First Measurement of the Small Scale Structure of the Intergalactic Medium

Tue, February 16, 2016

There is no such thing as empty space. Indeed, the most barren regions of the universe are the vast expanses between the galaxies, known as the intergalactic medium (IGM). Averaging just one lonely atom per cubic meter, this primordial gas left over from the Big Bang encodes fundamental information about our universe's history. About half a million years after the Big Bang, the plasma of primordial baryons recombined to form the first neutral atoms, releasing the cosmic microwave background and initiating the cosmic 'dark ages'. During this period primordial gas expanded and cooled to very low temperatures T~ 20 K, until the stars and black holes in the first galaxies emitted enough ionizing photons to reionize and reheat the universe. The thermal state of gas in the IGM is a relic of these reionization phase transitions, which can be measured via optical observations of bright distant quasars at cosmological lookback times of a few gigayears. On Mpc length scales, gas in the intergalactic medium traces density fluctuations in the underlying and gravitationally dominant dark matter, but on smaller scales of hundreds of kpc, fluctuations are suppressed because the ~10^4 Kelvin gas is pressure supported against gravity, analogous to the classical Jeans argument. This Jeans pressure smoothing scale thus quantifies the small scale structure of the IGM and provides a record of cosmic reionization and thermal evolution. Recently we have shown that it is possible to directly measure this scale by characterizing the coherence of correlated intergalactic absorption lines in the spectra of pairs of quasars, at small angular separation on the sky. I will describe a statistical method which quantifies correlated absorption in quasar spectra, which is highly sensitive to the pressure smoothing scale, and present its first-ever measurement with data collected from the Keck telescopes. Our preliminary results suggest that the pressure smoothing scale is smaller than expected from the standard models of reionization and the thermal evolution of the IGM.