Single metal-poor ultra compact dwarf galaxy at one kiloparsec distance from the low-mass elliptical galaxy FCC 47^⋆,⋆⋆

¹ European Southern Observatory, Karl Schwarzschild Straße 2, 85748 Garching bei München, Germany
e-mail: kfahrion@eso.org
² Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
³ Department of Astrophysics, University of Vienna, Türkenschanzstrasse 17, 1180 Wien, Austria
⁴ Dipartimento di Fisica e Astronomia “G. Galilei”, Universitá di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
⁵ INAF-Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁶ Armagh Observatory and Planetarium, College Hill, Armagh BT61 9DG, UK
⁷ Centre for Astrophysics Research, University of Hertfordshire, College Lane, Hatfield AL10 9AB, UK
⁸ Department of Physics and Astronomy, Macquarie University, North Ryde, NSW 2109, Australia
⁹ Sterrewacht Leiden, Leiden University, Postbus 9513, 2300 RA Leiden, The Netherlands
¹⁰ Max-Planck-Institut für extraterrestrische Physik, Gießenbachstraße 1, 85748 Garching bei München, Germany

Received: 21 December 2018
Accepted: 1 April 2019

Abstract

Context. Photometric surveys of galaxy clusters have revealed a large number of ultra compact dwarfs (UCDs) around predominantly massive elliptical galaxies. Their origin is still debated as some UCDs are considered to be the remnant nuclei of stripped dwarf galaxies while others seem to mark the high-mass end of the star cluster population.

Aims. We aim to characterize the properties of a UCD found at very close projected distance (r_wproj = 1.1 kpc) from the centre of the low-mass (M ∼ 10¹⁰ M_⊙) early-type galaxy FCC 47. This is a serendipitous discovery from MUSE adaptive optics science verification data. We explore the potential origin of this UCD as either a massive cluster or the remnant nucleus of a dissolved galaxy.

Methods. We used archival Hubble Space Telescope data to study the photometric and structural properties of FCC 47-UCD1. In the MUSE data, the UCD is unresolved, but we used its spectrum to determine the radial velocity and metallicity.

Results. The surface brightness of FCC 47-UCD1 is best described by a single King profile with low concentration C = R_t/R_c ∼ 10 and large effective radius (r_eff = 24 pc). Its integrated magnitude and blue colour (M_g = −10.55 mag, (g − z) = 1.46 mag) combined with a metallicity of [M/H] = −1.12 ± 0.10 dex and an age > 8 Gyr obtained from the full fitting of the MUSE spectrum suggests a stellar population mass of M_* = 4.87 × 10⁶ M_⊙. The low S/N of the MUSE spectrum prevents detailed stellar population analysis. Due to the limited spectral resolution of MUSE, we can only give an upper limit on the velocity dispersion (σ <  17 km s⁻¹), and consequently on its dynamical mass (M_dyn <  1.3 × 10⁷ M_⊙).

Conclusions. The origin of the UCD cannot be constrained with certainty. The low metallicity, old age, and magnitude are consistent with a star cluster origin, whereas the extended size is consistent with an origin as the stripped nucleus of a dwarf galaxy with a initial stellar mass of a few 10⁸ M_⊙.