STScI Preprint #1287

WFPC2 Imaging of the Globular Cluster System of the S0 Galaxy NGC 3115

Based on observations with the NASA/ESA Hubble Space Telescope, 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.

Using the WFPC2 on-board the HST we have identified 144 globular cluster candidates in the inner region of the S0 galaxy NGC 3115. The color distribution of the globular clusters is bimodal, with a blue peak at V-I = 0.96 mag and a red peak at V-I = 1.17 mag. There are roughly equal number of blue and red clusters. The mean [Fe/H] of the blue population is -1.36 dex while the red population has a mean [Fe/H] of -0.37 dex. Following Ashman & Zepf (1992), we interpret the bimodality as evidence of multiple epochs of cluster formation in the metal enhancement history of the galaxy. Assuming that the blue peak corresponds to a primordial, 15 Gyr old, metal poor population of globular clusters, we derive an age of 11±3 Gyr for the younger, red clusters. We also find that the age and metallicities of the globular clusters are consistent with the red giant stars in the field, suggesting that the two systems formed coevally. The spatial distribution of both the red and the blue clusters are flattened with major and minor axes roughly colinear with the galaxy light profile. While the red clusters follow the thick disk component of the galaxy, the blue cluster distribution is less flattened and mimics the bulge/halo light distribution. We find a local specific frequency of 1.3±0.1 in the central region of the galaxy, which is lower than the global specific of 2.3±0.5 reported in the literature, implying that the specific frequency increases with galactocentric distance. We have also observed the globular cluster luminosity function 2 magnitudes deeper than the turnover luminosity and derived a distance modulus of m-M =29.8±0.3 mag to NGC 3115. The average half light radii of the clusters is 2 pc and the red clusters are 20% smaller than the blue ones.

Status:

Appeared in: The Astronomical Journal, 116:2841-2853, 1998 December

Affiliations:

¹)Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
²) Astronomy Department, University of Maryland, College Park MD 20742, USA