A uniform analysis of the Lyman alpha forest at z = 0-5: IV. The clustering and evolution of clouds at z < 1.7

Abstract

We present results on the evolution and clustering of Ly-alpha lines at low z. The sample contains 1298 Ly-alpha lines from 165 quasar spectra mined from the archives of the HST/FOS. Our sample extends to z=1.7, slightly higher than the sample analyzed by the HST Quasar Absorption Line Key Project. We confirm the Key Project result that the number density evolution of Ly-alpha lines at low z can be described by a power law that is significantly flatter than that found at high z. We find gamma=0.54+/-0.21 for lines with equivalent widths greater than 0.24 A, and gamma=0.60+/-0.14 using a variable EQW threshold, somewhat steeper than obtained previously. We find that the difference is likely attributable to different coverage of the two samples. The results concerning gamma are not significantly affected if one includes Ly-alpha lines from metal systems. Object to object fluctuations in the number of lines detected are small, indicating a high degree of uniformity in the IGM on large scales. We find marginal evidence that weak and strong lines undergo different evolution. We find weak clustering for Ly-alpha lines at Delta(V)<500 km/s, weaker than earlier analysis by Ulmer of a subsample of the Key Project data. We see no correlations for metal system-Ly-alpha forest or extensive metal system-Ly-alpha forest combinations.

arXiv commentAccepted for publication in Ap.J.; AASTeX, 32 pages, 11 PS figures, 2 tables; available in: http://lithops.as.arizona.edu/~jill/QuasarSpectra/ or http://hea-www.harvard.edu/QEDT/QuasarSpectra/

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• The Evolution of Lyman alpha Absorbers in the Redshift Range 0.5<z<1.9
• Continuous statistics of the Lyman-alpha forest at 0 < z < 1.6: the mean flux, flux distribution, and autocorrelation from HST FOS spectra
• Modelling the IGM and the Lyalpha forest at high redshift from the dark matter distribution
• Cross correlation of Lyman-alpha absorbers with gas-rich galaxies
• The evolution of the low-density HI intergalactic medium from z=3.6 to 0: Data, transmitted flux and HI column density
• Cosmic Filaments in Superclusters
• Tracing the merger-driven evolution of active galaxies using the CJF sample
• The relation between Lyman-alpha absorbers and gas--rich galaxies in the local universe
• The End of the Rainbow: What Can We Say About the Extragalactic Sub-Megahertz Radio Sky?
• Quasar Absorption Lines