The Formation of Spheroids in Early-Type Spirals: Clues From Their Globular Clusters

TL;DR

This study uses Hubble data to analyze globular cluster systems in edge-on Sa spiral galaxies, revealing how their properties relate to bulge luminosity and suggesting different formation processes for bulges based on galaxy brightness.

Contribution

It introduces a new empirical relation between red globular clusters and bulge luminosity, linking globular cluster properties to galaxy formation mechanisms.

Findings

Blue globular cluster frequencies are similar to later-type spirals.

Red globular clusters increase with bulge luminosity.

Dissipative processes are more important in luminous bulge formation.

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 systems in four edge-on Sa spiral galaxies covering a factor of 4 in luminosity. The specific frequencies of the blue globular clusters 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 globular clusters 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 globular clusters 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 that dissipative processes played 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.