Chen Jian-sheng

Variation of QSO color indices with redshift

Abstract In this paper we discuss the effect of features in the spectrum of QSOs on their color indices and the consequent variation of the latter with the redshift. As the redshift increases, the various absorption features shortwards of the Lyα emission line enter into the visible region and alter the way in which the color indices vary with the redshift, while the effects of the intrinsic power-law spectrum and emission lines on the color indices become secondary. The largest effect is due to the Lyman limit absorption system. These results are checked using the color indices calculated from IUE spectra.

Star counts and Galactic structure

We use a model of the Galactic stellar distribution to analyse the BATC star count data toward two high Galactic latitude fields. Since star counts at high Galactic latitudes are not strongly related to the radial distribution, they are very suitable for the study of the vertical distribution of the Galaxy. The vertical density distribution of the stars shows the contribution of the thin disc, the thick disc and the stellar halo of the Galaxy. We give quantitative descriptions of these components in terms of exponential discs and a de Vaucouleurs halo. We find that the observed counts support an axial ratio of c/a~0.5, implying a more flattened halo. We consider that it is possible that the halo has two subpopulations, i.e. a flattened inner halo and a spherical outer halo in the Milky Way.

H-alpha + [NII] observations of the HII regions in M81

In a first of a series of studies of the Hα + [N II] emission from nearby spiral galaxies, we present measurements of Hα + [N II] emission from H II regions in M81. Our method uses large-field CCD images and long-slit spectra and is part of the ongoing Beijing-Arizona-Taipei-Connecticut Sky Survey (the BATC survey). The CCD images are taken with the National Astronomical Observatories of China (NAOC) 0.6/0.9 m f/3 Schmidt telescope at the Xinglong Observing Station, using a multicolor filter set. Spectra of 10 of the brightest H II regions are obtained using the NAOC 2.16 m telescope with a Tek 1024 × 1024 CCD. The continua of the spectra are calibrated by flux-calibrated images taken from the Schmidt observations. We determine the continuum component of our Hα + [N II] image via interpolation from the more accurately measured backgrounds (M81 starlight) obtained from the two neighboring (in wavelength) BATC filter images. We use the calibrated fluxes of Hα + [N II] emission from the spectra to normalize this interpolated, continuum-subtracted Hα + [N II] image. We estimate the zero-point uncertainty of the measured Hα + [N II] emission flux to be ~8%. A catalog of Hα + [N II] fluxes for 456 H II regions is provided, with those fluxes being on a more consistent linear scale than previously available. The logarithmically binned Hα + [N II] luminosity function of H II regions is found to have slope α = -0.70, consistent with previous results (which allowed α = -0.5 to -0.8). From the overall Hα + [N II] luminosity of the H II regions, the star formation rate of M81 is found to be ~0.68 M⊙ yr-1, modulo uncertainty with extinction corrections.

Studies of CIV associated absorption systems of QSOs (I) Samples

Abstract For the purpose of investigating the environment of high-redshift QSOs, we had carried out a statistic study of associated CIV absorption lines. This is the first paper of the study. In the current paper, we present an unbiased homogeneous CIV absorption line sample of 228 QSOs, which includes most of the published QSO spectra data involving CIV absorption lines with high or medium resolution. The CIV absorption systems are selected with 7 criteria. We extracted 229 as our underlying sample from a total of 534 CIV absorption doublets of 228 QSOs. 4 sub-samples are also defined.

A photometric study of the BAO-CCD BVRI system

Abstract The BAO Schmidt telescope with a Thomson CCD of 576 × 384 pixels was used for BVRI photometry. 9 secondary photometric standard stars in 3 CCD frames were observed on Oct. 31, 1989, to establish the color equations. Some problems relevant to the capability of CCD photometry are discussed.

Objective prism survey of QSO candidates in the field centered at 00H00M + 00° 00' paper v. discussion of survey results☆

Abstract This is the last in the series of papers on our visual search for QSO candidates, using low dispersion objective prism plates obtained with the UK Schmidt Telescope. We discuss and summarize some results at the current stage of this project. As a supplement to the Q ⩾ 4 candidates published in the previous papers we give in this paper the subset of Q = 3 candidates which have ultraviolet excess. Our survey may suffer from many types of contamination since any object was considered as a QSO candidate if we could not unambiguously classify it as a star, a galaxy or some other known type of object. Despite this, many of our candidates are interesting enough for follow-up slit spectroscopy. This will also provide us with useful information for improving our selection criteria.

A re-examination of the correlation between the Lyman-alpha absorption line density and the emission redshift in QSOs

Abstract Following our previous paper [1] we here re-examine the correlation between the absorption line density in Lα forests N ( Z abs ) and the emission redshift Z em using a sample of 15 QSOs with medium to high redshifts with Lα forests. The present sample covers a greater range in redshift than [1], the emission redshifts ranging between 1.715 and 3.780, and the absorption redshifts between 1.501 and 3.780. The statistical method used also differs somewhat. Results of analysis show once again that N ( Z abs ) clearly depends on Z em , not only in the sense that the mean density over the entire forest is the greater for greater z em , but more importantly in the sense that the density over a given range of Z abs increases with increasing Z em . This last result poses serious difficulty for the current “intervening hypothesis” for the origin of the absorption lines.