WIYN Imaging of the Globular Cluster Systems of the Spiral Galaxies NGC891 and NGC4013

TL;DR

This study combines ground-based WIYN and space-based HST imaging to analyze the globular cluster systems of two edge-on spiral galaxies, revealing their extent, total numbers, and relation to galaxy mass, highlighting the benefits of combined observational methods.

Contribution

It provides new measurements of globular cluster systems in NGC891 and NGC4013 using combined WIYN and HST data, improving accuracy over previous estimates.

Findings

NGC4013 has approximately 140 GCs, with specific frequencies indicating typical richness.

NGC891 has about 70 GCs, with fewer than the Milky Way, despite similar properties.

Blue GC frequency correlates with galaxy mass, following a general increasing trend.

Abstract

We present results from a WIYN 3.5m telescope imaging study of the globular cluster (GC) systems of the edge-on spiral galaxies NGC891 and NGC4013. We used the 10' x 10' Minimosaic Imager to observe the galaxies in BVR filters to projected radii of ~20 kpc from the galaxy centers. We combined the WIYN data with archival and published data from WFPC2 and ACS on the Hubble Space Telescope 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 NGC891 and NGC4013 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. NGC4013 has N_GC = 140+/-20, S_N = 1.0+/-0.2 and T = 1.9+/-0.5; our N_GC value is ~40% smaller than a previous determination from the literature. The HST data were especially useful for NGC891, because the GC system is concentrated toward the plane of the galaxy and was only weakly detected in our WIYN images. Although NGC891 is thought to resemble the Milky Way in 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 NGC891 and NGC4013 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.