Are Voids Present in the Lyman-Alpha Forest?

Abstract

Summary If voids like those seen in the low z galaxy distribution existed in the H I distribution at z ≈ 2, then high quality QSO spectra, with many Ly-α forest lines per unit z , could be used to discern the voids from the usual random fluctuations in observed number density of lines (≡ n ). Several such spectra have been obtained, and these show evidence for gaps in the Ly-α distribution on scales of 20 to 50 h −1 Mpc (comoving coordinates, with h = H 0 /66.7 km s −1 Mpc −1 , assuming q 0 = 0.1). These results are summarized in the table below. All QSO spectra with a line-of-sight n of Ly-α lines n > 80 per unit z and total number of lines N > 40 known to the author are included (except that of PKS 2000-330 [c.f. Carswell and Rees 1987], which is broken into five separate segments of Ly-α forest by gaps in the data and a broad absorption line). Excluded are portions of these spectra where n falls more than 25% below the mean due to instrumental bias. For each of these the distribution of gaps between nearest-neighbor Ly-α redshifts is computed as a function of gap size. If the distribution of redshifts were Poisson, the distribution of gaps should be a decreasing exponential function of gap size. For the two best spectra, large deviations from an exponential are found in the range of 20 to 50 h −1 Mpc (in the other four cases, it should be noted that a large number of gaps of such sizes are still expected from Poisson fluctuations). The probability that such deviations are statistically consistent with an exponential distribution is shown in the fifth column of the table.