First results from the Goodard High-Resolution Spectrograph: The Galactic halo and the Lyα forest at low redshift in 3C 273

We present results of initial spectroscopy using the Goddard High-Resolution Spectrograph and the Hubble Space Telescope on the bright low-redshift quasar 3C 273. We detect absorption lines produced by gas in the disk and halo of the Milky Way from numerous neutral and weakly ionized atoms and also from highly ionized atoms (Si IV, C IV, and N V). The detection of N V provides additional strong support for the existence of gas with T ∼ 2 × 10⁵ K at large distances away from the disk of the Milky Way. We also find 10 absorption lines with EW greater than 50 mÅ, and four possible lines with EW between 25 and 50 mÅ, in the region between 1235 and 1406 Å. These cannot be identified with any lines in the galactic interstellar medium and must therefore all be Lyα systems at very low redshifts. This number of lines is 5-10 times larger than expected from a simple extrapolation of the high-redshift evolution derived from ground-based observations and is consistent with no evolution in cloud properties since z = 2.