Missing Baryons

Simulated Colibrì and XMM-Newton Observations of the WHIM Absorption along the Line of Sight to a Bright Blazar

Ultimately the feedback from supernovae and AGN not only affects the star formation of the host galaxy but also energises the surrounding gas and pollutes it with material from the galaxy itself. Bright quasars illuminate the gas along the line of sight, i.e. the gas between the galaxies in intergalactic space. With its high spectral resolution and high sensitivity, Colibrì will enable detection and characterization of this material through absorption features.

Analysis of the density fluctuations in the cosmic microwave background, as well as observations of the high-redshift intergalactic medium (IGM) through Lyman-alpha absorption, provide an estimate of the fraction of baryonic matter in the Universe. Recent Planck results estimate this baryon fraction to be 4.86%. Of this fraction, about 10-20% is found in galaxies, groups and clusters, while the rest is expected to lie in the reservoirs of gas that make up the IGM. However, observations of the IGM at low redshift account only for about half of the remaining 80-90% of baryons. A big question then arises: where are the missing baryons in our current epoch?

Cosmological simulations suggest that the gas in the IGM at low redshift is distributed over a wide range of temperatures and densities, which are usually divided into two main components: a diffuse and photoionized phase and a condensed filamentary web that is heated by shocks. The diffuse phase can be traced by detecting Lyman-alpha absorption lines in the spectrum of background quasars and contains about 25% of the low-redshift baryons. The condensed and hot phase is called the warm-hot intergalactic medium (or WHIM), and it is thought to contain the remaining 60% of the gas in the IGM