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1992Apj. . .384. . .50A the Astrophysical Journal, 384:50-61,1992 January 1 © 1992. the American Astronomical Society. All Righ .50A . The Astrophysical Journal, 384:50-61,1992 January 1 .384. © 1992. The American Astronomical Society. All rights reserved. Printed in U.S.A. 1992ApJ. THE FORMATION OF GLOBULAR CLUSTERS IN MERGING AND INTERACTING GALAXIES Keith M. Ashman Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 AND Stephen E. Zepf1 Johns Hopkins University, Homewood Campus, Baltimore, MD 21218 Received 1991 April 19 ; accepted 1991 July 3 ABSTRACT We suggest that at least some globular clusters are formed during the interaction or merger of galaxies. Such events could explain the disk population of clusters in the Galaxy, the young globulars in the Magellanic Clouds, the excess of clusters around ellipticals relative to spirals of the same luminosity and the anomalously large globular cluster systems around some galaxies in the center of galaxy clusters. We show that if all proto- spirals contain subgalactic clouds with a similar mass spectrum, the specific frequency of globular clusters around spirals will be constant. Extending the argument allows us to predict, for a given cosmological spec- trum of density fluctuations, the number of globular clusters that form as a function of galactic mass. It also allows us to estimate the number of clusters formed in galaxy interactions and mergers. We show that our hypothesis is consistent with a number of current observations and describe possible tests of the model. These tests include direct observations of newly formed globular clusters in ongoing mergers and predictions about the number and metallicity of clusters as a function of radius in elliptical galaxies. Subject headings: galaxies: elliptical and lenticular, cD — galaxies: formation — galaxies: interactions — galaxies: Magellanic Clouds — globular clusters: general 1. INTRODUCTION In many ways globular clusters are the philosopher’s stone against which theories of the formation and early evolution of galaxies are judged. Because of their great ages, spatial distribution, kinematics, and metallicity, they stand out as observable products of the process of galaxy formation. Within the Milky Way, the similarities between the globular cluster system and the spheroid stars, as well as correlations between cluster metallicity and position within the Galaxy, strongly suggest these globular clusters formed as the Galaxy collapsed (e.g., Fall & Rees 1988; Zinn 1988). Models of the formation of globular clusters during protogalactic collapse are somewhat dependent on the assumed nature of protogalaxies. In the Eggen, Lynden-Bell, & Sandage (1962) picture a smooth, homogeneous collapse is envisaged. The Fall & Rees (1985) model for globular cluster formation fits naturally into this picture. In this scheme, the protogalactic gas is relatively smooth and at the virial temperature of the galactic halo. Small overdensities condense out of the gas via thermal instability. Under certain 4 6 conditions, these regions “hang-up” at a temperature of around 10 K and a corresponding Jeans mass of 10 M0. In this way the cooling history of clouds imprints a mass-scale characteristic of globular clusters on the original spectrum of inhomogeneities. Searle & Zinn (1978) proposed an alternative view of galaxy formation in which protogalaxies are very lumpy and galaxies are built from the hierarchical merging of smaller subunits. As a result of the more chaotic nature of the collapse in the Searle-Zinn model, the process occurs over a period of a few billion years, several times longer than in the original Eggen et al. model. Within the framework of the Searle & Zinn model, it is plausible that the collision and coalescence of the subunits could lead to conditions appropriate for the Fall-Rees model for globular cluster formation (cf. Kang et al. 1990). However, an emphasis on the lumpy nature of protogalaxies promotes the consideration of other ideas about globular cluster formation. Larson (1986) has noted that in observed star-forming regions the fraction of the parent cloud that ends up in stars is small. In complexes like Orion only about 10~3 of the original cloud will end up in a bound star cluster. The inference is that the progenitors 8 of globular clusters must have had masses in excess of 10 M0. Such objects may be identified with the protogalactic lumps of the Searle-Zinn picture. Larson (1986) also points out that some starburst galaxies have regions which may have similar properties to this view of a young globular cluster. In particular, he cites a “ super star cluster ” in NGC 1705 with an age in excess of 107 yr and a 6 mass ~5 x 10 M0- Zinnecker et al. (1988) and Freeman (1990) have suggested that the precursors to globular clusters may be the cores of the nucleated dwarf ellipticals. There are various observations that provide tentative support for this view, such as similarities in integrated spectra and velocity dispersions between the nucleated cores and globular clusters. This may be related to the idea of Larson (1986) discussed above, since NGC 1705 appears to be currently nucleating. Freeman (1990) suggests that glaxies originally are surrounded by many such nucleated dwarf ellipticals which subsequently get disrupted by some dynamical process. The tenuous outer regions of the dwarfs are easily removed, and Freeman suggests that the remaining dense cores are now observed as globular clusters. Most of the mass of the dwarf galaxies, which originally resides in the less tightly bound regions, is assumed to form the 1 Also Space Telescope Science Institute. 