The AIMSS Project I: Bridging the Star Cluster - Galaxy Divide

NORRIS, M.; KANNAPPAN, S.; FORBES, D.; ROMANOWSKY, A.; BRODIE, J.; FAIFER, F.; HUXOR, A.; MARASTON, C.; MOFFET, A.; PENNY, S.; POTA, V.; SMITH CASTELLI, A. V.; STRADER, J.; BRADLEY, D.; ECKERT, K.; FÖHRING, D.; MCBRIDE, J.; STARK, D.; VADUBESCU, O.

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY

WILEY-BLACKWELL PUBLISHING, INC

Lugar: Londres; Año: 2014 vol. 443 p. 1151 - 1172

0035-8711

We describe the structural and kinematic properties of the first 28 compact stellar systems discovered by the Archive of Intermediate Mass Stellar Systems (AIMSS) project. This survey identifies compact stellar systems in archival HST images of nearby (D < 200 Mpc) galaxies and uses spectroscopic follow-up to confirm their nature (success rate currently > 96%) and to measure their internal  kinematics and determine their stellar populations.    The objects presented here have e↵ective radii (~6 < Re [pc] < 500) and stellar masses (~2x10^6 < M* /M0 < 6x10^9 ) spanning the range displayed by massive globular clusters (GCs), ultra compact dwarfs (UCDs) and compact elliptical galaxies (cEs), and they completely fill the gap between star clusters and galaxies. Several ofthe objects have masses and sizes (and hence stellar densities) comparable to those of the prototypical cE, M32, making them the best M32 analogues known. These objects, which are more luminous/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. However, these new objects further strengthen the case for a universal "zone of avoidance" for dynamically hot old stellar systems in the mass vs size plane.    Examination of the luminosity/mass distribution of our compact stellar systems combined with literature samples, provides further support for a separation of such systems into two types: 1) those formed in-situ, in major star formation events (i.e. massive star clusters), and 2) a second population related to galaxies, either free-floating galaxy nuclei/bulges liberated by the tidal disruption of their hosts or lower mass analogues of classical ellipticals. The relative frequencies of these objects support the suggestion that in general, old UCDs/cEs more luminous than MV~ -13 are the result of stripping, whilst below this luminosity limit the population is increasingly dominated by massive star clusters.     We also see strong indications that the galaxy-type population divides into two groups, which we associate with objects formed by the stripping of nucleated dwarf galaxies (and found amongst the UCDs), and a second more massive population that formed from the stripping of galaxies with bulges or which are a lower mass extensionof the elliptical formation process (the densest UCDs and cEs). Finally, our compact stellar systems are found associated with galaxies in all environments, from isolated galaxies to the densest clusters, demonstrating that the processes responsible for com-pact stellar system formation are not limited to high density environments.