Master Thesis defense by Guozhen Ma

Title: A Complete CENSUS of Quasars

Abstract: The investigations of quasi-stellar objects (quasars) have suffered from selection effects of different color criteria for a long time, due to the limited samples to produce color criteria and inevitable imprecision caused by the degeneracy of spectral information in photometry. Here we point out the necessity and importance of a pure spectroscopic selection of quasars for a complete census of them and hence for a better view of the galaxy evolution and the universe, which is also feasible with the help of the forthcoming 4-metre Multi-Object Spectroscopic Telescope (4MOST).

We observe 50 quasar candidates (mainly red quasar candidates) with abnormal colors which make them unlikely to be regarded as quasars by common color criteria focusing on blue point sources, and find out that 45 of them are indeed quasars, including 23 normal quasars, 18 BAL quasars, 2 detached BAL quasars, and 2 peculiar quasars. The other 5 objects are expected to be stars as we simply select them close to the stellar locus. Such large quasar populations in the point sources with uncommon colors, especially in those which have red g − r and J − K colors, indicate that many quasars may be red or have peculiar colors and they have been overlooked by normal color criteria.

We elaborate our observation process, spectral reduction procedures, and spectral analysis methods in Chapter 2, Chapter 3, and Chapter 4 respectively. In Chapter 5, we present all the spectra we have analyzed and explain in detail how we determine their redshifts and amount of dust extinction. Besides, the overall results are displayed to highlight the incompleteness of quasar populations selected by color criteria.

In the end, we propose a quantitative way to measure the redshifts of the detached BAL quasars that don’t have any high ionization emission lines in their spectra. Moreover, we suggest a detailed dusty wind model including the required inner structure of the outflows to interpret the peculiar spectral features of the detached BAL quasars.


  • Johan Fynbo, University of Copenhagen, Niels Bohr Insitute


  • Jérôme Chenevez, DTU Space