The Study of QSO Absorption Lines using 3D Radiative Transfer

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Recently, it has been considered that the Lya absorption systems in QSO spectra can be accounted for in terms of the absorption by intergalactic overdense fluctuations. However, all of previous works have been based upon optically-thin or local optical-depth approximations. Here, we present 3D radiative transfer calculations on the cosmic reionization and the formation of QSO absorption line systems.The resultant ionization degrees in the universe are different from place to place by more than three orders of magnitude. Due to such inhomogeneous ionization structure, relatively low ionization regions could produce strong absorption in QSO spectra. To make a direct comparison with the observations, we have simulated absorption lines. First, we focus on Lya absorption. It has turned out that the so-called Gunn-Peterson effect appears as the overlapping effect of absorption lines just when low dispersion spectroscopy is made. In order to compare the simulations quantitatively with observa … More tions, we have assessed the continuum depression, D_A. It is found that any model with a constant UV intensity does not match the observed trend that D_A tends to grow quickly at higher redshifts up to 5. This implies that the metagalactic UV intensity must decrease rapidly with z at z > 5 by two orders of magnitude at least. If the well fitted value of UV intensity at z = 5 is unchanged at higher redshifts, the reionization epoch is estimated to be z_<reion> > 7- However, the Lya absorption is not appropriate to determine the reionization epoch more accurately, because Lyα is too strongly depleted even if the mean neutral fraction is less than 10^<-2>.Therefore, three conditions are required for a line in order to investigate the universe at z > 5 : (1) line emission is detectable, (2) it has lower line opacity than Lya, and (3) it has lower extinction against dust because young star-forming galaxies are often dust-enshrouded. The most favorable solution is Ha absorption lines. Actually, we have found that the Ha continuum depression traces the reionization history more accurately. Therefore, it is concluded that Ha forest is a more powerful tool to probe the universe at z > 5. The wavelengths of Ha forest drop on 3μm【less than or similar】λ_<Hα>【less than or similar】7μm at 4【less than or similar】z【less than or similar】10. Thus, the observations can be done with Subaru IRCS. Less