New Members of the Cluster Family in Nearby Lenticular Galaxies

Using spectra obtained with the Keck I Telescope, we have demonstrated conclusively that the faint (23 ≤ V ≤ 24 mag) and unusually extended objects we discovered in Hubble Space Telescope images of the lenticular galaxies NGC 1023 and NGC 3384 are star clusters associated with their respective galaxies. In the case of NGC 1023, we were further able to establish that these objects are old (≥7–8 Gyr) and moderately metal-rich ([Fe/H] = -0.58 ± 0.24) and, having a system rotation curve that is very similar to that of the host galaxy, are associated with the lenticular disk. The α-element to iron abundance ratios are highly supersolar with [α/Fe] between +0.3 and +0.6. With moderately high metallicities and luminosities and effective radii in the range 7–15 pc (compared with the 2–3 pc sizes typical of normal globular and open clusters), this population of clusters has no known analog in the Milky Way or elsewhere in the Local Group.