The Search for the Missing Baryons at Low Redshift

The baryon content of the universe is known from Big Bang nucleosynthesis and cosmic microwave background considerations, yet at low redshift, only about one-tenth of these baryons lie in galaxies or the hot gas seen in galaxy clusters and groups. Models posit that these “missing baryons” are in gaseous form in overdense filaments that connect the much denser virialized groups and clusters. About 30% of the baryons are cool (<10 K) and are detected in Lyα absorption studies, but about half the mass is predicted to lie in the 10-10 K regime, where detection is very challenging. Material has been detected in the 2-5×10 K range through OVI absorption studies, indicating that this gas accounts for about 7% of the baryons. Hotter gas (0.5-2×10 K) has been detected at zero redshift by OVII and OVIII Kα absorption at X-ray energies. However, this appears to be correlated with the Galactic soft X-ray background, so it is probably Galactic Halo gas, rather than a cosmologically significant Local Group medium. There are no compelling detections of the intergalactic hot gas (0.5-10×10 K) either in absorption or in emission. Early claims of intergalactic X-ray absorption lines have not been confirmed, but this is consistent with theoretical models, which predited equivalent widths below current detection thresholds. There have been many investigations for emission from this gas, within and beyond the virial radius of clusters, but the positive signals for this soft emission are largely artifacts of background subtraction and field-flattening. We discuss the various techniques that can be used to detect the missing baryons and show that it should be detectable with moderate improvements in sensitivity.