Globular Cluster Scale Sizes in Giant Galaxies: Orbital Anisotropy and Tidally Under-filling Clusters in M87, NGC 1399, and NGC 5128

Jeremy J. Webb; Alison Sills; William E. Harris; Matías Gómez; Maurizio Paolillo; Kristin A. Woodley; Thomas H. Puzia

doi:10.1093/mnras/stw1115

paper

Globular Cluster Scale Sizes in Giant Galaxies: Orbital Anisotropy and Tidally Under-filling Clusters in M87, NGC 1399, and NGC 5128

arXiv:1605.03191 · doi:10.1093/mnras/stw1115

Abstract

We investigate the shallow increase in globular cluster half-light radii with projected galactocentric distance $R_{g c}$ observed in the giant galaxies M87, NGC 1399, and NGC 5128. To model the trend in each galaxy, we explore the effects of orbital anisotropy and tidally under-filling clusters. While a strong degeneracy exists between the two parameters, we use kinematic studies to help constrain the distance $R_{β}$ beyond which cluster orbits become anisotropic, as well as the distance $R_{f α}$ beyond which clusters are tidally under-filling. For M87 we find $R_{β} > 27$ kpc and $20 < R_{f α} < 40$ kpc and for NGC 1399 $R_{β} > 13$ kpc and $10 < R_{f α} < 30$ kpc. The connection of $R_{f α}$ with each galaxy's mass profile indicates the relationship between size and $R_{g c}$ may be imposed at formation, with only inner clusters being tidally affected. The best fitted models suggest the dynamical histories of brightest cluster galaxies yield similar present-day distributions of cluster properties. For NGC 5128, the central giant in a small galaxy group, we find $R_{β} > 5$ kpc and $R_{f α} > 30$ kpc. While we cannot rule out a dependence on $R_{g c}$ , NGC 5128 is well fitted by a tidally filling cluster population with an isotropic distribution of orbits, suggesting it may have formed via an initial fast accretion phase. Perturbations from the surrounding environment may also affect a galaxy's orbital anisotropy profile, as outer clusters in M87 and NGC 1399 have primarily radial orbits while outer NGC 5128 clusters remain isotropic.

arXiv comment16 pages, 7 figures, 4 tables, Accepted for publication in MNRAS

Globular Cluster Scale Sizes in Giant Galaxies: Orbital Anisotropy and Tidally Under-filling Clusters in M87, NGC 1399, and NGC 5128

Abstract

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