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

Abstract

Fifty-seven nearby low surface brightness dwarf galaxies (-10⪆ M_V⪆ -16) 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 2{-}6 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, (V-I)_0=1.04±0.16 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 (V-I)₀ = 0.5 and 1.0 mag. Blue GCCs are presumably young with ages t⪉1 Gyr, while the red GCC population is likely to be older. The detected GCCs have absolute visual magnitudes between M_V=-10 and -5 mag. We find indications for an excess population of faint GCCs with M_V⪆-6.5 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.