Hubble Space Telescope imaging of globular cluster candidates in low surface brightness dwarf galaxies

¹ Special Astrophysical Observatory, Russian Academy of Sciences, N. Arkhyz, KChR, 369167, Russia
² Isaac Newton Institute, Chile, SAO Branch
³ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD21218, USA e-mail: tpuzia@stsci.edu

Received: 22 February 2005
Accepted: 26 May 2005

Abstract

Fifty-seven nearby low surface brightness dwarf galaxies () were searched for globular cluster candidates (GCCs) using Hubble Space Telescope WFPC2 imaging in V and I. The sample consists of 18 dwarf spheroidal (dSph), 36 irregular (dIrr), and 3 “transition” type (dIrr/dSph) galaxies with angular sizes less than 3.7 kpc situated at distances  Mpc in the field and in the nearby groups: M 81, Centaurus A, Sculptor, Canes Venatici I cloud. We find that ~50% of dSph, dIrr/dSph, and dIrr galaxies contain GCCs. The fraction of GCCs located near the center of dwarf spheroidal galaxies is 2 times higher than for dIrrs. The mean integral color of GCCs in dSphs,  mag, coincides with the corresponding value for Galactic globular clusters and is similar to the blue globular cluster sub-populations in massive early-type galaxies. The color distribution for GCCs in dIrrs shows a clear bimodality with peaks near and 1.0 mag. Blue GCCs are presumably young with ages Gyr, while the red GCC population is likely to be older. The detected GCCs have absolute visual magnitudes between and -5 mag. We find indications for an excess population of faint GCCs with mag in both dSph and dIrr galaxies, reminiscent of excess populations of faint globular clusters in nearby Local Group spiral galaxies. The measurement of structural parameters using King-profile fitting reveals that most GCCs have structural parameters similar to extended outer halo globular clusters in the Milky Way and M 31, as well as the recently discovered population of “faint fuzzy” clusters in nearby lenticular galaxies.