Journal of The Korean Astronomical Society http://dx.doi.org/10.5303/JKAS.2011.44.5.151 44: 151 ∼ 160, 2011 October ISSN:1225-4614 c 2011 The Korean Astronomical Society. All Rights Reserved. http://jkas.kas.org HUBBLE SPACE TELESCOPE IMAGING OF GLOBULAR CLUSTERS IN TWO FACE-ON LOW SURFACE BRIGHTNESS GALAXIES UGC 5981 AND UGC 6614 Ji Hoon Kim Center for the Exploration of the Origin of the Universe, Department of Physics and Astronomy, Seoul National University E-mail : jhkim@astro.snu.ac.kr (Received February 28, 2011; Revised August 5, 2011; Accepted August 9, 2011) ABSTRACT We present a study searching for globular cluster systems (GCSs) of two face-on low surface bright- ness galaxies (LSBGs), UGC 5981 and UGC 6614. Based on HST WFPC2 images of F555W and F814W filters, we detect 12 and 18 GC candidates for UGC 5981 and UGC 6614, respectively. Although these two LSBGs have very different bulge properties, they have very similar specific frequencies (S N ) of 0.1. However, S N ∼ 0.1 is quite small even for their late morphological types, albeit within errors. This suggests that LSBGs have had star formation histories lacking dominant initial starburst events while accumulating their stellar masses through sporadic star formation activities. Key words : galaxies: individual(UGC 5981, UGC 6614) — galaxies: formation — galaxies: star clusters 1. INTRODUCTION Globular clusters (GCs) in extragalactic systems have long been used to trace star formation history (SFH), dynamics, and structure of their host galax- ies (Larson 1992; Ashman & Zepf 1998). GCs are useful chronometers for timing the chemical evolution of galaxies; their luminosities and colors contain in- formation about their ages and metallicities. So far, most of the observational studies have been centered around globular cluster systems (GCSs) of elliptical galaxies, in which GCSs are easily identified against the smooth, dust-free background. These studies have not only shown the characteristic properties of GCSs, but also featured the characteristic changes of these prop- erties in galaxies. However, our knowledge of GCSs in spiral galaxies still needs to be improved (Harris 1991; Kissler-Patig et al. 1999; Goudfrooij et al. 2003) though, it has been vastly improved due to recent ob- servations of the Hubble Space Telescope. So far, the Milky Way GCSs has been playing the role of the only standard based on which the GCSs of other galaxies are interpreted. Yet it remains unclear how typical the Milky Way GCSs are either for metal rich early types or for very late types. The properties of GCSs accross all Hubble Types are an improtant probe and test of hierarchical galaxy formation theroy, espe- cially if mergers play an important role in triggering the formation of GCSs and the buildup of spheroidal components. In the hierarchical picture, one might expect the spe- cific frequency of GCSs to correlate with bulge fraction and/or environment. In a monolithic collapse picture, these properties may be unrelated, while in a delayed formation scenario, one might expect a rather low spe- cific frequency. GCSs have been detected in all nearby massive high surface brightness (HSB) galaxies and in many dwarf galaxies (Harris 1991; Miller et al. 1998) and seem to form naturally in many types of galaxies. However, the formation epoch and mechanism of GCSs are still unknown. Candidate scenarios are: 1) a pre-galactic formation (Peebles & Dicke 1968), 2) the formation during the turbulent early collapse of the galaxy from primordial fragments or in supergiant molecular clouds (Searle & Zinn 1978; Harris & Pudritz 1994), and 3) the formation due to perturbations induced by merg- ers or accretion (Ashman & Zepf 1992). While many cases of young GCSs in merging and interacting galax- ies are now known (e.g., Schweizer 1997), it is still be- lieved that most of the GCs were formed at early time (Chaboyer et al. 1996). Low surface brightness (LSB) galaxies make inter- esting test cases. One view is that LSB galaxies have been forming continuously over a Hubble time, but, as they are the most isolated type of spiral galaxies, have had suppressed star formation (Bothun et al. 1993; Mo et al. 1994). An alternative view to explain low sur- face brightness, low abundance, and blue colors in these galaxies is that they simply have delayed star formation histories (SFHs) (Impey & Bothun 1989; McGaugh, & Bothun 1994). Recently, Kim (2007) and Kim et al. (in preparation) suggest that LSBGs may have been experiencing sporadic star formation activities along Hubble time due to late gas infall based on their stellar – 151 –