Hot baryons in supercluster filaments

Summary

Most of the baryons in the local universe are “missing” in that they are not in galaxies or in the previously detected gaseous phases. These missing baryons are predicted to be in a moderately hot phase, 10⁵–10⁷ K, largely in the form of giant cosmic filaments that connect the denser virialized clusters and groups of galaxies. Models show that the highest covering fraction of such filaments occurs in superclusters. To determine whether such filaments exist, we have begun a project to search for UV absorption against AGNs projected behind possible supercluster filaments. Using data from the HST and FUSE archives along with new observations, we have detected UV absorption within about 1300 km s⁻¹ of seven supercluster sightlines out of a sample of eight. The likelihood of such detections being generated by chance is less than 10⁻⁴.

Introduction

A census of baryons in the local universe indicates that the majority of this normal matter is undetected, or “missing.” At high redshifts (z ∼ 3), big-bang nucleosynthesis models and QSO absorption line observations indicate a baryon mass fraction of Ω_b ∼ 0.04 (e.g., Fukugita, Hogan & Peebles 1998). The stars and gas detected in local galaxies account for only 20% of this (Ω_b ∼ 0.008). The absence of a local Lyα forest indicates that these baryons are likely in a hot (T > 10⁵ K), diffuse medium which has heretofore remained undetectable (e.g., Fukugita, Hogan & Peebles 1998; Cen & Ostriker 1999a; Davé et al. 2001).