Olimpo: A Balloon-Borne Instrument to Probe Dynamic Structure Formation and the WHIM

We describe OLIMPO, a proposed balloon-borne instrument to map 10 galaxy clusters and 4 emission bridges using the thermal and kinematic Sunyaev-Zel'dovich effects. OLIMPO will have a 2.6 m telescope, four frequency bands between 150 and 460 GHz, and a kilopixel array of kinetic inductance detectors. OLIMPO's measurements will for the first time directly probe internal intracluster medium gas motions from the cluster's center to its outskirts at a radius of 1.5 Mpc, giving unprecedented views of the dynamics of cluster formation. In addition, OLIMPO's mapping of emission bridges will transform the search for the missing baryons thought to reside in the filaments connecting clusters in the form of a warm hot intergalactic medium. The OLIMPO data will be complemented by state-of-the-art X-ray data from the eROSITA and XRISM satellites, radio data from ASKAP and MeerKAT, and from cosmological simulations. The science goals require X-ray data with high sensitivity to the diffuse gas in cluster outskirts and filaments, which are almost exclusively available only for low z, degree-scale objects best observed from above the atmosphere. With a single flight in late 2026, OLIMPO is expected to provide maps at least 17 times more sensitive to low z clusters than the 2030's survey by CMB-S4.