Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993apj...418..601z&link_type=abstract
Astrophysical Journal v.418, p.601
Astronomy and Astrophysics
Astronomy
51
Cosmology: Observations, Galaxies: Intergalactic Medium, Galaxies: Quasars: Absorption Lines, Galaxies: Quasars: General
Scientific paper
We have measured DA, the mean relative depression of quasar continuum between Lyα and Lyβ emission, for a large sample of quasars. We have carried out the measurements by simply adding the Lyα lines (excluding the lines in identified systems) together. The big advantage of this approach over the conventional method is that the continuum is fitted locally by employing high S/N and relatively high resolution spectra, and this leads to a more reliable estimate of the continuum level in the Lyα forest region. Also, the contamination of DA by metal lines is easily removed.
Extending the method proposed by Zuo (1993) and Lu & Zuo (1993), we have determined γint, the intrinsic evolutionary rate exponent of Lyα forest lines. For redshifts between 1.7 and 3.7, our best estimate is γint = 2.87±0.23 (for our Sample 2 QSOs). However, if we combine our measurements with the DA values determined for Zem > 4 QSOs by Schneider, Schmidt, & Gunn (1989a, b, 1991), a better fit is obtained by adopting a broken power-law form with the break point at Z = 3.11 and γ1 = 2.82 for Z < 3.11 and γ2 = 5.07 for Z > 3.11. Our method does not require line deblending and is quite robust to certain kinds of systematic errors in DA measurements.
We have explored how a simple Lyα cloud mode in which all (baryonic) clouds are expanding and at the same time keeping roughly a r-2 density profile, combined with the evolution of the ionizing background, can be used `to understand the observed DA evolution with redshift. This simple model offers a definite theory for the formation of absorption line profiles and leads to a line distribution η(Z, N) = AN-β(1 + Z)γint. We have calculated the line profiles for some impact parameter bimp and redshift Z, and have found that they can be roughly approximated by Gaussian profiles with a b = 30 km s-1. This approximation is especially good for low-N lines and at low Z.
Lu Limin
Zuo Lin
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