The Fornax3D project: Discovery of ancient massive merger events in the Fornax cluster galaxies NGC 1380 and NGC 1427

¹ Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai, 200030 PR China
e-mail: lzhu@shao.ac.cn
² Department of Astrophysics, University of Vienna, Türkenschanzstraße 17, 1180 Vienna, Austria
³ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
⁴ European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching bei München, Germany
⁵ Instituto de Astrofísica de Canarias, Calle Via Láctea s/n, 38200 La Laguna, Tenerife, Spain
⁶ Depto. Astrofísica, Universidad de La Laguna, Calle Astrofísico Francisco Sánchez s/n, 3826 La Laguna, Tenerife, Spain
⁷ Dipartimento di Fisica e Astronomia ‘G. Galilei’, Università di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
⁸ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁹ European Space Agency (ESA), European Space Research and Technology Centre (ESTEC), Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands
¹⁰ Department of Astronomy and Tsinghua Center for Astrophysics, Tsinghua University, Beijing, 100084 PR China
¹¹ Department of Physics and Astronomy, Macquarie University, North Ryde, NSW, 2109 Australia
¹² Armagh Observatory and Planetarium, College Hill, Armagh, BT61 9DG UK
¹³ Centre for Astrophysics Research, University of Hertfordshire, College Lane, Hatfield, AL10 9AB UK
¹⁴ Sterrewacht Leiden, Leiden University, Postbus 9513, 2300 RA Leiden, The Netherlands
¹⁵ Max-Planck-Institut für Extraterrestrische Physik, Gießenbachstraße 1, 85748 Garching bei München, Germany

Received: 14 January 2022
Accepted: 12 April 2022

Abstract

We report the discovery of ancient massive merger events in the early-type galaxies NGC 1380 and NGC 1427, members of the Fornax galaxy cluster. Both galaxies have been observed by the MUSE integral-field-unit instrument on the VLT as part of the Fornax3D project. By fitting recently developed population-orbital superposition models to the observed surface brightness, stellar kinematic, age, and metallicity maps, we obtain the stellar orbits, age, and metallicity distributions of each galaxy. We then decompose each galaxy into multiple orbital-based components, including a dynamically hot inner stellar halo component that is identified as the relic of past massive mergers. By comparing to analogs from cosmological galaxy simulations, chiefly TNG50, we find that the formation of such a hot inner stellar halo requires the merger with a now-destroyed massive satellite galaxy of $3.7_{-1.5}^{+2.7}\times {10}^{10}$ M_⊙ (about one-fifth of its current stellar mass) in the case of NGC 1380 and of $1.5_{-0.7}^{+1.6}\times {10}^{10}$ M_⊙ (about one-fourth of its current stellar mass) in the case of NGC 1427. Moreover, we infer that the last massive merger in NGC 1380 happened ∼10 Gyr ago based on the stellar age distribution of the regrown dynamically cold disk, whereas the merger in NGC 1427 ended t ≲ 8 Gyr ago based on the stellar populations in its hot inner stellar halo. The major merger event in NGC 1380 is the first one with both merger mass and merger time quantitatively inferred in a galaxy beyond the local volume. Moreover, it is the oldest and most massive merger uncovered in nearby galaxies so far.