Absorption diagnostics of the IGM


The Lyman-alpha Forest

Hydrogen in the intergalactic medium scatters light as it redshifts across its Lyman-α 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 z=3.6 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 much of the high-resolution 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).  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 extremely low density (1 atom per cubic meter today).  Studies of the hydrogen Lyman-alpha forest absorption have provided most of the knowledge we currently have regarding the IGM, but the Lyman-alpha forest only has a detectable optical depth and is unsaturated for 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 another 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 to date.  Nevertheless, the characteristics of this helium absorption appear to be much more complicated compared to our old friend hydrogen Lyman-alpha, especially because helium reionization appears to be ending at z=3 (as mentioned above).  I have been collaborating with Gabor Worseck , Xavier Prochaska, and Joe Hennawi to observe and interpret new helium Lyman-alpha forest sightlines --- a project made possible with a new spectrograph on the Hubble Space Telescope, COS, which was installed during the last Hubble re-servicing mission. 

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 Lyman-alpha absorption of singly ionized helium.  Note that the helium forest is much thicker than the hydrogen forest. Several studies had interestingly claimed that the helium absorption is not very correlated with features in the hydrogen Lyman-alpha forest.  This lack of correlation was attributed to factor-of-ten, Mpc-scale variations in the level of the ionizing backgrounds keeping the hydrogen and helium ionized.  Such a level of fluctuations is much larger than that anticipated in theoretical models.  However, these measurements are tricky as the optical depths in helium are ~30 times larger than in the hydrogen, making it so that the HeII absorption is saturated in all regions but in the deepest HI voids (where the HI optical depths are small and, hence, difficult to measure).  In a recent paper, we showed that the previous claims of large ionizing background fluctuations were erroneous and actually the ionizing background inferred from our comparison is almost consistent with a single, spatially independent ratio of intensities at 1 and 4 Rydberg (and also consistent with  theoretical expectations).


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, we 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.)

Other Absorption Diagnostics