A pearl in the shell: An ultra-compact dwarf within the tidal debris surrounding spiral galaxy NGC 7531

¹ Centro de Estudios de Física del Cosmos de Aragón (CEFCA), Unidad Asociada al CSIC, Plaza San Juan 1, 44001 Teruel, Spain
² ARAID Foundation, Avda. de Ranillas, 1-D, E-50018 Zaragoza, Spain
³ Department of Physics & Astronomy, San José State University, One Washington Square, San Jose, CA 95192, USA
⁴ Department of Astronomy & Astrophysics, University of California, Santa Cruz, CA 95064, USA
⁵ Instituto de Astrofísica de Andalucía, CSIC, Glorieta de la Astronomía, E-18080 Granada, Spain
⁶ School of Mathematics and Physics, University of Surrey, Guildford GU2 7XH, UK
⁷ Departamento de Física de la Tierra y Astrofísica, Universidad Complutense de Madrid, E-28040 Madrid, Spain
⁸ Leiden Observatory, Leiden University, Gorlaeus Building at Einsteinweg 55, NL-2333 CC, Leiden, The Netherlands
⁹ UAI – Unione Astrofili Italiani/P.I. Sezione Nazionale di Ricerca Profondo Cielo, 72024 Oria, Italy
¹⁰ Doc Greiner Research Observatory-Rancho Hidalgo, Animas, New Mexico, USA
¹¹ Perimeter Institute for Theoretical Physics, 31 Caroline St N, Waterloo, Canada

^★★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 17 September 2025
Accepted: 15 February 2026

Abstract

Context. Stellar substructures within tidal debris preserve information about their progenitor galaxies’ properties, offering insights into hierarchical mass assembly.

Aims. We examine a compact stellar system (CSS) around the nearby spiral galaxy NGC 7531, including the shell-like tidal debris around it. Our goals are to determine the nature of the CSS, reconstruct the accretion history, and understand how the large, diffuse shell-like structure formed.

Methods. We present photometric measurements of the shell-like debris and CSS using DESI Legacy Imaging Survey (LS) data. We obtained Keck/LRIS spectroscopic data for the CSS to confirm its association with NGC 7531 and to derive its star formation history (SFH). Deep (∼27.9 mag/arcsec²) amateur telescope images enabled us to make a complete characterisation of the tidal debris structure.

Results. We confirm that the CSS is associated with NGC 7531. We rename it NGC 7531-UCD1, since its stellar mass (6050⁺⁴⁶⁰₋₆₃₀ M_⊙), half-light radius (R_h = 0.13 ± 0.05 arcsec), and SFH place it as an ultra-compact dwarf galaxy (UCD). We find that NGC 7531-UCD1 experienced a star formation burst ∼1 Gyr ago. NGC 7531-UCD1 was likely a nuclear star cluster (NSC) that was tidally stripped into a UCD- this is further supported by the presence of tidal tails. We quantify the shell-like debris’ mass as M_★ ∼ 3–11 × 10⁸ M_⊙, implying a merger mass ratio of ∼300:1 to 10:1. Our amateur telescope images confirm new pieces of tidal debris, previously unclear in the DESI LS images. N-body simulations reproduce the tidal features, and require a near radial orbit of the progenitor dwarf galaxy with two pericentric passages. The first pericentre passage coincides with the measured star formation enhancement ∼1 Gyr ago.

Conclusions. Our findings agree with theoretical predictions about the NSC to UCD formation pathway via tidal stripping, and further confirm the presence of these objects outside of our Milky Way.