The MUSE eXtremely Deep Field: Detections of circumgalactic Si II* emission at z ≳ 2

Context. The circumgalactic medium (CGM) serves as a baryon reservoir that connects galaxies to the intergalactic medium and fuels star formation. The spatial distribution of the metal-enriched cool CGM has not yet been directly revealed at cosmic noon (z ≃ 2–4), as bright emission lines at these redshifts are not covered by optical integral field units. Aims. To remedy this situation, we performed the first-ever detections and exploration of extended Si II* emission in the low-ionization state (LIS), referred to as Si II* halos, at redshifts ranging from z = 2 to 4 as a way to trace the metal-enriched cool CGM. Methods. We used a sample of 39 galaxies with systemic redshifts of z = 2.1–3.9 measured with the [C III] doublet in the MUSE Hubble Ultra Deep Field catalog, whose integration times span from ≃30 to 140 hours. We searched for extended Si II* λ1265, 1309, 1533 emission (fluorescent lines) around individual galaxies. We also stacked a subsample of 14 UV-bright galaxies. Results. We report five individual detections of Si II* λ1533 halos. We also confirm the presence of Si II* λ1533 halos in stacks for the subsample containing UV-bright sources. The other lines do not show secure detections of extended emission in individual or in stacking analyses. These detections may imply that the presence of metal-enriched CGM is a common characteristic for UV-bright galaxies. To investigate whether the origin of Si II* is continuum pumping, as suggested in previous studies, we checked the consistency of the equivalent width (EW) of Si II* emission and the EW of Si II absorption for the individual halo object with the most reliable detection. We confirm the equivalence, suggesting that photon conservation works for this object and points toward continuum pumping as the source of Si II*. We also investigated Si II* lines in a RAMSES-RT zoom-in simulation including continuum pumping, and find the ubiquitous presence of extended halos. ⋆ Based on observations made with ESO telescope at the La Silla Paranal Observatory under the large program 1101.A-0127.