DDO 216-A1: A Central Globular Cluster in a Low-luminosity Transition-type Galaxy

Abstract

We confirm that the object DDO 216-A1 is a substantial globular cluster at the center of Local Group galaxy DDO 216 (the Pegasus dwarf irregular), using Hubble Space Telescope ACS imaging. By fitting isochrones, we find the cluster metallicity [M/H] = -1.6 ± 0.2, for reddening E(B-V) = 0.16 ± 0.02 the best-fit age is 12.3 ± 0.8 Gyr. There are ≈ 30 RR Lyrae variables in the cluster; the magnitude of the fundamental mode pulsators gives a distance modulus of 24.77 ± 0.08—identical to the host galaxy. The ratio of overtone to fundamental mode variables and their mean periods make DDO 216-A1 an Oosterhoff Type I cluster. We find a central surface brightness of 20.85 ± 0.17 F814W mag arcsec^-2, a half-light radius of 3\buildrel{\prime\prime}\over{.} 1 (13.4 pc), and an absolute magnitude M814 = -7.90 ± 0.16 (M/{M}_⊙ ≈ 10⁵). King models fit to the cluster give the core radius and concentration index, r _c = 2\buildrel{\prime\prime}\over{.} 1 ± 0\buildrel{\prime\prime}\over{.} 9 and c = 1.24 ± 0.39. The cluster is an “extended” cluster somewhat typical of some dwarf galaxies and the outer halo of the Milky Way. The cluster is projected ≲30 pc south of the center of DDO 216, unusually central compared to most dwarf galaxy globular clusters. Analytical models of dynamical friction and tidal destruction suggest that it probably formed at a larger distance, up to ∼1 kpc, and migrated inward. DDO 216 has an unexceptional specific cluster frequency, S _N = 10. DDO 216 is the lowest-luminosity Local Group galaxy to host a 10⁵ {M}_⊙ globular cluster and the only transition-type (dSph/dIrr) galaxy in the Local Group with a globular cluster.

Based on observations made with the NASA/ESA Hubble Space Telesope, 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. These observations were obtained under program GO-13768.