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Research Interests

Your main research interests lie in the inter-galactic medium.

For years, you have been fascinated by what lies in the gaps between the galaxies. Your colleagues often derided your fixation with empty space, but you remained convinced that this was an interesting and important field. After all, galaxies only occupy a tiny tiny fraction of the universe: if there is anything at all, however tenuous, in all those vast inter-galactic spaces, it could dominate the mass budget and dynamics of the whole universe.

The first big breakthrough occurred in the late 1960's, when high redshift QSOs were first discovered. If intergalactic space was full of neutral hydrogen, then all wavelengths shortward of Ly tex2html_wrap_inline92 (1216Å) in the rest-frame of the QSOs should have been absorbed (the Gunn-Peterson effect). It was not: this indicated either that intergalactic space at high redshifts is completely empty (which was very hard to understand theoretically), or that the gas was extraordinarily hot, and hence too higly ionised to absorb at the wavelengths of Ly tex2html_wrap_inline92 . Note that these absorption-lines occur in the UV: you can only observe this phenomenon in the early universe, where cosmological redshifts shift the relevant absorption-lines into the near-UV or optical.

This observation posed great problems: what is the energy source that heated up all this gas? Even at redshift 5, the inter-galactic medium is almost completely ionised: this means that a lot of very powerful ionising sources, hitherto undetected, most have existed way back then.

The second big breakthrough occurred about three years ago: Peter Jakobsen used the Hubble Space telescope to look for absorption due to He II in the far-UV (304Å). This would only show up if the inter-galactic medium was indeed very hot: strong absorption was seen, showing that there is a hot medium out there. This absorption has now been seen in four QSOs.

Very recently, HST observations of the He II Gunn-Peterson effect have shown that this hot gas is patchy: every now and then, voids are found in the hot gas: regions which may really be completely empty, or in which the intergalactic gas is either cold, or very very hot, so that He II is not found.

So: exciting days. The mystery of what lies between the galaxies is slowly being cracked, and the answer is turning out to be complex: remarkably hot gas, with a total mass probably exceeding that in all galaxies and denser gas clouds combined, and with these mysterious voids all over the place.

next up previous
Next: Proposed Facility Up: You are the NASA Previous: You are the NASA

Paul Francis
Sun Aug 30 11:56:56 EST 1998