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R.A. Simcoe, W.L.W. Sargent (Palomar Observatory, California Institute of Technology), M. Rauch (Observatories of the Carnegie Institution of Washington)
We report on a study of the Lyman alpha forest at intermediate redshift using a sample of quasars observed with the Keck I telescope + HIRES spectrograph. Until recently, this redshift range had remained an observational challenge because the observed absorption wavelengths reside at a low point in the sensitivities of both HST/STIS to the blue and ground-based telescopes to the red. However, the motivation for pushing these limits is strong, as the redshift interval in question is known to bracket an epoch of rapid change in the evolutionary trend of forest absorbers. Also, the comparatively low number of absorbers (relative to the high-z forest) allows for a more well-defined treatment of individual clouds using traditional profile-fitting methods.
These new high resolution, high S/N data probe neutral hydrogen column densities as low as logNHI=12.5, and provide continuous and homogeneous coverage over the region of the known break in dN/dz at higher NHI. Through Voigt profile decomposition of the forest, we search for changes in the physical properties of individual absorbers to distinguish between scenarios where dN/dz is driven predominantly by the evolution of bulk cloud properties as opposed to changes in the ionization state of the average cloud. We also combine the forest data with coverage of the C IV spectral region to explore the possibility that the strength of evolution in dN/dz is related to the presence of heavy metals.