Color-Redshift Relations and Photometric Redshift Estimations of Quasars in Large Sky Surveys

With a recently constructed composite quasar spectrum and the X2 minimization technique, we describe a general method for estimating the photometric redshifts of a large sample of quasars by deriving theoretical color-redshift relations and comparing the theoretical colors with the observed ones. We estimated the photometric redshifts from the 5-band SDSS photometric data of 18678 quasars in the first major data release of SDSS and compared them with their spectroscopic redshifts. The difference is less than 0.1 for 47% of the quasars and less than 0.2 for 68%. Based on the calculation of the theoretical color-color diagrams of stars, galaxies and quasars both on the SDSS system and on the BATC system, we expect that we would be able to select candidates of high redshift quasars more efficaciously with the latter than with the former, provided the BATC survey can detect objects with magnitudes fainter than 21.     

Hyperfine structure and interstellar curves of growth

The effects of the hyperfine structure (HFS) that is present in some interstellar absorption lines are investigated in the case of a single absorbing cloud. If the respective total equivalent widths of two or more unresolved HFS multiplets measured in relatively low-resolution spectra are analysed specifically by means of a curve of growth, the column density N (X) and the linewidth parameter b (X) inferred for absorbing species X in the cloud will generally be in error if the HFS is ignored. The fundamental physical effect is the reduced line saturation that arises because the total column density is divided among the HFS levels of the ground atomic level, each of which generally gives rise to an HFS line at a different wavelength. For nuclear spins   I = 3/2  and   I = 5/2  , theoretical curves of growth are calculated for some of the resonance lines of some alkali atoms, for each of four illustrative choices of the parameter  α=Δ/ b   , the ratio of the HFS splitting in the ground atomic level to the linewidth. Applications of the results to interstellar absorption by Na  i , K  i and Al  iii are emphasized. HFS is, fortunately, unimportant for most interstellar lines, however. Among the 35 elements that have been detected in diffuse clouds via interstellar absorption in the ultraviolet/optical spectral region, the most abundant isotope of each of 25 of these shows no HFS, because   I = 0  or, in the relevant ground atomic level,   J = 0  .     

Advances in radiative transfer

This review describes advances in radiative transfer theory since about 1985. We stress fundamental aspects and emphasize modern methods for the numerical solution of the transfer equation for spatially multidimensional problems, for both unpolarized and polarized radiation. We restrict the discussion to two-level atoms with noninverted populations for given temperature, density and velocity fields. Unfortunately this article was originally published with typesetter's errors: The correct publication date was 25 February 2006, not 3 January 2006. The content was not in the final form. The publishers wish to apologize for this mistake. The online version of the original version can be found at /10.1007/s00159-005-0025-8.