Evolution of Ly-alpha clouds - Observational biases due to line crowding

Astronomy and Astrophysics – Astronomy

Scientific paper

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Absorption Spectra, Hydrogen Clouds, Lyman Alpha Radiation, Quasars, Cosmology, Line Spectra, Red Shift

Scientific paper

The intrinsic properties and the evolution with redshift of Lyman-α absorption line clouds is estimated by counting lines as a function of redshift. The authors simulate spectra, with and without photon counting noise, in order to see what biases there are in deriving cloud properties. The line samples and the line counts are biased by the crowding of the lines. The bias results in an underestimate of the evolutionary rate exponent γ. The authors believe that the actual value is at least as large as γ ≈ 2, and may be as large as γ ≈ 3. An exponential distribution exp (-W/W0*) of intrinsic line equivalent widths W produces an apparent plateau in the distribution of inferred column densities. Both the crowding bias and the noise enhance this redshift dependent plateau and results in an overestimate of the scale of the distribution W0*.

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