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

TL;DR

The paper reports the discovery of two Lyα nebula satellite halos RO-1001-Sat and RO-0959-Sat around two giant LANs at z ≈ 2.92 and 3.09, analyzed with panoramic MUSE data to map Lyα emission, kinematics, and satellite content. It estimates satellite halo masses of log$(M_{\mathrm{h}}/M_\odot)\approx13.2\pm0.3$ and $12.8\pm0.3$, with central Lyman-α luminosities log$(L_{\mathrm{Ly}\alpha}/\mathrm{erg\,s^{-1}})\approx43.4$ and $42.9$, and finds very small velocity offsets (≈$100-300\ \mathrm{km\,s^{-1}}$) between each main–satellite pair, hinting at filamentary connections within the cosmic web. The study identifies a quiescent BGG in RO-1001-Sat and a dusty star-forming BGG in RO-0959-Sat, illustrating contrasting quenching histories in two similarly massive halos, with radio-mode AGN feedback posited as a plausible quenching mechanism in the former. A tentative Lyα filament between RO-1001 and RO-1001-Sat is reported, consistent in SB with predictions from cosmological simulations, underscoring Lyα emission as a tracer of large-scale structure at high redshift. Comparisons with MultiDark and TNG300 simulations show that such satellite halos and filaments are broadly expected in the early universe, supporting the view that LANs efficiently trace massive halos and their assembly within the cosmic web, with future deep spectroscopic and ALMA follow-ups to connect gas reservoirs to halo growth.

Abstract

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. They are found neighbouring two out of four known giant LANs at $z\sim3$ in our MUSE follow-up observations, reinforcing the idea that Ly$α$ 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 $\simeq80\times160$ and $80\times100~\mathrm{pkpc}^2$, the two nebulae are both $\sim$300pkpc from the main LANs. The Ly$α$ emission is only shifted by $\simeq100-300$ 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$(M_\mathrm{h}/M_\odot)\simeq13.2\pm0.3$ and $12.8\pm0.3$, 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 $\sim10^{11}M_\odot$, 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$α$ 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.

Figures (16)

• Figure 1: Colour image of RO-1001 and RO-1001-Sat from F444W, F277W and F115W bands of JWST NIRCam. The cyan box shows the $1.1\arcmin\times 1.2\arcmin$ FOV of the MUSE mosaic. The orange circles show the virial radii of the two halos with the errors illustrated by the pink segment on each circle, being $197\pm45$ and $269\pm41$ kpc for RO-1001-Sat and RO-1001. The contours show the Ly$\alpha$ SB distributions in both nebulae displayed in steps of log$\rm (SB_{Ly\alpha}/\mathrm{erg~s}^{-1}\mathrm{cm}^{-2}\mathrm{arcsec}^{-2})$ = (-18.5, -18.0, -17.5, -17.0). The lowest contour level corresponds to S/N=5. Candidate member galaxies of the satellite halo in Table \ref{['tab:galaxies_ro1001']} are marked by green circles. The central brightest galaxy (log$(M_*/M_\odot)\simeq11.3$) is highlighted by the pink circle. The radii of the green circles are proportional to the stellar masses of the galaxies, ranging from log$(M_*/M_\odot)\simeq9.5-10.3$.
• Figure 2: Colour image of RO-0959 and RO-0959-Sat from F444W, F277W and F115W bands of JWST NIRCam. The cyan box shows the $1.1\arcmin\times 1.1\arcmin$ FOV of the MUSE mosaic. The orange circles show the virial radii of the two halos with the errors illustrated by the pink segment on each circle, being $139\pm32$ kpc for both RO-0959-Sat and RO-0959. The contours show the Ly$\alpha$ SB distributions in both nebulae displayed in steps of log$\rm (SB_{Ly\alpha}/\mathrm{erg~s}^{-1}\mathrm{cm}^{-2}arcsec^{-2})$ = (-18.5, -18.0, -17.5, -17.0). The lowest contour level corresponds to S/N=3. The central and the second candidate member galaxies in Table \ref{['tab:galaxies_RO0959']} with log$(M_*/M_\odot)\simeq11.2$ and 9.9 are highlighted by the pink and green circles.
• Figure 3: Ly$\alpha$ spectra integrated from the main and satellite nebulae of RO-1001 (left) and RO-0959 (right) after noise re-normalisation. Dashed red lines mark the flux-weighted peak of each spectrum. The dashed green line in the lower right panel shows the expected position of the Ly$\alpha$ peak from the CO(3-2) emission in RO-0959-Sat.
• Figure 4: Ly$\alpha$ SB (top), velocity (middle) and velocity dispersion (bottom) maps of RO-1001-Sat. The velocity map is referenced to the flux-weighted Ly$\alpha$ peak as the zero point. The contours in each map show the Ly$\alpha$ SB of the same levels as in Figure \ref{['fig:rgb_ro1001']}. The dashed ellipse at the centre of each map shows the location and approximate shape of the BGG as seen in Figure \ref{['fig:rgb_ro1001']}.
• Figure 5: Ly$\alpha$ SB (top left), velocity (top right), and velocity dispersion (bottom) maps of RO-0959-Sat. The velocity map is referenced to the flux-weighted Ly$\alpha$ peak as the zero point. The contours in each map show the Ly$\alpha$ SB of the same levels as in Fig. \ref{['fig:rgb_ro0959']}. The dashed ellipse at the centre of each map shows the location and approximate shape of the BGG as seen in Fig. \ref{['fig:rgb_ro0959']}.