Ly-alpha emission reveals two satellite halos around massive groups at z ∼ 3: The puzzling case of a quiescent central galaxy

¹ CEA, IRFU, DAp, AIM, Université Paris-Saclay, Université de Paris, Sorbonne Paris Cité, CNRS, F-91191 Gif-sur-Yvette, France
² Instituto de Física, Pontificia Universidad Católica de Valparaíso, Casilla 4059 Valparaíso, Chile
³ Cosmic Dawn Center (DAWN), Denmark
⁴ DTU Space, Technical University of Denmark, Elektrovej 327, DK-2800 Kgs. Lyngby, Denmark
⁵ Department of Physics and Astronomy, UCLA PAB 430 Portola Plaza Los Angeles, CA 90095-1547, USA
⁶ Instituto de Estudios Astrofícos, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Av. Ejército 441, Santiago, Chile
⁷ Millenium Nucleus for Galaxies (MINGAL), Santiago, Chile
⁸ Leiden Observatory, Leiden University, PO Box 9513 2300 RA Leiden, The Netherlands
⁹ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, I-40129 Bologna, Italy
¹⁰ Université Paris Cité, CNRS(/IN2P3), Astroparticule et Cosmologie, F-75013 Paris, France
¹¹ INAF- Osservatorio di Astrofisica e Scienza dello Spazio, Via Gobetti 93/3, I-40129 Bologna, Italy
¹² Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, People’s Republic of China
¹³ Kavli Institute for the Physics and Mathematics of the Universe, The University of Tokyo, Kashiwa 277-8583, Japan
¹⁴ California Institute of Technology, 1200 East California Boulevard, MC 278-17, Pasadena, CA 91125, USA
¹⁵ European Southern Observatory, Karl-Schwarzschild-Str. 2, D85748 Garching bei Munchen, Germany
¹⁶ School of Astronomy and Space Science, Nanjing University, Nanjing 210093, China
¹⁷ Key Laboratory of Modern Astronomy and Astrophysics (Nanjing University), Ministry of Education, Nanjing 210093, China

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Received: 3 June 2025
Accepted: 23 October 2025

Abstract

We present the discovery and characterisation of two Lyα nebulae (LANs), RO-1001-Sat and RO-0959-Sat, as satellite structures of two giant LANs at z = 2.920 and 3.092. These two satellite LANs are found neighbouring two out of four known giant LANs at z ∼ 3 in our Multi-Unit Spectroscopic Explorer (MUSE) follow-up observations, reinforcing the idea that Lyα emission can be used as a tracer of massive dark matter halos at high z. This high occurrence of massive satellite halos is in agreement with simulations. With sizes of ≃80 × 160 and 80 × 100 pkpc², the two nebulae are both ∼300 pkpc from the main LANs. With Lyα emission only shifted by ≃100 − 300 km s⁻¹ between each of the two pairs, these two satellite structures are likely connected to their main halos by the large-scale structure. RO-1001-Sat and RO-0959-Sat are estimated to have log(M_h/M_⊙)≃13.2 ± 0.3 and 12.8 ± 0.3, putting them potentially close to the regime of cold-mode accretion according to several models, which suggests that cold streams should be able to penetrate the virial radii to reach the halo centres without being shock-heated. The central brightest galaxies in the two halos are morphologically distinct despite having a similar stellar mass of ∼10¹¹ M_⊙, one being an elliptical quiescent galaxy in RO-1001-Sat and the other being a dusty star-forming spiral in RO-0959-Sat. Intriguingly, the quiescent galaxy aligns well with the peak of the LAN as well as the potential well of the host halo, making it the first clear-cut case in which the cold gas ought to be accreting onto the galaxy but with no observable star formation, either due to morphological quenching or, more likely, radio-mode feedback from an active galactic nucleus, as supported by excess yet weak radio emission. Finally, we show a tentative detection of a Lyα filament connecting RO-1001 and RO-1001-Sat. This work shows how panoramic MUSE (and in the future, BlueMUSE) observations of massive halo seeds can be used to efficiently search for additional halos, unveiling their large-scale structure and enabling the study of Lyα-selected galaxy groups.