Small-scale structure in the Lyα forest at high redshift

We have obtained spectra of the four components of the system B1422 + 231 with HST's Faint Object Spectrograph in order to study spatial inhomogeneities and velocity variations in the absorbers which are pierced by the four lines of sight. The four images are well resolved, and the spectra clearly show the system is a lensed quasar; a new redshift based on the Ly alpha and N v emission lines is measured to be 3.6261. The following are results from the analysis of the four individual lines of sight that probe redshifts 2.9 less than or equal to z less than or equal to 3.5 and transverse dimensions 0 - 0.14 h(100)(-1) kpc: (1) In the region between Ly beta and Ly alpha (less 6000 km s(-1) to allow for the proximity effect: 4745 - 5511 Angstrom), the limiting rest equivalent width (1 sigma(lim)) is measured to be 0.059, 0.052, 0.034, and 0.22 Angstrom for components A, B, C, and D, respectively. (2) The total number of 5 sigma(W) lines found in this region are 83, 85, 102, and 44 for A-D, respectively; for A-C 11, 9, and 13 are identified with metal line systems and the remaining 72, 76, and 89 are assumed to be Ly alpha lines. Between paired lines of sight A and B to C, and A paired to B, there are 62, 68, and 62 coincidences and 3, 1, and 1, anticoincidences for the Ly alpha lines. For the metal lines there are 8, 9, and 8 coincidences for AC, BC, and AB, respectively, and each has one anticoincidence. (3) Scatter plots of the equivalent widths for the coincident lines for both Ly alpha and metals are highly correlated and all pairs match within 100 and 75 km s(-1), respectively. (4) Maximum-likelihood analysis limits spatial inhomogeneity for Ly alpha absorbers in components A, B, and C to no more than similar to 15 % of the equivalent width of an average (W = 2 Angstrom) line. This corresponds to a limit on the mean variation in column density, assuming unsaturated lines, of log (Delta N-H) = 13.1 cm(-2). (5) A method of detecting spatial inhomogeneities independent of continuum-fitting and line-profile measurement is developed yielding an upper limit on the column density difference for unsaturated lines of log (Delta N-H) = 12.8. This is approximately a factor of 2 smaller than the limits obtained from the line pairs. (6) The distribution of the velocity differences for AC, BC, and AB for the Ly alpha lines and the metal line pairs have a mean close to zero and rms similar to 40 km s(-1). Maximum-likelihood analysis gives an upper limit on the variations in velocity of similar to 40 km s(-1) for Ly alpha absorbers in paired lines of sight. The overall result of this work is a set of stringent limits on the inhomogeneity of low column density hydrogen absorbers on subkiloparsec scales at high redshift.