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 .″ 1 (13.4 pc), and an absolute magnitude M814 = −7.90 ± 0.16 ( M / M ☉ ≈ 10 5 ). King models fit to the cluster give the core radius and concentration index, r c  =  2 .″ 1  ±  0 .″ 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 10 5   M ⊙ globular cluster and the only transition-type (dSph/dIrr) galaxy in the Local Group with a globular cluster.