VST-SMASH: VST Survey of Mass Assembly and Structural Hierarchy

I. Survey presentation and deep photometry of IC 5332: Tracing the mass assembly in the challenging faintest-end regime

¹ INAF – Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, I-80131, Napoli, Italy
² INAF – Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125, Firenze, Italy
³ Sterrenkundig Observatorium, Universiteit Gent, Krijgslaan 281 S9, 9000, Gent, Belgium
⁴ Department of Astronomy, Indiana University, 727 East Third Street, Bloomington, IN, 47405, USA
⁵ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, I-40129, Bologna, Italy
⁶ Università di Salerno, Dipartimento di Fisica “E.R. Caianiello”, Via Giovanni Paolo II 132, 84084, Fisciano (SA), Italy
⁷ INFN – Gruppo Collegato di Salerno – Sezione di Napoli, Dipartimento di Fisica “E.R. Caianiello”, Università di Salerno, Via Giovanni Paolo II, 132, 84084, Fisciano (SA), Italy
⁸ Department of Physics and Astronomy, University of Padua, 35131, Padua, Italy
⁹ Max Planck Institute for Astronomy, Künigstuhl 17, D-69117, Heidelberg, Germany

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

Received: 4 November 2025
Accepted: 1 April 2026

Abstract

Context. Understanding the formation and evolution of late-type galaxies requires deep imaging for tracing the faintest stellar components in their outskirts (i.e. accreted material, diffuse halos, and tidal streams). Despite their crucial role in the build-up of stellar mass, these low-surface-brightness features remain largely unexplored due to observational limitations. The VST Survey of Mass Assembly and Structural Hierarchy (VST-SMASH) is designed to fill this gap, providing deep, wide-field optical imaging for a volume-limited sample of nearby spiral galaxies (D < 11 Mpc), overlapping with the Euclid Wide Survey in the southern hemisphere.

Aims. This paper aims to introduce the VST-SMASH survey and showcase its scientific potential through the analysis of IC 5332, a late-type spiral galaxy observed in the g, r, and i bands. The main goal is to demonstrate the depth, quality, and diagnostic power of the dataset in tracing low-surface-brightness features and structural components in galactic outskirts.

Methods. We carried out detailed surface photometry of IC 5332, using elliptical isophotal fitting and growth curve analysis to extract radial surface brightness and colour profiles down to μ_r ∼ 30 mag arcsec⁻². We performed multi-component Sérsic decompositions in each band and constructed stellar mass surface density profiles. Furthermore, we identified and characterized faint stellar streams visible in the very deep and smoothed images, estimating their colours and comparing them with adjacent galactic regions.

Results. While the internal (< 1 effective radius) negative colour gradients can be explained by dissipative (monolithic) collapses and supernovae outflows, the colour profiles at larger radii reveal a significant gradient towards redder g − r and g − i, spatially consistent with the presence of accreted populations in the outskirts. Nevertheless, we also find bluer r − i, which , together with the behaviour of g − r and g − i, could be explained by strong Hα emission. Single-component Sérsic fits fail to reproduce the observed light distribution, especially in the outer regions, requiring a two-component model. In the outskirts, multiple faint stellar streams are detected, with integrated g − r colours consistent with disrupted dwarf and/or low-surface-brightness satellites. However, spectroscopic follow-ups would be needed to detect possible strong emission lines, which could affect the r-band photometry. These findings support a scenario of ongoing stellar mass assembly through accretion and highlight the capability of VST-SMASH to uncover faint structures in nearby galaxies.