Wal Sargent

The First Heavy Elements and the End of the Dark Ages

Due to the efforts of many astronomers, both observers and theorists, we now have a general idea as to how the universe has evolved from the Big Bang 13.7 billion years ago to the present day. We know that about 300,000 years after the Big Bang the hot, expanding, ionized hydrogen and helium gas in the universe had cooled sufficiently for these elements to become neutral. The ensuing period (for which I invented the term “the Dark Ages” in 1985) lasted until the first stars and galaxies began to form and reionize the gas.

My main current interest is to study the evolution of large scale structure and the physics of the intergalactic medium when the first stars and galaxies were forming and the baryons in the universe were largely in the form of hydrogen and helium gas. My main collaborators are Michael Rauch (OCIW), George Becker (Kavli Cosmology Institute, Cambridge), and Bryan Penprase (Pomona College). Most of our work involves observations of quasar absorption lines using the spectrographs on the two 10-meter Keck Telescopes in Hawaii and the￼ 6.5-meter Magellan Telescopes at the Las Campanas Observatory in Chile.

When I came to Caltech as a postdoc in 1959 I became an observer and learned about stellar spectroscopy and element abundance determinations. At that time the first “metal poor” old stars were being discovered and we increasingly wondered through the 1960s why no stars (designated Population III) with primordial abundances (just H and He) had been found. I moved on to work on many areas of galactic and extragalactic spectroscopy over the ensuing years. I find it particularly satisfying that 50 years later we can directly observe the products of nucleosynthesis by the first stars in the intergalactic gas at very high redshifts so that in some sense my career has come around full circle.

I have had few but very successful graduate students. They are all Manchester United supporters and devotees of Elgar's music.

[Image credits: Bob Paz; W. Sargent]