Hydrogen in the intergalactic medium scatters light as it redshifts across its Lyman-alpha resonance at 1216 Angstrom.  This scattering creates a forest of absorption lines in the continuum spectra of high-redshift objects.  The above figure shows this forest in the spectrum of a famous lensed quasar (taken by Michael Rauch using the Keck telescopes in Hawaii).  I have studied the signatures of intensity and temperature fluctuations on these spectra, which can inform models of the intergalactic medium and reionization (especially of helium).  In a study led by Adam Lidz, we used data to search for these signatures. 

Recently, I have thought about how we can use multiple Lyman-alpha forest sightlines to measure sightline-to-sightline correlations and what can  be learned from such a measurement (the most exciting of which would be the properties of the Dark Energy...if you do not know what this is, I’m not going to get into it here, but the truth is we are a bit clueless anyway).  Upcoming surveys will provide hundreds of thousands of spectra like in the above figure (albeit with a bit lower signal-to-noise ratios).

The intergalactic medium (IGM) is one of the most difficult regions in the Universe to observe owing to its extreme low density (It is 1 atom per cubic meter today.) and because it is only weakly enriched with heavier elements than helium.  Studies of hydrogen Lyman-alpha forest absorption have provided most of the knowledge we currently have regarding the IGM, but the Lyman-alpha forest only probes a limited range in density, densities near the cosmic average at z=3.  Part of my recent work has been in the direction of investigating other probes of the IGM.

Recently I have been working on a different Lyman-alpha absorption line, this time that of singly ionized helium at 2 < z < 3.5.  The study of this line is in a more nascent state than the study of hydrogen Lyman-alpha, with only a few studied helium sightlines at this point.  Nevertheless, the characteristics of this helium absorption appear to be much more complicated compared to our old friend hydrogen Lyman-alpha, and helium reionization appears to be ending at z=3 (as mentioned above).  I have been collaborating with Gabor Worseck and Xavier Prochaska to observe and interpret new helium Lyman-alpha forest sightlines --- a project made possible with a new spectrograph on the Hubble Space Telescope that was installed during the Hubble re-servicing mission a couple years ago.  The above figure shows two of the sightlines we (primarily Gabor) have analyzed and reported here.  The red is the hydrogen Lyman-alpha transmission and the black is the coeval helium Lyman-alpha absorption.  Note that the helium forest of absorption lines is much thicker and also does not trace very well the structure in the hydrogen forest.  We are working to understand the implications of these differences. 

Another frontier in IGM research involves testing our models of the IGM at higher densities than probed by the hydrogen and helium Lyman-alpha forests.  Such tests could inform theories of structure formation and of the interplay between galaxies and the IGM.  In collaboration with Peng Oh and Claude-Andre Faucher-Giguere, I have looked at how well models of the IGM reproduce the observed abundance of Lyman-limit absorption systems, systems with hydrogen column densities of 1017-1019 cm-2.  These, like the Lyman-alpha forest absorption, can be measured in quasar spectra.  The lines in the panel on the right, taken from this paper, show our models (calculated

using a cosmological simulation)  for f(NHI) --  the number of absorption lines per unit redshift per hydrogen column, NHI.  The plotted NHI correspond to systems with densities that are  more than a hundred times larger than probed with the Lyman-alpha forest.  The highlighted regions are the constraints on f(NHI) at z=4 from quasar absorption observations.    Thus, cosmological simulations are able to reproduce the observed properties, at least at the factor of 2-level.  We found that this conclusion was relatively robust to different uncertainties in the astrophysics that we explored. (The inset shows the observed and predicted mean free path for hydrogen-ionizing photons, which also show decent agreement.)