DR. GIRISH KULKARNI

At redshifts 2 to 5, the intergalactic medium (IGM) is a relic of the epoch of hydrogen reionization and is likely undergoing Helium reionization. It traces dark matter fluctuations on Mpc scales, but on smaller scales of hundreds of kpc, fluctuations are suppressed as the reheated gas is pressure supported against gravity, according to the classical Jeans argument. This pressure smoothing scale thus quantifies the small-scale structure of the IGM and has crucial cosmological implications: it provides a record of cosmic reionization and thermal evolution, measures IGM clumping, and sets minimum mass for galaxy formation. In this talk, the speaker will show that classical descriptions of the Jeans scale fail at z=2--5. He will then present a generalised theoretical characterisation of the Jeans scale that is valid at these redshifts. He will also discuss the first measurements of the Jeans filtering scale at 2 < z < 3 based on an unprecedented dataset of hundreds of close quasar pair spectra, and argue that it provides a crucial observable for resolving various inconsistencies in our current understanding of reionization and the concomitant thermal evolution of the IGM. Finally, he will discuss small-scale structure in the IGM at higher redshifts (z > 6) and how it affects the predicted 21 cm emission from the IGM.