Understanding entropy in massive halos: The role of baryon decoupling

¹ INAF – IASF Milano, Via A. Corti 12, I-20133 Milano, Italy
² Dipartimento di Scienza e Alta Tecnologia, Università dell’Insubria, Via Valleggio 11, I-22100 Como, Italy
³ DIFA – Università di Bologna, Via Gobetti 93/2, I-40129 Bologna, Italy
⁴ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, I-23807 Merate (LC), Italy
⁵ Department of Physics, Informatics and Mathematics, University of Modena and Reggio Emilia, I-41125 Modena, Italy
⁶ Dipartimento di Fisica, Università degli Studi di Milano, Via G. Celoria 16, I-20133 Milano, Italy
⁷ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi, I-50122 Firenze, Italy

^⋆ Corresponding author: silvano.molendi@inaf.it

Received: 4 December 2024
Accepted: 13 February 2025

Abstract

Aims. The goal of the work presented in this paper is to use observed entropy profiles to infer constraints on the accretion process in massive halos.

Methods. We compare entropy profiles from various observational samples with those generated by an updated version of the semi-analytical models developed in the early 2000s, that have been modified to reflect recent advancements in our understanding of large-structure formation.

Results. Our model reproduces the growing departure from self-similarity observed in data when moving inward in individual profiles and down in mass across different profiles. These deviations stem from a phase of extremely low gas content centered around 10¹³ M_⊙. According to our model, halos at this mass scale are missing between 50% and 90% of their baryons, corresponding to a gas fraction ranging between 2% and 8%.

Conclusions. Baryon decoupling, the mechanism at the heart of our model, proves effective in explaining much of the behavior we sought to understand.