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

We present the discovery and characterisation of two Ly α 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 Ly α 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 pkpc 2 , the two nebulae are both ∼300 pkpc from the main LANs. With Ly α 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 ∼10 11 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 Ly α 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 Ly α -selected galaxy groups.