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

Abstract

We present the discovery and characterisation of two Ly$\hat{I}\pm$ nebulae (LANs), RO-1001-Sat and RO-0959-Sat, as satellite structures of two giant LANs at $z=2.920$ and 3.092. They are found neighbouring two out of four known giant LANs at $z\sim 3$ in our MUSE follow-up observations, reinforcing the idea that Ly$\hat{I}\pm$ emission can be used to trace massive dark matter halos at high-$z$. This high occurrence of massive satellite halos agrees with simulations. With sizes of $\simeq 80\times 160$ and $80\times 100{\mathrm{pkpc}}^2$, the two nebulae are both $\sim$300pkpc from the main LANs. The Ly$\hat{I}\pm$ emission is only shifted by $\simeq 100-300$ km s${}^{-1}$ between each of the two pairs, suggesting connections via large-scale structure. RO-1001-Sat and RO-0959-Sat are estimated to have log$(M_{\mathrm{h}}/M_{\odot })\simeq 13.2\pm 0.3$ and $12.8\pm 0.3$, putting them potentially close to the regime of cold-mode accretion. The central brightest galaxies in the two halos are morphologically distinct despite having similar stellar mass $\sim 10^{11}{M}_{\odot }$, 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 where 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. Finally, we show a tentative detection of a Ly$\hat{I}\pm$ 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$\hat{I}\pm$-selected galaxy groups.

19 pages, 16 figures, A&A in press