WIYN IMAGING OF THE GLOBULAR CLUSTER SYSTEMS OF THE SPIRAL GALAXIES NGC 891 AND NGC 4013

We present results from a WIYN 3.5 m telescope imaging study of the globular cluster (GC) systems of the edge-on spiral galaxies NGC 891 and NGC 4013. We used the 10' x 10' Minimosaic Imager to observe the galaxies in BVR filters to projected radii of similar to 20 kpc from the galaxy centers. We combined the WIYN data with archival and published data from the WFPC2 and Advanced Camera for Surveys on the Hubble Space Telescope (HST) to assess the contamination level of the WIYN GC candidate sample and to follow the GC systems further in toward the galaxies' centers. We constructed radial distributions for the GC systems using both the WIYN and HST data. The GC systems of NGC 891 and NGC 4013 extend to 9 +/- 3 kpc and 14 +/- 5 kpc, respectively, before falling off to undetectable levels in our images. We use the radial distributions to calculate global values for the total number (N(GC)) and specific frequencies (S(N) and T) of GCs. NGC 4013 has N(GC) = 140 +/- 20, S(N) = 1.0 +/- 0.2, and T = 1.9 +/- 0.5; our N(GC) value is similar to 40% smaller than a previous determination from the literature. The HST data were especially useful for NGC 891 because the GC system is concentrated toward the plane of the galaxy and was only weakly detected in our WIYN images. Although NGC 891 is thought to resemble the Milky Way in terms of its overall properties, it has only half as many GCs, with N(GC) = 70 +/- 20, S(N) = 0.3 +/- 0.1, and T = 0.6 +/- 0.3. We also calculate the galaxy-mass-normalized number of blue (metal-poor) GCs in NGC 891 and NGC 4013 and find that they fall along a general trend of increasing specific frequency of blue GCs with increasing galaxy mass. Given currently available resources, the optimal method for studying the global properties of extragalactic GC systems is to combine HST data with wide-field, ground-based imaging with good resolution. The results here demonstrate the advantage gained by using both methods when possible.