Identification of low-redshift groups and clusters of galaxies in the X-CLASS survey and the X-ray luminosity-temperature relation^★

¹ IRAP, Université de Toulouse, CNRS, CNES, 14 Avenue Belin, 31400 Toulouse, France
² LAM, Aix-Marseille Université, CNRS, CNES, 38, rue Frédéric Joliot-Curie, 13388 Marseille Cedex 13, France
³ ESAC/ESA, Camino Bajo del Castillo, s/n., Urb. Villafranca del Castillo, 28692, Villanueva de la Cañada, Madrid, Spain
⁴ Institute for Astronomy & Astrophysics, National Observatory of Athens, 15236 Palaia Penteli, Greece
⁵ Univ. Grenoble Alpes, CNRS, Grenoble INP, LPSC-IN2P3, 53, Avenue des Martyrs, 38000 Grenoble, France
⁶ Telespazio UK for ESA, European Space Astronomy Centre, Operations Department, E-28691 Villanueva de la Cañada, Spain
⁷ Ulugh Beg Astronomical Institute of Uzbekistan Academy of Sciences, 33 Astronomicheskaya Str., Tashkent 100052, Uzbekistan
⁸ Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, 91191 Gif-sur-Yvette, France
⁹ Leiden Observatory, Leiden University, P.O. Box 9513, 2300 RA, Leiden, The Netherlands
¹⁰ National Research Institute of Astronomy and Geophysics (NRIAG), 11421 Helwan, Egypt
¹¹ Sorbonne Université, CNRS, Institut d’Astrophysique de Paris, 98 bis boulevard Arago, 75014 Paris, France

^★★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 18 December 2025
Accepted: 20 March 2026

Abstract

Context. Properties of the hot intracluster and intragroup medium are mostly set by the underlying gravitational potential well, although complex astrophysical processes at play during their buildup may leave a significant imprint. Observational constraints on the degree and scales of such nongravitational processes require well-selected samples of objects and deep observations of their gas content.

Aims. We aim to study the scaling relation between two global properties of the hot gas, namely its soft-band X-ray luminosity (L_X) and its temperature (T), by studying a sample of low-mass systems associated with precise redshifts, simultaneously accounting for sample selection biases and associated measurement uncertainties.

Methods. This work takes as input a large catalog of X-ray-selected galaxy clusters (X-CLASS). We performed a thorough revision of the redshifts of sources using deep photometric data from the Legacy Surveys and our own tailored spectroscopic follow-up of 52 low-redshift systems. We devised a spectroscopically complete sample of 155 low-redshift (0.07 < z < 0.2) systems, and we measured properties of their X-ray emitting gas, with median ¯T = 1.7 keV and median ¯L_X = 10⁴³ erg s⁻¹. We inferred the relation between L_X and T in a Bayesian framework.

Results. Our sample of groups and clusters with a median total mass of ∼6 × 10¹³ M_⊙ reveals a relation L_X − T steeper than that predicted by the self-similar model, with a slope of B = 3.2 ± 0.1. This result fits well within recent studies that together indicate a trend of an increasing slope with decreasing median halo mass.

Conclusions. This work supports a scenario of a stronger decrease in luminosity with decreasing mass in the group regime than for massive galaxy clusters. This effect is possibly due to strong and sustained feedback expelling gas efficiently from their shallower potential wells. We release the list of updated redshifts (photometric and spectroscopic) for the full X-CLASS sample and the gas properties of the low-redshift sample. The cluster photometric redshift code presented in the paper photXclus is made publicly available (https://codeberg.org/fsarron/photXclus).