The Formation of Spheroids in Early-type Spirals: Clues from Their Globular Clusters

Abstract

We use deep Hubble Space Telescope images taken with the Advanced Camera for Surveys (ACS) in the F475W and F814W filters to investigate the globular cluster (GC) systems in four edge-on Sa spiral galaxies covering a factor of four in luminosity. The specific frequencies of the blue GCs in the galaxies in our sample fall in the range 0.34-0.84, similar to typical values found for later-type spirals. The number of red GCs associated with the bulges generally increases with the bulge luminosity, similar to what is observed for elliptical galaxies, although the specific frequency of bulge clusters is a factor of 2-3 lower for the lowest luminosity bulges than for the higher-luminosity bulges. We present a new empirical relation between the fraction of red GCs and total bulge luminosity based on the elliptical galaxies studied by ACSVCS (ACS Virgo Cluster Survey) and discuss how this diagram can be used to assess the importance of dissipative processes in building spiral bulges. Our results suggest a picture where dissipative processes, which are expected during gas-rich major mergers, were more important for building luminous bulges of Sa galaxies, whereas secular evolution may have played a larger role in building lower-luminosity bulges in spirals.

Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.