XLSSC 122 caught in the act of growing up

Spatially resolved SZ observations of a z  = 1.98 galaxy cluster

¹ European Southern Observatory (ESO), Karl-Schwarzschild-Strasse 2, Garching 85748, Germany
² Astronomy Unit, Department of Physics, University of Trieste, Via Tiepolo 11, Trieste 34131, Italy
³ INAF – Osservatorio Astronomico di Trieste, Via Tiepolo 11, Trieste 34131, Italy
⁴ IFPU – Institute for Fundamental Physics of the Universe, Via Beirut 2, 34014 Trieste, Italy
⁵ David A. Dunlap Department of Astronomy & Astrophysics, University of Toronto, 50 St. George St, Toronto, ON M5S 3H4, Canada
⁶ Department of Aeronautics and Astronautics, Massachusetts Institute of Technology, 77 Massachusetts Avenue, 33-207 Cambridge, MA 02139, USA
⁷ Department of Astronomy, Cornell University, Ithaca, NY 14853, USA
⁸ Sapienza – University of Rome – Physics department, Piazzale Aldo Moro 5, 00185 Rome, Italy
⁹ Canadian Institute for Theoretical Astrophysics, University of Toronto, 60 St. George St., Toronto, ON M5S 3H8, Canada
¹⁰ Department of Physics and Astronomy, University of Pennsylvania, 209 South 33rd Street, Philadelphia, PA 19104, USA
¹¹ Department of Physics and Astronomy, Rutgers, The State University of New Jersey, Piscataway, NJ 08854-8019, USA
¹² Joseph Henry Laboratories of Physics, Jadwin Hall, Princeton University, Princeton, NJ 08544, USA
¹³ Department of Astrophysical Sciences, Peyton Hall, Princeton University, Princeton, NJ 08544, USA
¹⁴ Centre for Radio Astronomy Techniques and Technologies, Department of Physics and Electronics, Rhodes University, PO Box 94 Makhanda 6140, South Africa
¹⁵ South African Radio Astronomy Observatory, 2 Fir Street, Observatory 7925, South Africa
¹⁶ Wits Centre for Astrophysics, School of Physics, University of the Witwatersrand, Private Bag 3, 2050 Johannesburg, South Africa
¹⁷ Astrophysics Research Centre and the School of Mathematics, Statistics and Computer Science, University of KwaZulu-Natal, Durban 4001, South Africa
¹⁸ Institute of Theoretical Astrophysics, University of Oslo, Oslo, Norway
¹⁹ Department of Physics and Astronomy, Haverford College, Haverford, PA 19041, USA
²⁰ Instituto de Física, Pontificia Universidad Católica de Valparaíso, Casilla 4059, Valparaíso, Chile
²¹ NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA

Received: 9 October 2023
Accepted: 4 June 2024

Abstract

Context. How protoclusters evolved from sparse galaxy overdensities to mature galaxy clusters is still not well understood. In this context, detecting and characterizing the hot intracluster medium (ICM) at high redshifts (z ∼ 2) is key to understanding how the continuous accretion from the filamentary large-scale structure and the mergers along it impact the first phases of cluster formation.

Aims. We study the dynamical state and morphology of the z = 1.98 galaxy cluster XLSSC 122 with high-resolution observations (≈5″) of the ICM through the Sunyaev–Zeldovich (SZ) effect. XLSSC 122 is the highest redshift optically confirmed galaxy cluster found in an unbiased, widefield survey.

Methods. Via Bayesian forward modeling, we mapped the ICM on scales from the virial radius down to the core of the cluster. To constrain such a broad range of spatial scales, we employed a new technique that jointly forward-models parametric descriptions of the pressure distribution to interferometric ACA and ALMA observations and multiband imaging data from ACT.

Results. We detect the SZ effect with 11σ significance in the ALMA+ACA observations and find a flattened inner pressure profile that is consistent with a noncool core classification with a significance of ≥3σ. In contrast to the previous works, we find better agreement between the SZ effect signal and the X-ray emission as well as the cluster member distribution. Further, XLSSC 122 exhibits an excess of SZ flux in the south of the cluster where no X-ray emission is detected. By reconstructing the interferometric observations and modeling in the uv-plane, we obtain a tentative detection of an infalling group or filamentary-like structure in the southeast that is believed to boost and heat up the ICM while the density of the gas is still low. In addition to characterizing the dynamical state of the cluster, we provide an improved SZ mass estimate M_500,c = 1.66_−0.20^+0.23 × 10¹⁴ M_⊙.

Conclusions. Altogether, the observations indicate that we see XLSSC 122 in a dynamic phase of cluster formation while a large reservoir of gas is already thermalized.