科研之友
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摘要
Using numerical simulations of structure formation, we investigate several methods for determining the strength of the proximity effect in the H I Ly alpha forest. We analyze three high-resolution (similar to 10 kpc) redshift snapshots ((z) over bar = 4, 3, and 2.25) of a Hydro-Particle-Mesh simulation to obtain realistic absorption spectra of the H I Ly alpha forest. We model the proximity effect along the simulated sight lines with a simple analytical prescription based on the assumed quasar luminosity and the intensity of the cosmic UV background (UVB). We begin our analysis investigating the intrinsic biases thought to arise in the widely adopted standard technique of combining multiple lines of sight when searching for the proximity effect. We confirm the existence of these biases, albeit smaller than previously predicted with simple Monte Carlo simulations. We then concentrate on the analysis of the proximity effect along individual lines of sight. After determining its strength with a fiducial value of the UVB intensity, we construct the proximity effect strength distribution (PESD). We confirm that the PESD inferred from the simple averaging technique accurately recovers the input strength of the proximity effect at all redshifts. Moreover, the PESD closely follows the behaviors found in observed samples of quasar spectra. However, the PESD obtained from our new simulated sight lines presents some differences to that of simple Monte Carlo simulations. At all redshifts, we find a smaller dispersion of the strength parameters, the source of the corresponding smaller biases found when combining multiple lines of sight. After developing three new theoretical methods for recovering the strength of the proximity effect on individual lines of sight, we compare their accuracy to the PESD from the simple averaging technique. All our new approaches are based on the maximization of the likelihood function, albeit invoking some modifications. The new techniques presented here, in spite of their complexity, fail to recover the input proximity effect in an unbiased way, presumably due to some (unknown) higher order correlations in the spectrum. Thus, employing complex three-dimensional simulations, we provide strong evidence in favor of the PESD obtained from the simple averaging technique, as a method of estimating the UVB intensity, free of any intrinsic biases.
- 出版日期2010-10-10
