The Globular Cluster Systems of the Sculptor Group

摘要

We use CTIO 4 m Mosaic II images taken with the Washington CM and Harris R filters to identify candidate globular clusters in the six major galaxies of the Sculptor group: NGC 45, 55, 247, 254, 300, and 7793. From follow-up spectroscopy with Hydra-CTIO, we find 19 new globular clusters in NGC 55, 247, 253, and 300, bringing the total number of known Sculptor group globular clusters to 36. The newly discovered clusters have spectroscopic ages consistent with those of old Milky Way globular clusters, and the majority are metal-poor. Their luminosity function closely resembles that of the Milky Way's globular clusters; their metallicity distribution is somewhat more metal-rich, but this may be the result of our color selection of candidates. The mean [α/Fe] ratio in the clusters is -0.2 ± 0.3, which is lower than the Milky Way average. The specific frequencies SN are similar to those of other late-type galaxies. However, if we calculate the specific frequency using the K-band total magnitudes of the host galaxies, we find values that are more than a factor of 2 higher. The kinematics of the globular cluster systems are consistent with rotation with the H I disk in each of the four galaxies; however, only in NGC 253 is this result based on more than seven objects. We suggest that the Sculptor group galaxies add to evidence indicating that many of the first-generation globular clusters formed in disks, not halos.