HUBS: A dedicated hot circumgalactic medium explorer

TL;DR

HUBS is a proposed X-ray observatory designed to detect and characterize the elusive hot baryons in the circum-galactic medium, aiming to solve the missing baryon problem and enhance understanding of galaxy evolution.

Contribution

This paper introduces the design and technological development status of HUBS, a dedicated mission to observe the hot circum-galactic medium and locate missing baryons.

Findings

HUBS's design optimizes detection of soft X-ray emissions from hot baryons.

The mission aims to characterize physical and chemical properties of circum-galactic baryons.

Potential to observe missing baryons in galaxy cluster outskirts and inform galaxy evolution models.

Abstract

The Hot Universe Baryon Surveyor (HUBS) mission is proposed to study "missing" baryons in the universe. Unlike dark matter, baryonic matter is made of elements in the periodic table, and can be directly observed through the electromagnetic signals that it produces. Stars contain only a tiny fraction of the baryonic matter known to be present in the universe. Additional baryons are found to be in diffuse (gaseous) form, in or between galaxies, but a significant fraction has not yet been seen. The latter (missing baryons) are thought to be hiding in low-density warm-hot ionized medium (WHIM), based on results from theoretical studies and recent observations, and be distributed in the vicinity of galaxies (i.e., circum-galactic medium) and between galaxies (i.e., intergalactic medium). Such gas would radiate mainly in the soft X-ray band and the emission would be very weak, due to its very low density. HUBS is optimized to detect the X-ray emission from the hot baryons in the circum-galactic medium, and thus fill a void in observational astronomy. The goal is not only to detect the missing baryons, but to characterize their physical and chemical properties, as well as to measure their spatial distribution. The results would establish the boundary conditions for understanding galaxy evolution. Though highly challenging, detecting missing baryons in the intergalactic medium could be attempted, perhaps in the outskirts of galaxy clusters, and could shed significant light on the large-scale structures of the universe. The current design of HUBS will be presented, along with the status of technology development.