1999/11/30 by Steven V. Penton, J. Michael Shull, John T. Stocke · 4 citations
Chemistry · Physics and Astronomy · #Analytical Chemistry (journal) #Astronomy #Astrophysics #Astrophysics and Star Formation Studies #Baryon #Chemistry #Distribution (mathematics) #Doppler effect #Emission spectrum #Equivalent width #Extrapolation #Full width at half maximum #Galaxies: Formation, Evolution, Phenomena #Galaxy #Number density #Optics #Physics #Redshift #Spectral line #Statistics #Stellar, planetary, and galactic studies #astro-ph
paper · pdf · doi:10.1086/317179
published as Astrophys.J. 544 (2000) 150 · 59 pages, 22 figures, 7 tables, ApJ accepted, scheduled to appear Dec 1, 2000. Changes to the GHRS linelist (astro-ph/9911117) are reflected in this revison
arxiv created 2000/07/12 · openalex publication_date 2000/11/20 · arxiv updated 2009/12/01 · openalex created_date 2019/06/27 · openalex updated_date 2026/08/05
In Paper I of this series we described observations of 15 extragalactic targets taken with the Hubble Space Telescope +GHRS+G160M grating for studies of the low- z Lyα forest. We reported the detection of 111 Lyα absorbers at significance level (SL) ≥3 σ, 81 with SL ≥ 4 σ, in the redshift range 0.002 < z < 0.069, over a total path length c Δ z = 116,000 km s -1 (Δ z = 0.387). In this second paper, we evaluate the physical properties of these Lyα absorbers and compare them to their high- z counterparts. The SL ≥ 4 σ distribution of Doppler parameters is similar to that at high redshift, with ⟨ b ⟩ = 38.0 ± 15.7 km s -1 . The true Doppler parameter may be somewhat lower, owing to component blends and nonthermal velocities. The distribution of equivalent widths exhibits a significant break at ≤ 133 mÅ, with an increasing number of weak absorbers (10 mÅ < < 100 mÅ). Adopting a curve of growth with b = 25 ± 5 km s -1 and applying a sensitivity correction as a function of equivalent width and wavelength, we derive the distribution in column density, N for 12.5 ≤ log ≤ 14.0. We find no redshift evolution within the current sample at z < 0.07, but we do see a significant decline in d / dz compared to values at z > 1.6. Similar to the high equivalent width ( > 240 mÅ) absorbers, the number density of low- absorbers at z = 0 is well above the extrapolation of d / dz from z > 2, but we observe no difference in the mean evolution of d / dz between absorbers of high ( > 240 mÅ) and low ( ≤ 100 mÅ) equivalent width. While previous work has suggested slower evolution in number density of lower absorbers, our new data do not support this conclusion. A consistent evolutionary pattern is that the slowing in the evolution of the low column density clouds occurs at lower redshift than for the higher column density clouds. A 4-5 σ signal in the two-point correlation function of Lyα absorbers for velocity separations Δ v ≤ 150 km s -1 is consistent with results at high z , but with somewhat greater amplitude. Applying a photoionization correction, we find that the low- z Lyα forest may contain ~20% of the total number of baryons, with closure parameter Ω Lyα = (0.008 ± 0.001) h , for a standard absorber size and ionizing radiation field. Some of these clouds appear to be primordial matter, owing to the lack of detected metals (Si III) in a composite spectrum, although current limits on composite metallicity are not strong.