The statistical and physical properties of the low-redshift Lyα forest observed with the Hubble Space Telescope/STIS

Romeel Davé, Todd M. Tripp

Research output: Contribution to journalArticlepeer-review

72 Scopus citations


We examine the Lyα absorber population at z < 0.3 detected in spectra of the quasars PG 0953+415 and H1821+643 taken with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope. We compare their statistical properties to those in carefully constructed mock quasar spectra drawn from a cosmological hydrodynamic simulation of a Λ-dominated cold dark matter universe. We find very good agreement in the column density and b-parameter distributions, down to the smallest observable absorbers with NHI ≈ 1012.3 cm-2. The observed absorber population is complete for NHI ≳ 1013 cm-2, with a column density distribution slope of β = 2.04 ± 0.23 and a median b-parameter of 21 km s-1 above this limit. The intergalactic gas giving rise to these weak absorbers is analogous to that at high redshift, located in diffuse large-scale structures that are highly photoionized by the metagalactic UV flux, though a greater number arise within shock-heated warm gas. The density, temperature, and column density of these absorbers follow similar relationships of those at high redshift, though with substantially larger scatter due to the shock-heated gas. The b-parameters typically have a significant contribution from thermal broadening, which facilitates a measurement of the low-z intergalactic medium temperature as traced by Lyα absorbers. From our simulation we estimate TIGM ∼ 5000 K, with an upper limit of 104 K, at the mean density. The agreement in predicted and observed amplitude of the column density distributions allows us to measure the H I photoionization rate at z̄ = 0.17 to be ΓHI = 10-13.3±0.7 s-1 (estimated modeling uncertainty), close to predictions based on quasar properties.

Original languageEnglish (US)
Pages (from-to)528-537
Number of pages10
JournalAstrophysical Journal
Issue number2 PART 1
StatePublished - Jun 1 2001


  • Cosmology: observations
  • Cosmology: theory
  • Intergalactic medium
  • Quasars: absorption lines

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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