Evidence for Large, Quiescent Lyman-Alpha Clouds from HST UV Spectroscopy of the Quasar Pair Q0107-025A,B

Abstract

We present ultraviolet spectra of the Lyα forest of the quasar pair Q0107-025A,B (z _em = 0.956, 0.952; angular separation 1.44′) obtained with the Hubble Space Telescope, in which we detect four absorption lines common to both spectra in the redshift range 0.5 ≤ z ≤ 0.9, and six lines which are seen in the spectrum of one quasar but not the other. Assuming that these are Lyα lines, the directly-measured lower limit on the characteristic radii of the clouds is between 160 and 180 h ⁻¹₁₀₀ kpc (where h ₁₀₀ = H ₀/100 km s⁻¹ Mpc⁻¹, q ₀ = 0.5). The rms velocity difference between the common absorption lines along the two lines of sight is only about 100 km s⁻¹. These direct measurements lead to a picture of absorbing clouds that are both larger in extent and more quiescent than can easily be explained by current theoretical models.