A Uniform Analysis of the Lyα Forest atz= 0–5. IV. The Clustering and Evolution of Clouds atz≤ 1.7

Abstract

We present results on the evolution and clustering of Ly lines at low redshift as part of our series '' A Uniform Analysis of the Ly Forest at z ¼ 0 5.'' The sample analyzed in this paper contains 1298 Ly absorption lines from 165 quasar spectra mined from the archives of the Faint Object Spectrograph on the Hubble Space Telescope (HST). Our sample extends to z % 1:7, slightly higher than the sample analyzed by the HST Quasar Absorption Line Key Project. We confirm the result from the Key Project that the number density evolution of Ly lines at low redshifts can be described by a power law that is significantly flatter than that found at high z. We find that the evolution is somewhat steeper than obtained previously. Specifically, we find ¼ 0:54 AE 0:21 for lines with equivalent widths greater than 0.24 Å and ¼ 0:60 AE 0:14 using a variable equivalent width threshold. We find that the difference between our and Key Project results is likely attributable to different redshift coverage of the two samples. The results concerning the number density evolution are not significantly affected if one includes Ly lines which are members of metal systems. Object-to-object fluctuations in the number of lines detected are small, indicating a high degree of uniformity in the intergalactic medium on large scales. We find marginal evidence that weak and strong lines undergo different evolution. We find weak clustering for Ly lines at velocity separations DV 500 km s À1 , weaker than the level predicted from an earlier analysis by Ulmer of a small subsample of the Key Project data. We see no correlations for metal system-Ly forest or extensive metal system-Ly forest combinations.

A high-resolution study of the evolution of the Lyman α forest in the redshift interval $0.9 < z < 1.7$

“…previously claimed (Impey et al 1996;Weymann et al 1998;Dobrzycki et al 2002). The large spread of our data points result from the poor statistics of a single line of sight.…”

“…We use the column density range 13.10 ≤ log N H i ≤ 14.00 for the weak lines, while the interval 13.64 ≤ log N H i ≤ 16.00 defines the strong ones. These boundaries are frequently used in the literature (Weymann et al 1998;Kim et al 2001;Dobrzycki et al 2002) allowing a direct comparison.…”

A Composite Extreme‐Ultraviolet QSO Spectrum fromFUSE

“…Because the result of Davé & Tripp (2001) is larger than that typically inferred for from lower resolution data (Penton et al 2000;Dobrzycki et al 2002) or at high redshift (Kim et al 2001), we reduce it from the fiducial value of 2.0 to 1.5. This increases by 0.3.…”

“…This increases by 0.3. The composite is fairly insensitive to the redshift distribution parameter, , which we varied from 0.15 (Weymann et al 1998) to 0.54 (Dobrzycki et al 2002). Raising and lowering the upper column density limits for the Ly forest correction to log N ¼ 16:9 and 16.5 changes by +0.06 and À0.04, respectively.…”

On the sizes of neutral hydrogen regions giving rise to damped Lyα absorption systems

“…In fact, as noted earlier, in cosmological simulations they stem from gas overdensities and thus take many forms. The properties of the forest evolve from high to low redshift (Weymann et al 1998; Dobrzycki et al 2002), in large part due to evolution of the ionizing background radiation (e.g. see Davé et al 1999).…”