The Physical State of the Intergalactic Medium or Can We Measure Y?

Details The Physical State of the Intergalactic Medium or Can We Measure Y? The Physical State of the Intergalactic Medium or Can We Measure Y?

1995-06-26

arxiv.org/abs/astro-ph/9506125v1

Astronomy and Astrophysics

Astrophysics

Proceedings of `Unveiling the Cosmic Infrared Background', April 23-25, 1995, Maryland. Self-unpacking uuencoded, compressed t

Scientific paper

We present an argument for a {\it lower limit} to the Compton-$y$ parameter describing spectral distortions of the cosmic microwave background (CMB). The absence of a detectable Gunn-Peterson signal in the spectra of high redshift quasars demands a high ionization state of the intergalactic medium (IGM). Given an ionizing flux at the lower end of the range indicated by the proximity effect, an IGM representing a significant fraction of the nucleosynthesis-predicted baryon density must be heated by sources other than the photon flux to a temperature $\go {\rm few} \times 10^5\, K$. Such a gas at the redshift of the highest observed quasars, $z\sim 5$, will produce a $y\go 10^{-6}$. This lower limit on $y$ rises if the Universe is open, if there is a cosmological constant, or if one adopts an IGM with a density larger than the prediction of standard Big Bang nucleosynthesis.

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