Wide-Field Imaging from Space of Early-Type Galaxies and Their Globular Clusters

Wide-field imaging from space will reveal a wealth of information about the globular cluster systems of any galaxies in the local universe that are observed by such a mission. Individual globular clusters around galaxies in the local universe have compact sizes that are ideal for the excellent spatial resolution afforded by spacebased imaging, while systems of these globular clusters have large spatial extent that can only be fully explored by wide-field imaging. One example of the science return from such a study is the determination of the major formation epoch(s) of galaxies from the ages of their globular clusters determined via their optical to nearinfrared colors. A second example is determining the sites of metal-poor globular cluster formation from their cosmological bias, which constrains the formation of structures early in the universe. 1 Why JDEM Imaging of Globular Cluster Systems Globular clusters are invaluable fossil records of the early formation history of the galaxy in which they are located. Composed of roughly one million stars that formed at the same time with similar composition and that have remained bound for up to a Hubble time, each globular cluster provides an observable record of the age, metallicity, and kinematics at the time it was formed. Because these quantities are determined for individual globulars, studies of globular cluster systems can constrain the distribution of the ages and metallicities rather than the global average of these key galaxy quantities revealed by most studies of the integrated light of galaxies. Globular clusters are also observed to form in major starbursting episodes in the local universe, so their properties provide a way to trace the major formation episodes in their host galaxies. Moreover, because typical globular clusters contain roughly one million stars, they are bright and can be readily observed as individual objects out to sufficient distances (∼ 15 Mpc) to obtain a significant sample of Preprint submitted to Elsevier Science 2 February 2008 elliptical galaxies. This allows the investigation of the formation history of massive early-type galaxies that are absent in the Local Group, but make up the majority of stellar mass in the local universe (e.g. Hogg et al. 2002). Consequently, studies of globular cluster systems have long played an important role in constraining how and when the major formation episodes in galaxies occurred (e.g. Ashman & Zepf 1998, Harris 2001). The study of extragalactic globular cluster systems has been revolutionized by space-based imaging with the Hubble Space Telescope. This is in large part because the size of extragalactic globular clusters is very well-matched to diffraction limited optical imaging with a 2-m class telescope. Specifically, a typical globular cluster half-light radius of several pc at a distance of 10 Mpc corresponds to ∼ 0.05 on the sky. However, in one critical aspect, the study of extragalactic globular cluster systems is not well suited to HST imaging. The systems of globular cluster systems around massive early-type galaxies extend into the halos of the galaxies, covering tens of arcminutes on the sky (e.g. Rhode & Zepf 2004 and references therein). Therefore, wide fields of view are required in order to accurately determine total properties of globular cluster systems around galaxies. Moreover, the outer halos of galaxies may hold unique clues to the assembly history of galaxies, and globular clusters are one of the few probes available in these regions. In this contribution, we highlight two key science questions that would be addressed by wide-field, space-based imaging of galaxies in the local universe and their globular cluster systems. 2 Constraints on the Formation Epoch(s) of Elliptical Galaxies Globular cluster systems are key tools for determining the formation history of elliptical galaxies because the ages and metallicites of their globular clusters can potentially be determined, thereby yielding the major formation epoch(s) of the host galaxies. Studies in optical colors have revealed that globular cluster systems of elliptical galaxies often have color distributions with two or more peaks (e.g. Kundu & Whitmore 2001, Larsen et al. 2001). This is one of the clearest signs that elliptical galaxies form eposidically, and is consistent with earlier predictions for elliptical galaxies formed by the mergers of disk galaxies (Ashman & Zepf 1992). While the optical colors of globular cluster systems indicate an episodic formation history, they do not significantly constrain when these events occur. This is because optical colors alone can not generally distinguish between different age and metallicity combinations (the age-metallicity degeneracy). In some specific cases with strongly bimodal color distributions, the cluster systems must be mostly old and have a bimodal metallicity distribution to account for the red and blue populations (e.g. Zepf & Ashman 1993). However, a general understanding of the age and metallicity distribution of globular cluster systems requires the breaking of