50 © American Astronomical Society • Provided by the NASA Astrophysics Data System .50A . FORMATION OF GLOBULAR CLUSTERS 51 .384. visible spheroid of spiral galaxies. In the context of the Searle-Zinn picture of galaxy formation one could identify these dwarf galaxies with the lumps within the protogalaxy. All of these schemes for globular cluster formation must explain the similarity of clusters in many different environments. These similarities include the independence of the characteristic mass of clusters from host galaxy mass (Harris 1987), as well as the 1992ApJ. constancy of the number of clusters per unit galaxy luminosity around galaxies of the same morphological type (Harris & Racine 1979). While the similarities in globular cluster properties undoubtedly provide insights into the formation process, we suggest in this paper that there are important clues to the formation of globular clusters to be found in the differences between globular cluster systems. These differences fall under two main categories. First, it is apparent that, while globular cluster formation in many galaxies terminated at an early epoch, there are some galaxies where globular clusters are still forming. The best-studied examples are the Magellanic Clouds (e.g., Westerlund 1990, and references therein), although there is still some debate whether the young globulars in these and similar galaxies are “ true ” globular clusters. The second difference relates to globular cluster systems and the observation that the number of globular clusters around ellipticals exceeds that around spirals of the same luminosity. Harris & van den Bergh (1981) have quantified this effect by defining the specific frequency of clusters S, which is the number of clusters per unit (Mv = —15) galaxy luminosity. For normal ellipticals the typical value of S is about a factor of 2 higher than in spirals (Harris & Racine 1979; Harris 1988). An equally striking and perhaps related observation is a correlation between S and galaxy environment, in the sense that galaxies of the same morphological type generally have higher values of S in clusters than in the field (Bridges & Hanes 1990 and references therein). In this paper we attempt to explain these two sets of observations by a common mechanism. We suggest that at least some globular clusters form during galaxy mergers and interactions. As Schweizer (1986) points out, this hypothesis can explain the increase in the number of clusters around ellipticals provided, as has been suggested from independent considerations, such galaxies are merger products. The environmental dependence of S arises naturally in this scheme, provided regions of high galaxy density have a relatively high frequency of mergers and interactions. Perhaps most importantly, young globular clusters are mostly found in galaxies which show signs of interaction. The plan of this paper is as follows. In § 2 we review observations of young clusters in the Magellanic Clouds and in some other nearby galaxies. We suggest that galaxies in which globular cluster formation is currently occurring are usually systems which are interacting with larger galaxies. We also describe indirect evidence that some of the globular clusters in massive galaxies formed as the result of merger or interaction. In § 3 we develop our theory and present an argument to explain the constancy of the specific frequency of globular clusters around spiral galaxies. We also estimate the number of globular clusters that form in galaxy interactions as a function of the available gas mass. In § 4 we compare our theoretical predictions to observed globular cluster systems and find reasonable agreement. These considerations lead to observational tests of the model which we describe in § 5. We discuss other implications of our model in § 6 and present our conclusions. 2. OBSERVATIONAL CLUES TO GLOBULAR CLUSTER FORMATION 2.1. Globular Cluster Formation at the Present Epoch There is no doubt that objects similar to the old globular clusters of the Galactic halo are currently forming in galaxies such as the Magellanic Clouds. However, there has been some debate as to whether these very young clusters are truly analogous to their much older counterparts. For instance, Larson (1988) has noted that the young LMC clusters are generally less massive than the halo clusters of the Galaxy, although the oldest LMC clusters have masses comparable to those of halo clusters. While this may suggest some alteration in the conditions of cluster formation over the past several Gyr, it need not imply that the young clusters are fundamentally different from the old ones.
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