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

We compare nuclear globular clusters (nGCs) in dwarf galaxies and Galactic GCs with extended (hot) horizontal branches (EHBs-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 HST/ACS imaging we derive the present-day escape velocities (υesc) 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〉 = 0.9, magnitudes 〈MV〉 < −9 mag, ellipticities 〈ǫ〉 = 0.11) to EHB-GCs. The nGCs populate the same MV vs. rh region as EHB-GCs, although they do not reach the sizes of the largest EHB-GCs like ωCen and NGC 2419. We argue that during accretion the rh 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 υesc and their metallicity as well as (V−I)0 colour. The similar υesc , (V−I)0 distribution of nGCs and EHB-GCs implies that nGCs could also have complex stellar populations. The υesc–[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 υesc can be used as 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.