CfA Colloquium: October 31, 2002

Speaker: Fabrizio Nicastro (CfA)

Title: Tracking the Missing Baryons in the Local Group

Abstract: The deficit of baryons in already virialized structures of the low-redshift Universe (z <~ 1), compared to the amount of baryonic matter observed at higher redshifts, is one of the most puzzling, and yet unsolved, mystery in astronomy. Missing baryons can amount up to 2/3 of the total baryonic mass in the local Universe. Hydro-dynamical simulations for the formation of structures, all agree in predicting that a large fraction of these baryons at z <~1 be distributed in a web of very tenuous filaments, connecting already virialized structures (i.e. galaxies, groups and clusters of galaxies) and shock-heated during their collapse onto the potential well of the dark matter concentrated in the nodes of these systems. This matter would have escaped identification so far, due to its high temperature (logT ~ 5.5-7 K), and low density (overdensity of 10-50, compared to the average density in the Universe), that makes its detection in emission very challenging with the current UV and X-ray instruments. The advent of high resolution Far-UV (i.e. FUSE) and X-ray (i.e. Chandra and Newton-XMM) spectroscopy, however, allows, for the first time, these filaments to be detected through OVI, OVII, OVIII and NeIX-X absorption against bright cosmological sources as quasars and X-ray afterglow of GRBs.

Here I present the very first detection of the "missing baryons", showing physical and dynamical evidences that unambiguously demonstrate the association of this matter with diffuse primordial gas pervading our own Local Group of galaxies. These baryons have a temperature of about logT = 5.8, and are distributed with a density of ~ 4e-6 cm^-3 along a filament of ~ 1-2 Mpc in length, possibly elongated along the direction of the barycenter of the Local Group. Assuming a transverse size of 1 Mpc, we estimate a total baryonic mass of about 1e12 solar masses, comparable to the entire mass of the most massive constituents of the Local Group: M 31 and the Milky Way, and sufficient to stabilize the Local Group.

Reference for students:

Nicastro et al., 2002, ApJ, 573, 157