The old globular cluster system of the dIrr galaxy NGC1427A in the Fornax cluster

We present a study of the old globular cluster (GC) population of the dwarf irregular galaxy NGC 1427A using multi-wavelength VLT observations in U, B, V, I, H_alpha, J, H, and Ks bands under excellent observing conditions. We applied color and size selection criteria to select old GC candidates and made use of archival ACS images taken with the Hubble Space Telescope to reject contaminating background sources and blended objects from the GC candidates' list. The H_alpha observations were used to check for contamination due to compact, highly reddened young star clusters whose colors and sizes could mimic those of old GCs. After accounting for contamination we obtain a total number of 38+/-8 GC candidates with colors consistent with an old (~10 Gyr) and metal-poor (Z < 0.4xZ_solar) population as judged by simple stellar population models. Our contamination analysis indicates that the density distribution of GCs in the outskirts of the Fornax central cD galaxy NGC1399 may not be spherically symmetric. We derive a present-day specific frequency S_N of 1.6+/-0.23 for NGC 1427A, a value significantly larger than what is observed in the Local Group dwarf irregular galaxies and comparable with the values found for the same galaxy types in the Virgo and Fornax clusters. Assuming a universal globular cluster luminosity function turnover magnitude, we derive a distance modulus to NGC 1427A of 31.01+/-0.21 mag which places it 3.2+/-2.5(statistic)+/-1.6(systematic) Mpc in front of the Fornax central cD galaxy NGC 1399. The implications of this result for the relationship between NGC 1427A and the cluster environment are briefly discussed.Comment: 16 pages, 2 tables, 14 figures; accepted for publication in A&

Hubble Space Telescope Imaging of Globular Cluster Candidates in Low Surface Brightness Dwarf Galaxies

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 2-6 Mpc in the field and in the nearby groups: M81, 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 that for dIrrs. The mean integral color of GCCs in dSphs, V-I = 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 = 0.5 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 M31, as well as the recently discovered population of "faint fuzzy" clusters in nearby lenticular galaxies.Comment: A&A accepted, 17 page