Detecting Missing Baryons in the Universe

Picture Credit: Illustris Collaboration



The goals of this ISSI-BJ forum are to discuss the problems of missing baryons on scales of galaxies and large-scale structures and to make plans for X-ray missions that are dedicated to solving these problems. The missing baryons are deemed to constitute a major component of the circumgalactic medium (CGM) and intergalactic medium (IGM), but they are difficult to observe due to the limitations of the current instrumentation, and thus, they represent a key obstacle to our understanding of the galaxy evolution. During this activity, the forum participants will discuss the results yielded from cosmological hydrodynamical simulations on the properties of the hot CGM and IGM, the observational evidence for their existence, and the implications on galaxy evolution. Moreover, it will be discussed how to fill the observational gap of directly detecting the missing baryons and thus measuring their properties and distributions. Since they are postulated to exist in the form of low-density, hot CGM/IGM and to radiate mainly in soft X-rays, the discussions will center on effective X-ray means. To this end, a number of proposed mission concepts will be presented. Consensus will be sought on an optimal strategy to collectively push for a high-resolution X-ray spectroscopic mission and to sustain the momentum over the long run. Besides the subject of missing baryons, the participants will also discuss some important scientific issues related to the topic that can benefit from high-resolution X-ray spectroscopic observations, including diffuse X-ray background, supernova remnants, and cluster outskirts, as well as techniques of modeling the high-resolution X-ray spectra obtained.


  • Census of cosmic baryons 
  • Theoretical understanding of missing baryons 
  • Recent observations of WHIM and implications 
  • High resolution X-ray spectroscopic observations and modeling 
  • Synergies and collaborations

Date: 20-21 July 2023


Bregman Joel  University of Michigan, USA
Cui Wei Tsinghua University, China
Falanga Maurizio  ISSI-BJ, China
Fang Taotao  Xiamen University, China
McCammon Dan  University of Wisconsin-Madison, USA
Mitsuda Kazuhisa ISAS/JAXA, Japan
Simionescu Aurora SRON, The Netherlands
Yamasaki Noriko ISAS/JAXA, Japan
Yuan Feng Shanghai Astronomical Observatory, CAS, China
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