2006AJ....132.2099N

We compare a new R=120,000 spectrum of PG 1634+706 (z_QSO=1.337 and m_V=14.9) obtained with the HDS instrument on Subaru to a R=45,000 spectrum obtained previously with the Keck HIRES. In the strong Mg II system at z=0.9902 and the multiple-cloud, weak Mg II system at z=1.0414, we find that at the higher resolution, additional components are resolved in a blended profile. We find that two single-cloud, weak Mg II absorbers were already resolved at R=45,000, with b=2-4 km/s. The narrowest line that we measure in the R=120,000 spectrum is a component of the Galactic Na I absorption, with b=0.90±0.20 km/s. We discuss expectations of similarly narrow lines in various applications, including studies of damped Lyα absorbers, the Mg I phases of strong Mg II absorbers, and high-velocity clouds. By applying Voigt profile fitting to synthetic lines, we compare the consistency with which line profile parameters can be accurately recovered at R=45,000 and 120,000. We estimate the improvement gained from superhigh resolution in resolving narrowly separated velocity components in absorption profiles. We also explore the influence of isotope line shifts and hyperfine splitting in measurements of line profile parameters, and the spectral resolution needed to identify these effects. Superhigh-resolution spectra of quasars, which will be routinely possible with 20 m class telescopes, will lead to greater sensitivity for absorption-line surveys, and to the determination of more accurate physical conditions for cold phases of gas in various environments.