Dwarf galaxies in the MATLAS survey: Hubble Space Telescope observations of nuclear star clusters

¹ Space Physics and Astronomy Research Unit, University of Oulu, P.O. Box 3000 FI-90014 Oulu, Finland
² Institute für Astro- und Teilchenphysik, Universität Innsbruck, Technikerstraße 25/8, Innsbruck A-6020, Austria
³ Université de Strasbourg, CNRS, Observatoire astronomique de Strasbourg, UMR 7550, F-67000 Strasbourg, France
⁴ UK Astronomy Technology Centre, Royal Observatory Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK
⁵ Dept. of Physics, Astronomy, Geology, and Environmental Sciences, Youngstown State University, Youngstown, OH 44555, USA
⁶ Department of Astronomy and Center for Galaxy Evolution Research, Yonsei University, Seoul 03722, Republic of Korea
⁷ INAF – Astronomical Observatory of Abruzzo, Via Maggini, 64100 Teramo, Italy
⁸ Institute of Physics, Laboratory of Astrophysics, École Polytechnique Fédéale de Lausanne (EPFL), 1290 Sauverny, Switzerland
⁹ Institute of Astronomy, Madingley Rd, Cambridge CB3 0HA, UK
¹⁰ Visiting Fellow, Clare Hall, University of Cambridge, Cambridge, UK
¹¹ Division of Science Education, Kangwon National University, Chuncheon, Republic of Korea
¹² Department of Astronomy, Yonsei University, 50 Yonsei-ro Seodaemun-gu, Seoul 03722, Republic of Korea
¹³ Univ. Lyon, ENS de Lyon, Univ. Lyon 1, CNRS, Centre de Recherche Astrophysique de Lyon, UMR5574, 69007 Lyon, France

^⋆ Corresponding author: melina.poulain@oulu.fi, melina.poulain45@gmail.com

Received: 28 March 2025
Accepted: 22 September 2025

Abstract

In dwarf galaxies, nuclear star clusters (NSCs) are believed to primarily form from the migration and merger of globular clusters (GCs), with a possible contribution from in situ star-forming activity triggered by gas infall. We present the study of NSCs in 41 MATLAS survey dwarf galaxies including ultra-diffuse galaxies (UDGs), as part of a large follow-up imaging program with the Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) using the F606W and F814W filters. The sample is biased toward low surface brightness and large dwarfs, i.e., UDG-like galaxies, and includes two galaxies with a double nucleus; 13 newly identified nucleated dwarfs, thanks to HST’s high spatial resolution; and five candidate ultra-compact dwarf progenitors. We modeled the NSCs with a Sérsic profile and derived their structural properties and photometry. We find the NSC Sérsic index to increase with the luminosity and stellar mass, while no obvious trend is seen for the effective radius and ellipticity. The faint NSCs tend to have a constant color profile, whereas the bright ones have a bluer center, suggesting that the most massive NSCs in our sample might have experienced a mixed formation scenario, including in situ star formation. A significant portion of our NSCs tend to be more massive than for other galaxy samples of similar stellar mass, which could be due to some dwarfs undergoing tidal disruption or an initial formation of massive NSCs from multiple GC mergers and in situ star-forming activity. More observations of resolved NSCs are needed to be able to infer their formation scenario from the structural properties and photometry in dwarfs.