Ultraviolet spectroscopy of the quasar pair LB 9605, LB 9612 with the Hubble Space Telescope:: Evolution in the size of the Lyα absorbers?

Ultraviolet spectroscopy has been obtained with the faint object spectrograph of the Hubble Space Telescope in the Ly alpha forest of the quasar pair, LB 9605 (z(cm) = 1.834) and LB 9612 (z(cm) = 1.898), in order to measure the size of the Ly alpha absorbers. The quasars are separated by 1.65 on the sky corresponding to a proper separation of 412 h(-1) kpc at z = 1.83 (where h drop H(0)/100 km s(-1) Mpc(-1) q(0) = 0.5). We detect five Ly alpha absorption lines common to both spectra within a velocity difference of 400 km s(-1) in the redshift range 1.06 < z < 1.69, and 20 lines which are seen in the spectrum of one quasar but not the other. The number of coincidences expected for randomly distributed absorbers in this redshift interval is 3.2 +/- 1.8, implying a less than 2 sigma significance for the observed coincidences. If none of the observed coincidences are real, then we can place an upper limit on the absorber radius with 95% confidence of similar to 285 h(-1) kpc for redshifts 1.06 < z < 1.69. If, on the other hand, all of the observed coincidences are real then a maximum likelihood estimate of the characteristic absorber radius in the context of identical, spherical clouds give a most probable radius of 380 h(-1) kpc with 95% confidence that the characteristic radius lies in the range 305 < R < 595 h(-1) kpc. Taken together with a low redshift estimate and new ground-based estimates, the upper limit on the absorber size provides tantalizing evidence for evolution in the radius of the Ly alpha absorbers with cosmic time, in the sense that the characteristic size of the Ly alpha absorbers increases with decreasing redshift.