Journal Article

Families of dynamically hot stellar systems over 10 orders of magnitude in mass

I. Misgeld and M. Hilker

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 414, issue 4, pages 3699-3710
Published in print July 2011 | ISSN: 0035-8711
Published online July 2011 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2011.18669.x
Families of dynamically hot stellar systems over 10 orders of magnitude in mass

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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 Reff≥ 2.24 × 10−6M4/5 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 (Reff∼ 1.0 kpc) as well as GCs (Reff∼ 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 < 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 Σeff= 3.17 × 1010M−3/5 M pc−2, 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.

Keywords: globular clusters: general; galaxies: dwarf; galaxies: fundamental parameters; galaxies: star clusters: general

Journal Article.  8050 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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