Work

A Lower Bound on the Cosmic Baryon Density

Year: 1997

Type: article

Abstract: We derive lower bounds on the cosmic baryon density from the requirement that the high-redshift intergalactic medium (IGM) contain enough neutral hydrogen to produce the observed Lyα absorption in quasar spectra. These analytic bounds follow from a key theoretical assumption—that absorbing structures are on average no more extended in redshift space than in real space—which is likely to hold in the gravitational instability picture of the Lyα forest, independently of the details of the cosmological model. The other ingredients that enter these bounds are an estimate of (or lower limit to) the intensity of the photoionizing UV background from quasars, a temperature T ~ 10 4 K for the "warm" photoionized IGM that produces most of the Lyα absorption, a value of the Hubble constant, and observational estimates of the mean Lyα flux decrement or, for a more restrictive bound, the distribution function P (τ) of Lyα optical depths. With plausible estimates of the quasar UV background and , the mean decrement bound implies a baryon density parameter Ω b ≳ 0.0125 h -2 , where h ≡ H 0 /(100 km s -1 Mpc -1 ). A recent observational determination of P (τ) implies that Ω b ≳ 0.0125 h -2 even for a conservative estimate of the quasar UV background, and Ω b ≳ 0.018 h -2 for a more reasonable estimate. These bounds are consistent with recent low estimates of the primordial deuterium-to-hydrogen ratio (D/H) P , which imply that Ω b ≈ 0.025 h -2 when combined with standard big bang nucleosynthesis. Since the bounds account only for baryons in the warm IGM, their combination with the nucleosynthesis constraint implies that most of the baryons in the universe at z ~ 2-4 were distributed in diffuse intergalactic gas rather than in stars or compact dark objects. The P (τ) bound on Ω b is incompatible with some recent high estimates of (D/H) P , unless one drops the assumptions of standard big bang nucleosynthesis or abandons the idea that Lyα forest lines originate in the smooth, large-scale structures of photoionized gas that arise in gravitational instability theories.

Source: The Astrophysical Journal

DOI: 10.1086/304893

Authors: David H. Weinberg, Jordi Miralda‐Escude, Lars Hernquist, Neal Katz

Institutions: The Ohio State University, University of Pennsylvania, University of California, Santa Cruz, University of California Observatories, University of Massachusetts Amherst

Language: English

Topic: Galaxies: Formation, Evolution, Phenomena

Subfield: Astronomy and Astrophysics

Field: Physics and Astronomy

Domain: Physical Sciences

Open Access status: bronze

Funders: National Science Foundation, National Aeronautics and Space Administration

Awards: QUASAR ABSORPTION IN THE UV: PROBING THE INTERGALA, The Formation of Galaxies and Large-Scale Structure