The amplitude of mass density fluctuations at z~3.25 from the Lyα forest of Q1422+231
Abstract
The realspace opticaldepth distribution along the line of sight to the QSO Q1422+231 is recovered from two HIRES spectra using a modified version of the inversion method proposed by Nusser & Haehnelt. The first two moments of the truncated opticaldepth distribution are used to constrain the densityfluctuation amplitude of the intergalactic medium (IGM) assuming that the IGM is photoionized by a metagalactic UV background and obeys a temperaturedensity relation. The fluctuation amplitude and the powerlaw index α of the relation between gas and neutral hydrogen (Hi) density are degenerate. The rms of the IGM density at z~3.25 estimated from the first spectrum is α√(exp[(1.8±0.27)^{2}&alpha^{2M}];  1), with 1.56<α<2 for plausible reionization histories. This corresponds to 0.9<~σ<~2.1 with σ(α=1.7)=1.44+/0.3. The values obtained from the second spectrum are higher by ~20per cent. If the IGM density traces the dark matter (DM) as suggested by numerical simulations we have measured the fluctuation amplitude of the DM density at an effective Jeans scale of a few 100kpc. For cold dark matter (CDM)like power spectra the amplitude of dark matter fluctuations on these small scales depends on the cosmological density parameter Ω. For power spectra normalized to reproduce the space density of presentday clusters and with a slope parameter of Γ=0.21 consistent with the observed galaxy power spectrum, the inferred Ω can be expressed as: Ω=0.61(α/1.7)^{1.3}(x_{J}/0.62)^{ 0.6} for a flat universe, and Ω=0.91(α/1.7)^{1.3}(x_{J}/0.62)^{ 0.7} for a λ=0 universe. x_{J} is the effective Jeans scale in (comoving) h^{1}Mpc. Based on a suite of detailed mock spectra the 1σ error is ~25per cent. The estimates increase with increasing Γ. For the second spectrum we obtain 15per cent lower values.
 Publication:

Monthly Notices of the Royal Astronomical Society
 Pub Date:
 April 2000
 DOI:
 10.1046/j.13658711.2000.03263.x
 arXiv:
 arXiv:astroph/9906406
 Bibcode:
 2000MNRAS.313..364N
 Keywords:

 INTERGALACTIC MEDIUM;
 QUASARS: ABSORPTION LINES;
 COSMOLOGY: OBSERVATIONS;
 COSMOLOGY: THEORY;
 DARK MATTER;
 LARGESCALE STRUCTURE OF UNIVERSE;
 Astrophysics
 EPrint:
 14 pages, submitted to MNRAS