AVID: A near-major post-merger of late-type dwarfs beneath a regularly rotating H I disk (VCC 693)

¹ Department of Astronomy, University of Science and Technology of China, Hefei 230026, China
² School of Astronomy and Space Science, University of Science and Technology of China, Hefei 230026, China
³ Institute of Deep Space Sciences, Deep Space Exploration Laboratory, Hefei 230026, China
⁴ Centre for Astrophysics Research, University of Hertfordshire, College Lane, Hatfield AL10 9AB, UK
⁵ Department of Astronomy and Space Science, Sejong University, Seoul 05006, Republic of Korea
⁶ Department of Physics and Astronomy, Sejong University, Seoul 05006, Republic of Korea
⁷ Departamento de Fisica, Universidad Tecnica Federico Santa Maria, Avenida España 1680 Valparaíso, Chile
⁸ Millenium Nucleus for Galaxies (MINGAL), Valparaíso, Chile
⁹ Institute for Astrophysics, School of Physics, Zhengzhou University, Zhengzhou 450001, China
¹⁰ National Research Council of Canada, Herzberg Astronomy and Astrophysics, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada
¹¹ Aix-Marseille Univ., CNRS, CNES, LAM, Marseille, France
¹² Université de Strasbourg, CNRS, Observatoire astronomique de Strasbourg (ObAS), UMR 7550, F-67000 Strasbourg, France
¹³ Università di Milano-Bicocca, Piazza della Scienza 3, 20100 Milano, Italy
¹⁴ Department of Astronomy, Yonsei University, Seoul 03722, Republic of Korea
¹⁵ Center for Galaxy Evolution Research, Yonsei University, Seoul 03722, Republic of Korea
¹⁶ NSF NOIRLab, 950 N. Cherry Avenue, Tucson, AZ 85719, USA
¹⁷ Instituto de Astrofísica, Pontificia Universidad Católica de Chile, 7820436 Macul, Santiago, Chile
¹⁸ Isaac Newton Group of Telescopes, Apartado 321, E-38700 Santa Cruz de la Palma, Tenerife, Spain
¹⁹ University of Calgary, 2500 University Drive NW, Calgary, Alberta T2N 1N4, Canada
²⁰ Yunnan Observatories, Chinese Academy of Sciences, Kunming 650011, People’s Republic of China
²¹ State Key Laboratory of Radio Astronomy and Technology, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, China

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Received: 30 September 2025
Accepted: 21 February 2026

Abstract

In the periphery of galaxy clusters, the combination of moderately high galaxy number densities and modest velocity dispersions favors interactions and mergers that affect the galaxy evolution prior to cluster infall. Observational studies of this important phase of galaxy evolution, particularly in the dwarf galaxy regime, are still rare. We present a high-resolution case study of VCC 693 (stellar mass ∼ 2.8 × 10⁸ M_⊙), a merger remnant in the outskirts of the Virgo cluster, using observations from the project Atomic gas in Virgo Interacting Dwarf galaxies (AVID). We explored the consequences of the merger on the star formation and structural evolution of VCC 693, based on a joint analysis of high-resolution Karl G. Jansky Very Large Array and high-sensitivity Five-Hundred-meter Aperture Spherical radio Telescope H I emission line observations, optical broadband images, narrowband Hα images, and optical spectra. We also employed hydrodynamical simulations of dwarf-dwarf mergers to aid in interpreting the observations of VCC 693. Our analysis favors a near-major merger between two late-type dwarf galaxies with a stellar mass ratio of approximately 3:1–4:1, in which one of the progenitors is likely relatively gas poor (i.e., a damp merger). The optical appearance of VCC 693 is dominated by complex tidal structures spanning the whole system, whereas the H I gas has settled to a regular rotating disk with a normal surface density profile. Compared to dwarfs of similar mass, the star formation and gas-phase metallicity are moderately enhanced in the center. The global star formation rate, H I gas content, and H I-to-optical size ratio of VCC 693 are broadly consistent with those of typical late-type dwarfs of similar mass, although they fall at the lower side of the distributions. By decomposing the H I rotation curve into the baryonic and dark matter component, we found that the dark matter halo is characterized by an unusually high concentration or core density. This implies that the dark matter halo might have relaxed into a more centrally peaked distribution following the merger event. Together with two other recent studies of AVID post-merger systems, our findings reinforce the emerging view that even major mergers between dwarfs can produce remnants whose overall stellar structures, apart from tidal features, are indistinguishable from those of ordinary dwarfs, and that the diverse environmental effects experienced by galaxies in cluster outskirts can promote damp or mixed mergers, which constitute an integral part of galactic preprocessing.