Metal-poor nuclear star clusters in two dwarf galaxies near Centaurus A suggesting formation from the in-spiraling of globular clusters^⋆

¹ European Southern Observatory, Karl Schwarzschild Straße 2, 85748 Garching bei München, Germany
e-mail: kfahrion@eso.org
² Observatoire Astronomique de Strasbourg (ObAS), Universite de Strasbourg – CNRS, UMR, 7550 Strasbourg, France
³ Department of Astrophysics, University of Vienna, Türkenschanzstrasse 17, 1180 Wien, Austria
⁴ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁵ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
⁶ Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611, Australia

Received: 15 November 2019
Accepted: 19 December 2019

Abstract

Studies of nucleated dwarf galaxies can constrain the scenarios for the formation and evolution of nuclear star clusters (NSC) in low-mass galaxies and give us insights on the origin of ultra compact dwarf galaxies (UCDs). We report the discovery of a NSC in the dwarf galaxy KKs58 and investigate its properties together with those of another NSC in KK197. Both NSCs are hosted by dwarf elliptical galaxies of the Centaurus group. Combining ESO VLT MUSE data with photometry from VLT FORS2, CTIO Blanco DECam, and HST ACS, as well as high-resolution spectroscopy from VLT UVES, we analyse the photometric, kinematic and stellar population properties of the NSCs and their host galaxies. We confirm membership of the NSCs based on their radial velocities and location close to the galaxy centres. We also confirm the membership of two globular clusters (GCs) and detect oblate rotation in the main body of KK197. Based on high signal-to-noise spectra taken with MUSE of the NSCs of both KKs58 and KK197 we measure low metallicities, [Fe/H] = −1.75 ± 0.06 dex and [Fe/H] = −1.84 ± 0.05 dex, and stellar masses of 7.3 × 10⁵ M_⊙ and 1.0 × 10⁶ M_⊙, respectively. Both NSCs are more metal-poor than their hosts that have metallicities of −1.35 ± 0.23 dex (KKs58) and −0.84 ± 0.12 dex (KK197). This can be interpreted as NSC formation via the in-spiral of GCs. The masses, sizes and metallicities of the two NSCs place them among other NSCs, but also among the known UCDs of the Centaurus group. This indicates that NSCs might constitute the progenitors of a part of the low-mass UCDs, although their properties are almost indistinguishable from typical GCs.