According to current models, baryonic matter makes up about 5% of all matter in the Universe. This matter composed of "ordinary" particles should in principle be detectable either in emission or in absorption against background sources. In fact, whatever observations and methods are used to derive the mass of detected baryonic matter, galaxies only account for half of the expected quantity.
It is therefore assumed a large part of the mass fraction of this baryonic matter in the Universe resides outside galaxies, probably in the form of a diffuse and hot (on the order of 100,000 K) medium called the Warm-Hot Intergalactic Medium (WHIM). Simulations suggest the WHIM is composed of large-scale structures such as filaments.
Comparison of an image of the emerging galaxy SSA22 obtained with the Cosmic Web Imager instrument at Mount Palomar Observatory with a simulation. Three gas filaments can be seen flowing into the Lyman alpha blob (arrows). Credit: Christopher Martin, Robert Hurt
Moreover, this intergalactic medium interacts with galaxies as matter flows into and feeds them along these filaments, while conversely galaxies emit jets and winds generated notably by their central black hole and exploding supernovae. It is suspected these interactions play a key role in galaxy evolution. In particular, they are thought to regulate the rate of star formation, which could explain why it has slowed unexpectedly in the current epoch. Observing and characterizing the circumgalactic medium—in terms of velocities, temperatures, and so on—are therefore priority objectives for astronomers.
Some rare cases of OV, OVI and OVII ions detected in absorption against distant quasars have been extracted from observations by HST/COS and Chandra. However, it is not certain they belong to the WHIM and searching for baryons in absorption is limited by the number of usable quasars.
Fireball is designed to look for baryon emission in Lyman alpha and OVI spectral lines in low-redshift circumgalactic regions. It will acquire spectral imagery in the 200-nanometre window accessible from the stratosphere of regions selected on the basis of intensive simulations (ANR !BINGO).
The experiment is also intended to validate the concept and technologies (notably the detector) for a more-ambitious dedicated space mission previously proposed to NASA (ISTOS, MIGAL) and likely to be resubmitted in response to the agency’s next request for proposals.