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The far-ultraviolet signature of the ‘missing’ baryons in the Local Group of galaxies


The number of baryons detected in the low-redshift (z < 1) Universe is far smaller than the number detected in corresponding volumes at higher redshifts. Simulations1,2,3 of the formation of structure in the Universe show that up to two-thirds of the ‘missing’ baryons may have escaped detection because of their high temperature and low density. One of the few ways to detect this matter directly is to look for its signature in the form of ultraviolet absorption lines in the spectra of background sources such as quasars. Here we show that the amplitude of the average velocity vector of ‘high velocity’ O vi (O5+) absorption clouds detected in a survey4 of ultraviolet emission from active galactic nuclei decreases significantly when the vector is transformed to the frames of the Galactic Standard of Rest and the Local Group of galaxies. At least 82 per cent of these absorbers are not associated with any ‘high velocity’ atomic hydrogen complex in our Galaxy, and are therefore likely to result from a primordial warm–hot intergalactic medium pervading an extended corona around the Milky Way or the Local Group. The total mass of baryons in this medium is estimated to be up to 1012 solar masses, which is of the order of the mass required5 to dynamically stabilize the Local Group.

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Figure 1: The velocity range of HV-O vi absorbers greatly exceeds typical rotational velocities in the Galaxy.
Figure 2: Velocity segregation of the HV-O vi absorbers.


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F.N. thanks P. Kaaret and G. C. Perola for discussions and comments. We thank W.B. Burton for comments that helped improve the Letter. This work was partly supported by NASA-Chandra grants and a NASA-Chandra X-ray Center contract.

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Correspondence to Fabrizio Nicastro.

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Nicastro, F., Zezas, A., Elvis, M. et al. The far-ultraviolet signature of the ‘missing’ baryons in the Local Group of galaxies. Nature 421, 719–721 (2003).

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