Globular cluster systems in nearby dwarf galaxies - II. Nuclear star clusters and their relation to massive Galactic globular clusters

Abstract

We compare nuclear globular clusters (nGCs) in dwarf galaxies and Galactic GCs with extended (hot) horizontal branches (EHB-GCs) in order to test the suggested external origin of the latter and the conditions at which GC self-enrichment can operate. Using luminosities and structural parameters of nGCs in low-mass (mainly late-type) dwarf galaxies from Hubble Space Telescope (HST)/Advanced Camera for Surveys (ACS) imaging, we derive the present-day escape velocities (υ_esc) of stellar ejecta to reach the cluster tidal radius and compare them with those of EHB-GCs. We show that nGCs in dwarf galaxies are very similar in their photometric and structural properties (colour <V - I> = 0.9mag, magnitudes <M_V> < -9mag, ellipticities <ɛ> = 0.11) to EHB-GCs. The nGCs populate the same M_V versus r_h region as EHB-GCs, although they do not reach the sizes of the largest EHB-GCs like ωCen and NGC2419. We suggest that during accretion the r_h of an nGC could increase due to significant mass loss in the cluster vicinity and the resulting drop in the external potential in the core once the dwarf galaxy dissolves. For EHB-GCs, we find a correlation between the present-day υ_esc and their metallicity as well as (V - I)₀ colour. The similar υ_esc, (V - I)₀ distribution of nGCs and EHB-GCs imply that nGCs could also have complex stellar populations. The υ_esc-[Fe/H] relation could reflect the known relation of increasing stellar wind velocity with metallicity, which in turn could explain why more metal-poor clusters typically show more peculiarities in their stellar population than more metal-rich clusters of the same mass do. Thus, the cluster υ_esc can be used as a parameter to describe the degree of self-enrichment. All our findings support the scenario in which Galactic EHB-GCs have originated in the centres of pre-Galactic building blocks or dwarf galaxies that were later accreted by the Milky Way.

This study is based on archival data of the NASA/ESA Hubble Space Telescope (HST), which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.

E-mail: iskren@astro.uni-bonn.de