The GECKOS survey: Identifying kinematic sub-structures in edge-on galaxies

¹ European Southern Observatory, Karl-Schwarzschild-Straße 2, Garching 85748, Germany
² ARC Centre of Excellence for All Sky Astrophysics in Three Dimensions (ASTRO-3D), Australia
³ School of Physics, University of New South Wales, NSW 2052, Australia
⁴ Centre for Extragalactic Astronomy, Department of Physics, Durham University, South Road, Durham DH1 3LE, UK
⁵ National Research Council of Canada, Herzberg Astronomy and Astrophysics Research Centre, 5071 W. Saanich Rd., Victoria, BC V9E 2E7, Canada
⁶ Centre for Astrophysics and Supercomputing, Swinburne University of Technology, PO Box 218 Hawthorn, VIC 3122, Australia
⁷ Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, UK
⁸ Sub-department of Astrophysics, Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
⁹ Max-Planck-Institut für Extraterrestrische Physik, Gießenbachstraße 1, 85748 Garching, Germany
¹⁰ International Centre for Radio Astronomy Research (ICRAR), The University of Western Australia, M468, 35 Stirling Highway, Crawley, WA 6009, Australia
¹¹ Research School of Astronomy and Astrophysics, Australian National University, Cotter Road, Weston Creek, ACT 2611, Australia
¹² Sydney Institute for Astronomy, School of Physics, A28, The University of Sydney, NSW 2006, Australia
¹³ Department of Astrophysics, University of Vienna, Türkenschanzstraße 17, 1180 Vienna, Austria
¹⁴ Observatoire de Paris, LERMA, CNRS, PSL University, Sorbonne University, 75014 Paris, France
¹⁵ Collège de France, 11 Pl. Marcelin Berthelot, 75231 Paris, France
¹⁶ Cardiff Hub for Astrophysics Research & Technology, School of Physics & Astronomy, Cardiff University, Queens Buildings, Cardiff CF24 3AA, UK
¹⁷ Instituto de Astrofísica de Canarias, Calle Vía Láctea s/n, E-38205 La Laguna, Tenerife, Spain
¹⁸ Departamento de Astrofísica, Universidad de La Laguna, Av. del Astrofísico Francisco Sánchez s/n, E-38206 La Laguna, Tenerife, Spain
¹⁹ Institute for Computational Cosmology, Department of Physics, Durham University, South Road, Durham DH1 3LE, UK
²⁰ Homer L. Dodge Department of Physics & Astronomy, University of Oklahoma, 440 W. Brooks St., Norman, OK 73019, USA
²¹ Research Centre for Astronomy, Astrophysics, and Astrophotonics, Department of Physics and Astronomy, Macquarie University, NSW 2109, Australia
²² European Southern Observatory, Alonso de Córdova 3107, Vitacura, Región Metropolitana, Chile
²³ Universidade Cidade de São Paulo/Universidade Cruzeiro do Sul, Rua Galvão Bueno 868, São Paulo-SP 01506-000, Brazil
²⁴ Universitäts-Sternwarte, Fakultät für Physik, Ludwig-Maximilians-Universität München, Scheinerstr. 1, 81679 München, Germany
²⁵ Department of Physics and Astronomy, University of Utah, Salt Lake City, UT 84112, USA

^⋆ Corresponding authors: a.fraser-mckelvie@eso.org

Received: 5 November 2024
Accepted: 12 June 2025

Abstract

The vertical evolution of galactic discs is governed by the sub-structures within them. Several of these features, including bulges and kinematically distinct discs, are best studied in edge-on galaxies, as the viewing angle allows the easier separation of component light. For this work, we examined the diversity of kinematic sub-structure present in the first 12 galaxies observed from the GECKOS survey, a VLT/MUSE large programme providing a systematic study of 36 edge-on Milky Way-mass disc galaxies. Employing the NGIST analysis pipeline, we derived the mean luminosity-weighted line-of-sight stellar velocity (V_⋆), velocity dispersion (σ_⋆), skew (h₃), and kurtosis (h₄) for the sample, and examined 2D maps and 1D line profiles. Common clear kinematic signatures were observed: all galaxies display h₃ – V_⋆ sign mismatches in the outer disc regions consistent with a (quasi-)axisymmetric, rotating disc of stars. After scrutinising visual morphologies, we found that the majority of this sample (8/12) possess boxy-peanut bulges and host the corresponding kinematic structure predicted for stellar bars viewed in projection. Inferences were made on the bar viewing angle with respect to the line of sight from the strength of these kinematic indicators; we found one galaxy whose bar is close to side-on with respect to the observer, and two that are close to end-on. Four galaxies exhibit strong evidence for the presence of nuclear discs, including central h₃–V_⋆ profile anti-correlations, croissant-shaped central depressions in σ_⋆ maps, strong gradients in h₃, and positive h₄ plateaus over the expected nuclear disc extent. The strength of the h₃ feature corresponds to the size of the nuclear disc, measured from the h₃ turnover radius, taking into account geometric effects. We can explain the features within the kinematic maps of the four unbarred galaxies via disc structure(s) alone. We do not find any need to invoke the existence of dispersion-dominated bulges in any of the sample galaxies. Obtaining the specialised data products for this paper and the broader GECKOS survey required significant development of existing integral field spectroscopic (IFS) analysis tools. Therefore, we also present the NGIST pipeline: a modern, sophisticated, and easy-to-use pipeline for the analysis of galaxy IFS data, and the key tool employed by the GECKOS survey for producing value-added data products. We conclude that the variety of kinematic sub-structures seen in GECKOS galaxies requires a contemporary view of galaxy morphology, expanding on the traditional view of galaxy structure, and uniting the kinematic complexity observed in the Milky Way with the extragalactic.