2011MNRAS.414.3699M

Dynamically hot stellar systems, whether star clusters or early-type galaxies, follow well-defined scaling relations over many orders of magnitudes in mass. These Fundamental Plane relations have been subject of several studies, which have been mostly confined to certain types of galaxies and/or star clusters so far. Here, we present a complete picture of hot stellar systems ranging from faint galaxies and star clusters of only a few hundred solar masses up to giant ellipticals (gEs) with 10¹² M_☉, in particular including – for the first time – large samples of compact ellipticals (cEs), ultracompact dwarf galaxies (UCDs), dwarf ellipticals (dEs) of nearby galaxy clusters and Local Group ultrafaint dwarf spheroidals (dSphs). For all those stellar systems we show the effective radius luminosity, effective radius stellar mass and effective mass surface density stellar mass plane. Two clear families of hot stellar systems can be differentiated: the `galaxian' family, ranging from gEs over ellipticals (Es) and dEs to dSphs, and the `star cluster' family, comprising globular clusters (GCs), UCDs and nuclear star clusters (NCs). Interestingly, massive Es have a similar size–mass relation as cEs, UCDs and NCs, with a clear common boundary towards minimum sizes, which can be approximated by R_eff≥ 2.24 {x} 10^–6M^4/5_{Ux2605} pc. No object of either family is located in the `zone of avoidance' beyond this limit. Even the majority of early-type galaxies at high redshift obeys this relation. The sizes of dEs and dSphs (R_eff∼ 1.0 kpc) as well as GCs (R_eff∼ 3 pc) barely vary with mass over several orders of magnitude. We use the constant galaxy sizes to derive the distances of several local galaxy clusters. The size gap between star clusters and dwarf galaxies gets filled in by low mass, resolving star clusters and the faintest dSphs at the low mass end, and by GCs/UCDs, NCs and cEs in the mass range 10⁶ < M_{Ux2605}< 10⁹ M_☉. In the surface density–mass plane the sequences of star clusters and galaxies show the same slope, but are displaced with respect to each other by 10³ in mass and 10² in surface density. Objects that fall in between both sequences include cEs, UCDs, NCs and ultrafaint dSphs. Both, galaxies and star clusters, do not exceed a surface density of Σ_eff= 3.17 {x} 10¹⁰M^–3/5_{Ux2605}M_☉/pc2, causing an orthogonal kink in the galaxy sequence for Es more massive than 10¹¹ M_☉. The densest stellar systems (within their effective radius) are NCs.