The ACT bridge census: A search for hot gas between massive galaxy clusters with pair stacking

¹ Dipartimento di Fisica, Sapienza Università di Roma, Piazzale Aldo Moro, 5, 00185, Rome ( RM), Italy
² INFN Sezione Roma1, Piazzale Aldo Moro, 2, 00185, Rome ( RM), Italy
³ INAF OAC, Via della Scienza, 5, 09047, Selargius (CA), Italy

^* e-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.

Published online: 20 February 2026

Abstract

Baryonic matter only accounts for 5% of the mass-energy density of our universe, where the other 95% is shared between dark energy and dark matter. Measurements of the early universe predict a certain baryon fraction, that is not detected in the later stages of the cosmic evolution, and this discrepancy between predicted and observed baryons is known as the “missing baryon” problem. Cosmological simulations predict that a significant fraction of the missing baryons could be found in the form of the warm–hot intergalactic medium (WHIM) trapped inside large scale structures that constitute the cosmic web. WHIM can be detected using the thermal Sunyaev-Zel’dovich effect, as the amplitude of its spectral distortion scales linearly with the electron pressure integrated along the line of sight. This effect has been extensively used by space and ground based CMB experiments to study clusters and filament candidates. In this work, we present results on the detection of hot gas between pairs of candidate interacting galaxy clusters obtained with the latest high resolution (1.65′), high sensitivity Compton-y maps from ACT. We consider a set of candidate double cluster systems extracted from a preliminary ACT-DR6 catalog of clusters blindly detected with a matched filter approach. We then aligned and stacked these cluster pairs in order to increase the SNR and detect the faint SZ signal due to warm-hot filaments of gas connecting them. This sample focuses on short filaments with a projected length ℓ_fil < 10 Mpc and typical halo mass M₅₀₀ ~ 2 × 10¹⁴M_⊙. Additionally, we study individual pairs of clusters to identify promising candidates for follow-up observations using higher (10 − 20′′) resolution millimeter cameras or X-ray satellites.