The AMIGA sample of isolated galaxies

XIV. Disc breaks and interactions through ultra-deep optical imaging^⋆

¹ Instituto de Astrofísica de Canarias, c/ Vía Láctea s/n, 38205 La Laguna, Tenerife, Spain
e-mail: pmsa.astro@gmail.com
² Departamento de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain
e-mail: jromanastro@gmail.com
³ Instituto de Astrofísica de Andalucía (CSIC), Granada, Spain
⁴ Kapteyn Astronomical Institute, University of Groningen, PO Box 800 9700 AV Groningen, The Netherlands
⁵ Department of Physics & Astronomy, University of California Los Angeles, 430 Portola Plaza, Los Angeles, CA 90095-1547, USA
⁶ Departamento de Física Teórica y del Cosmos Universidad de Granada, 18071 Granada, Spain
⁷ Instituto Universitario Carlos I de Física Teórica y Computacional, Universidad de Granada, 18071 Granada, Spain
⁸ ESA NEO Coordination Centre, Via Galileo Galilei, 00044 Frascati (RM), Italy
⁹ GMV, Isaac Newton 11, Tres Cantos, 28760 Madrid, Spain
¹⁰ Centro de Astronomía (CITEVA), Universidad de Antofagasta, Avda. U. de Antofagasta 02800, Antofagasta, Chile

Received: 20 April 2023
Accepted: 29 June 2023

Abstract

Context. In the standard cosmological model of galaxy evolution, mergers and interactions play a fundamental role in shaping galaxies. Galaxies that are currently isolated are thus interesting because they allow us to distinguish between internal and external processes that affect the galactic structure. However, current observational limits may obscure crucial information in the low-mass or low-brightness regime.

Aims. We use optical imaging of a subsample of the AMIGA catalogue of isolated galaxies to explore the impact of different factors on the structure of these galaxies. In particular, we study the type of disc break as a function of the degree of isolation and the presence of interaction indicators such as tidal streams or plumes, which are only detectable in the ultra-low surface brightness regime.

Methods. We present ultra-deep optical imaging in the r band of a sample of 25 low-redshift (z < 0.035) isolated galaxies. Through careful data processing and analysis techniques, the nominal surface brightness limits achieved are comparable to those to be obtained on the ten-year LSST coadds (μ_r,lim ≳ 29.5 mag arcsec⁻² [3σ; 10″ × 10″]). We place special emphasis on preserving the low surface brightness features throughout the processing.

Results. The extreme depth of our imaging allows us to study the interaction signatures of 20 galaxies since Galactic cirrus is a strong limiting factor in the characterisation of interactions for the remaining 5 of them. We detect previously unreported interaction features in 8 (40% ± 14%) galaxies in our sample. We identify 9 galaxies (36% ± 10%) with an exponential disc (Type I), 14 galaxies (56% ± 10%) with a down-bending (Type II) profile, and only 2 galaxies (8% ± 5%) with up-bending (Type III) profiles. Isolated galaxies have considerably more purely exponential discs and fewer up-bending surface brightness profiles than field or cluster galaxies. We find clear minor merger activity in some of the galaxies with single exponential or down-bending profiles, and both of the galaxies with up-bending profiles show signatures of a past interaction.

Conclusions. We show the importance of ultra-deep optical imaging in revealing faint external features in galaxies that indicate a probable history of interaction. We confirm that up-bending profiles are likely produced by major mergers, while down-bending profiles are probably formed by a threshold in star formation. Unperturbed galaxies that slowly evolve with a low star formation rate could induce the high rate of Type I discs in isolated galaxies.