Deep imaging of the galaxy Malin 2 shows new faint structures and a candidate satellite dwarf galaxy

¹ Instituto de Astrofísica de Canarias, Vía Láctea s/n, E-38205 La Laguna, Spain
² Departamento de Astrofísica, Universidad de La Laguna, E-38206 La Laguna, Spain
³ Light Bridges S.L., Observatorio del Teide, Carretera del Observatorio s/n, E-38500 Guimar, Tenerife, Canarias, Spain
⁴ Université de Strasbourg, CNRS, Observatoire Astronomique de Strasbourg, UMR 7550, 67000 Strasbourg, France

^⋆ Corresponding author: junais@iac.es

Received: 23 July 2025
Accepted: 10 August 2025

Abstract

Giant low-surface-brightness (GLSB) galaxies represent an extreme class of disk galaxies characterized by exceptionally large sizes and low stellar densities. Their formation and evolutionary pathways remain poorly constrained, primarily due to the observational challenges associated with detecting their faint stellar disks. In this work, we present deep, multiband optical imaging of Malin 2, a prototypical GLSB galaxy, obtained with the newly commissioned Two-meter Twin Telescope (TTT) at the Teide Observatory. Our g-, r-, and i-band observations reach surface brightness depths of 30.3, 29.5, and 28.2 mag arcsec⁻² (3σ, in areas equivalent to 10″ × 10″), respectively, enabling us to trace the stellar disk of Malin 2 out to a radius of ∼110 kpc for the first time. We observe new diffuse stellar structures, including a prominent stellar emission toward the northwest region of Malin 2. This emission coincides well with the H I gas distribution in this region. We also identify a faint spiral arm-like structure in the southeast of Malin 2. Additionally, we report the discovery of a very faint dwarf galaxy, TTT-d1 (μ_0, g ∼ 26 mag arcsec⁻²), located at a projected distance of ∼130 kpc southeast of Malin 2. If physically associated with Malin 2, it would represent the first known satellite ultra-diffuse galaxy of a GLSB galaxy. We perform a multidirectional wedge photometric analysis of Malin 2 and find that the galaxy has significant azimuthal variations in its stellar emission. A comparison of the stellar mass surface density profiles of Malin 2 with those of a large sample of nearby spiral galaxies and other GLSB galaxies shows that Malin 2 lies at the extreme end of both these classes of objects in its radial extent and stellar mass surface density distribution. The spatial overlap between the asymmetric stellar emission and a lopsided H I distribution suggests that Malin 2’s GLSB disk has contributions from tidal interactions. Our results highlight the importance of ultradeep, wide-field imaging in understanding the structural complexity of GLSB galaxies. Upcoming surveys such as LSST will be crucial to determine whether the features we observe in Malin 2 are common to other GLSB disk galaxies.