The AIMSS Project – I. Bridging the star cluster–galaxy divide

Abstract We describe the structural and kinematic properties of the first compact stellar systems discovered by the Archive of Intermediate Mass Stellar Systems project. These spectroscopically confirmed objects have sizes (∼6 < Re [pc] < 500) and masses (∼2 × 106 < M*/M⊙ < 6 × 109) spanning the range of massive globular clusters, ultracompact dwarfs (UCDs) and compact elliptical galaxies (cEs), completely filling the gap between star clusters and galaxies. Several objects are close analogues to the prototypical cE, M32. These objects, which are more massive than previously discovered UCDs of the same size, further call into question the existence of a tight mass–size trend for compact stellar systems, while simultaneously strengthening the case for a universal ‘zone of avoidance’ for dynamically hot stellar systems in the mass–size plane. Overall, we argue that there are two classes of compact stellar systems (1) massive star clusters and (2) a population closely related to galaxies. Our data provide indications for a further division of the galaxy-type UCD/cE population into two groups, one population that we associate with objects formed by the stripping of nucleated dwarf galaxies, and a second population that formed through the stripping of bulged galaxies or are lower mass analogues of classical ellipticals. We find compact stellar systems around galaxies in low- to high-density environments, demonstrating that the physical processes responsible for forming them do not only operate in the densest clusters. galaxies: dwarf, galaxies: evolution, galaxies: formation, galaxies: kinematics and dynamics 1 INTRODUCTION In the past 15 years there has been a revolution in the study of low-mass stellar systems. It began with the discovery (Hilker et al. 1999; Drinkwater et al. 2000) in the Fornax cluster of a population of generally old and compact objects with luminosity/mass and size intermediate between those of globular clusters (GCs) and the few then-known compact elliptical galaxies (cEs). These objects, known as ultracompact dwarfs (UCDs; Phillipps et al. 2001), became the first in a series of stellar systems found to exist with properties between star clusters and galaxies. They were followed by a zoo of objects inhabiting slightly different regions of the size–luminosity parameter space. These new objects included extended star clusters such as ‘Faint Fuzzies’ (Larsen & Brodie 2000, 2002) and ‘Extended Clusters’ (Huxor et al. 2005, 2011a; Brodie et al. 2011; Forbes et al. 2013), additional Milky Way (MW) and M31 dwarf spheroidals and ultrafaint dwarf galaxies (e.g. Willman et al. 2005; Zucker et al. 2006, 2007; Belokurov et al. 2007), and a host of new cEs (Mieske et al. 2005; Chilingarian et al. 2007; Smith Castelli et al. 2008; Chilingarian et al. 2009; Price et al. 2009) that fill the gap between M32 and ‘normal’ elliptical galaxies. These discoveries have broken the simple division thought to exist between star clusters and galaxies, with UCDs displaying properties that lie between those of ‘classical’ GCs and early-type galaxies. Naturally, this situation has led to a search for unifying scaling relations, and therefore formation scenarios, for the various compact stellar systems (CSSs) and early-type galaxy populations. Initial indications of a tight mass–size relation for the UCD and cE populations that extend from the massive GC (i.e. stellar mass >2 × 106 M⊙) to elliptical galaxy regime (Haşegan et al. 2005; Kissler-Patig, Jordán & Bastian 2006; Dabringhausen, Hilker & Kroupa 2008; Murray 2009; Misgeld & Hilker 2011; Norris & Kannappan 2011) have been called into question by the discovery of extended but faint star clusters that broaden the previously observed tight relation for UCDs at fainter magnitudes (see e.g. Brodie et al. 2011; Forbes et al. 2013). Investigating the reality of such trends requires a more systematic and homogeneous sample of CSSs than currently exists. In this paper series, we present the archive of intermediate mass stellar systems (AIMSS) survey. The goal of this survey is to produce a comprehensive catalogue of spectroscopically confirmed CSSs of all types which have resolved sizes from Hubble Space Telescope (HST) photometry, as well as homogeneous stellar mass estimates, spectroscopically determined velocity dispersions, and stellar population information. This catalogue will then be used to systematically investigate the formation of CSSs and their relationships with other stellar systems. In order to achieve this goal, we have undertaken a search of all available archival HST images to find CSS candidates. We have deliberately broadened the selection limits traditionally used to find CSSs, both to probe the limits of CSS formation and to avoid producing spurious trends in CSS properties. One of the first results of the AIMSS survey presented in this paper is the discovery of further examples of a class of extremely dense stellar systems which broaden the previously suggested mass/luminosity–size trend to brighter magnitudes. The AIMSS survey also includes a key additional parameter – central velocity dispersion. The central velocity dispersion of stars has been shown to be one of the best predictors of galaxy properties (e.g. Forbes & Ponman 1999; Cappellari et al. 2006; Graves, Faber & Schiavon 2009). It can also provide clues to the evolutionary history of a galaxy since, for example, tidal stripping will tend to reduce both the size and luminosity of a galaxy but its velocity dispersion will remain largely unchanged (see e.g. Bender, Burstein & Faber 1992; Chilingarian et al. 2009) and hence will remain a reliable signature of its past form. In fact Chilingarian et al. (2009) showed that when their simulated disc galaxy on a circular orbit around a cluster potential is stripped severely enough to lose ∼75 per cent of its original mass, the global velocity dispersion is essentially unaffected (their fig. 1). This is because as stripping progresses it is increasingly the tightly bound central stellar structure (either nucleus or bulge) that comes to dominate the global light distribution of the galaxy, and the dispersion of this is relatively unaffected by the loss of an outer dark matter halo. The central velocity dispersion, which is always dominated by the stellar component of the galaxy, is likely to be less affected by stripping, at least until the point where the central mass component itself begins to lose mass. Figure 1. Open in new tabDownload slide Implied luminosity–size plot for objects detected in an HST ACS pointing centred on NGC 4649, created assuming that all objects detected are at the same distance as NGC 4649. The grey dots are all objects detected by SExtractor in the ACS image. The shaded region indicates the selection region, the dashed horizontal line is the HST resolution limit at the distance of NGC 4649. The dot–dashed line shows the edge of the ‘zone of avoidance’ for ear (...truncated)