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

¹ Observatoire de Genève, Université de Genève, 51 Chemin de Pégase, 1290 Versoix, Switzerland
² National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
³ Kapteyn Astronomical Institute, University of Groningen, PO Box 800 9700 AV Groningen, The Netherlands
⁴ Univ. Lyon, Univ. Lyon1, Ens de Lyon, CNRS, Centre de Recherche Astrophysique de Lyon UMR5574, F-69230 Saint-Genis-Laval, France
⁵ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
⁶ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
⁷ Department of Astronomy, The University of Texas at Austin, 2515 Speedway, Stop C1400, Austin, TX 78712-1205, USA
⁸ The Oskar Klein Centre, Department of Astronomy, Stockholm University, AlbaNova, SE-10691 Stockholm, Sweden
⁹ Institut de Recherche en Astrophysique et Plan, Toulouse, 14 Avenue E. Belin 31400, France
¹⁰ Inter-University Centre for Astronomy and Astrophysics, Ganeshkind, Post Bag 4, Pune 41007, India
¹¹ Department of Astronomy, University of Wisconsin-Madison, 475 N. Charter St., Madison, WI 53706, USA
¹² Aix Marseille Univ, CNRS, CNES, LAM, Marseille, France
¹³ Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Hawthorn, Victoria 3122, Australia
¹⁴ Institute for Cosmic Ray Research, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8582, Japan
¹⁵ Kavli Institute for the Physics and Mathematics of the Universe (WPI), University of Tokyo, Kashiwa, Chiba 277-8583, Japan
¹⁶ Department of Astronomical Science, SOKENDAI (The Graduate University for Advanced Studies), Osawa 2-21-1, Mitaka, Tokyo 181-8588, Japan
¹⁷ Leiden Observatory, Leiden University, PO Box 9513 NL-2300 RA Leiden, The Netherlands

^⋆⋆ Corresponding author; haruka.kusakabe.takeishi@gmail.com, haruka.kusakabe@nao.ac.jp

Received: 6 June 2024
Accepted: 17 September 2024

Abstract

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.