A z = 1.85 galaxy group in CEERS: Evolved, dustless, massive intra-halo light and a brightest group galaxy in the making

¹ CEA, IRFU, DAp, AIM, Université Paris-Saclay, Université Paris Cité, CNRS, 91191 Gif-sur-Yvette, France
e-mail: rcoogan.astrophysics@gmail.com
² NSF’s National Optical-Infrared Astronomy Research Laboratory, 950 N. Cherry Ave., Tucson, AZ 85719, USA
³ University of Massachusetts Amherst, 710 North Pleasant Street, Amherst, MA 01003-9305, USA
⁴ Department of Physics and Astronomy, University of California, 900 University Ave, Riverside, CA 92521, USA
⁵ Department of Astronomy, The University of Texas at Austin, Austin, TX 78712, USA
⁶ Department of Physics & Astronomy, University of California, Irvine, 4129 Reines Hall, Irvine, CA 92697, USA
⁷ Department of Astronomy and Steward Observatory, University of Arizona, Tucson, AZ 85721, USA
⁸ Division of Science, National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
⁹ Dipartimento di Fisica e Astronomia “G.Galilei”, Universitá di Padova, Via Marzolo 8, 35131 Padova, Italy
¹⁰ INAF–Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
¹¹ Aix-Marseille Univ., CNRS, CNES, LAM, 13388 Marseille Cedex 13, France
¹² INAF – Osservatorio Astronomico di Roma, Via di Frascati 33, 00078 Monte Porzio Catone, Italy
¹³ Department of Physics, University of Helsinki, Gustaf Hällströmin katu 2, 00014 Helsinki, Finland
¹⁴ Space Telescope Science Institute, 3700 San Martin Dr., Baltimore, MD 21218, USA
¹⁵ Centro de Astrobiología (CAB), CSIC-INTA, Ctra. de Ajalvir km 4, Torrejón de Ardoz, 28850 Madrid, Spain
¹⁶ Astrophysics Science Division, NASA Goddard Space Flight Center, 8800 Greenbelt Rd, Greenbelt, MD 20771, USA
¹⁷ Laboratory for Multiwavelength Astrophysics, School of Physics and Astronomy, Rochester Institute of Technology, 84 Lomb Memorial Drive, Rochester, NY 14623, USA
¹⁸ Department of Physics and Astronomy, Texas A&M University, College Station, TX 77843-4242, USA
¹⁹ Space Telescope Science Institute, Baltimore MD 21210, USA
²⁰ Astronomy Centre, University of Sussex, Falmer, Brighton BN1 9QH, UK
²¹ Institute of Space Sciences and Astronomy, University of Malta, Msida MSD 2080, Malta

Received: 15 February 2023
Accepted: 15 May 2023

Abstract

We present CEERS JWST/NIRCam imaging of a massive galaxy group at z = 1.85, to explore the early JWST view on massive group formation in the distant Universe. The group contains ≳16 members (including six spectroscopic confirmations) down to log₁₀(M_⋆/M_⊙) = 8.5, including the brightest group galaxy (BGG) in the process of actively assembling at this redshift. The BGG is comprised of multiple merging components extending ∼3.6″ (30 kpc) across the sky. The BGG contributes 69% of the group’s total galactic stellar mass, with one of the merging components containing 76% of the total mass of the BGG and a star formation rate > 1810 M_⊙ yr⁻¹. Most importantly, we detected intra-halo light (IHL) in several HST and JWST/NIRCam bands, allowing us to construct a state-of-the-art rest-frame UV-NIR spectral energy distribution of the IHL for the first time at this high redshift. This allows stellar population characterisation of both the IHL and member galaxies, as well as the morphology distribution of group galaxies versus their star formation activity when coupled with Herschel data. We created a stacked image of the IHL, giving us a sensitivity to extended emission of 28.5 mag arcsec⁻² at rest-frame 1 μm. We find that the IHL is extremely dust poor (A_v ∼ 0), containing an evolved stellar population of log₁₀(t₅₀/yr) = 8.8, corresponding to a formation epoch for 50% of the stellar material 0.63 Gyr before z = 1.85. There is no evidence of ongoing star formation in the IHL. The IHL in this group at z = 1.85 contributes ∼10% of the total stellar mass, comparable with what is observed in local clusters. This suggests that the evolution of the IHL fraction is more self-similar with redshift than predicted by some models, challenging our understanding of IHL formation during the assembly of high-redshift clusters. JWST is unveiling a new side of group formation at this redshift, which will evolve into Virgo-like structures in the local Universe.