The early-type dwarf galaxy population in nearby galaxy clusters
Final Report Abstract
The main topic of this project is the study of the properties of early-type dwarf galaxies, ultra-compact dwarf galaxies (UCDs) and globular clusters (GCs) in nearby galaxy clusters, in particular the Hydra I galaxy cluster. The first part of the project concentrated on the spectroscopic confirmation of 118 UCDs and bright GCs in Hydra I. We showed that this cluster has one of the largest populations of UCDs. Their kinematics and spatial distribution di↵ers from that of the globular clusters, indicating that some UCDs might have had a different formation history than the GCs. The colours and magnitudes of several UCDs coincide with those of nuclear star clusters, suggesting that they might be the descendants of nucleated dwarf galaxies that have been disrupted in the cluster potential except for their remnant nuclei. The 118 GCs/UCDs in Hydra I were used as moving test particles to probe the gravitational potential of the cluster core. Their kinematics can constrain the dark halo of the central giant elliptical galaxy NGC 3311. With simplifying assumptions for our dynamical model (equilibrium, sphericity, NGC 3311 at rest), we have found that cuspy dark matter halos of the NFW type, which obey the relation between concentration and virial mass found in cosmological simulations and explain the globular cluster velocity dispersion, do not satisfactorily account for the smooth rise in the stellar velocity dispersion at small radii under isotropy. Halos with a core can more accurately reproduce the stellar and globular cluster kinematics observed for NGC 3311. In the second part of the project, we took advantage of the availability of large datasets on dynamically hot stellar systems in the local universe. We combined our own results on UCDs, GCs and early-type dwarfs with those from the literature to present a complete picture of hot stellar systems ranging over 10 orders in magnitude in mass from faint galaxies and star clusters of only a few hundred solar masses up to giant ellipticals (gEs) with 10^12 MΘ. For all those stellar systems we showed 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 dwarf ellipticals (dEs) to dwarf spheroidals (dSphs), and the ‘star cluster’ family, comprising GCs, UCDs and nuclear star clusters (NCs). Interestingly, massive Es have a similar sizemass relation as compact ellipticals (cEs), UCDs and NCs, with a clear common boundary towards minimum sizes. 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. 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. The densest stellar systems (within their effective radius) are NCs. Finally, we quantified the specific frequency of UCDs in a range of environments and at related it to the frequency of star clusters and potential progenitor dwarf galaxies. We found that the specific frequencies derived for UCDs match those of GCs very well, to within 10-50%. This implies that not more than ~50% of UCDs were formed from dwarf galaxies. We show that such a scenario would require ≥90% of primordial dwarfs in galaxy cluster centers (<100 kpc) to have been stripped of their stars. We concluded that the number counts of UCDs are fully consistent with them being the bright tail of the GC population. From a statistical point of view there is no need to invoke an additional formation channel.
Publications
- 2011, A&A, 531, 119: The dark halo of the Hydra I galaxy cluster: core, cusp, cosmological? Dynamics of NGC 3311 and its globular cluster system
Richtler, T., Salinas, R., Misgeld, I., Hilker, M., Hau, G.K.T., Romanowsky, A.J., Schuberth, Y., Spolaor, M.
- 2011, A&A, 531, 4: A large population of ultra-compact dwarf galaxies in the Hydra I cluster
Misgeld, I., Mieske, S., Hilker, M., Richtler, T., Georgiev, I.Y., Schuberth, Y.
- 2011, in ‘A Universe of Dwarf Galaxies’, eds. M. Koleva, P. Prugniel & I. Vauglin, EAS Publications Series, 48, p. 207: Faint Dwarf Galaxies in Nearby Clusters
Misgeld, I., Hilker, M., Mieske, S.
- 2011, in ‘A Universe of Dwarf Galaxies’, eds. M. Koleva, P. Prugniel & I. Vauglin, EAS Publications Series, 48, p. 219: A Census of Ultra-Compact Dwarf Galaxies in Nearby Galaxy Clusters
Hilker, M.
- 2011, MNRAS, 414, 3699: Families of dynamically hot stellar systems over 10 orders of magnitude in mass
Misgeld, I., Hilker, M.
- 2012, A&A, 537, 3: The specific frequencies of ultracompact dwarf galaxies
Mieske, S., Hilker, M., Misgeld, I.
(See online at https://doi.org/10.1051/0004-6361/201117634)
