with those in the Milky Way
which would be detected in the same metric area. 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 GCs represents a `universal’, old halo population that is present around
each galaxy. Most galaxies in our sample have S_N values that are consistent
with a scenario in which GC systems are made up of (i) the aforementioned halo
population plus (ii) a population that is associated with bulges, which grows
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 (1990), 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 (NGC 7814) with a low S_N value that is consistent
with those of the latest-type 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. (abridged)

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