Physical Society Colloquium
Interview for Faculty Position
UV and X-ray Detections of the
`Missing Baryons':
from the Local Group Outwards
Fabrizio Nicastro
Harvard-Smithsonian Institute
The majority of the baryons in the local (i.e. redshift, z < 1-2) Universe,
is supposed to hide in a filamentary network of tenuous (baryon overdensities
of ~ 5-100, in units of the average density in the Universe), relatively
metal-rich (metallicity in the WHIM, Z ~ 0.05-0.1 times Solar), warm-to-hot
(temperatures of T ~ 1-10 million degrees) gas, shock-heated during the
collapse of density perturbation: the so called Warm Hot Intergalactic Medium
(WHIM).
This matter had eluded observation until very recently, due to its extreme
low density (except around the rare and dense nodes of this web), and to the
lack of high resolution soft-X-ray spectrometers, needed to detect weak
resonant absorption lines from the most abundant metal ions in the gas
(mainly OVII and OVIII K-alpha transitions) in the spectra of background
sources. This situation has dramaticaly changed in the past 2-3 years, with
the advent of high resolution X-ray spectroscopy.
In this talk I will review the current observational evidence of the WHIM in
the X-rays, focusing mainly on (a) the spectroscopic and dynamical lines of
evidence for the existence of a WHIM filament in our own Local Group, and (b)
the first detection of two WHIM filaments at redshift larger than zero, in
the intergalactic space along the line of sight to the blazar Mkn 421, for
which exceptionally high quality data have been collected with the Chandra
Low Energy Transmission Grating (LETG). Finally I will discuss possible
future prospects for WHIM studies.
Thursday, January 29th 2004, 16:00
Ernest Rutherford Physics Building, R.E. Bell Conference Room (room 103)
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