(News & views on the paper by Xue-Bing Wu et al., RAA, 2012, vol.12, 1185-1190)
Identifying High-Redshift Quasars with Near-Infrared Surveys
(Kavli Institute for Astronomy and Astrophysics, Peking University and Steward Observatory, University of Arizona)
Understanding the evolution of supermassive black holes (BHs) in the universe and the interaction between BH growth and galaxy evolution is one of the key goals of galaxy formation and observational cosmology. Supermassive BHs at the center of massive galaxies obtain most of their masses through Eddington-limited accretion during an active phase as quasars or active galactic nuclei (e.g., Yu & Tremaine 2002, Marconi et al. 2004, Barger et al. 2005). Surveys of high-redshift quasars provide the fundamental data set to study the evolution of supermassive BHs across cosmic time. Observations show that the number density of quasars peaks at z=2-3, and declines rapidly towards lower and higher redshift (e.g., Boyle et al. 2000, Fan et al. 2001, Richards et al. 2006). Quasar surveys at z > 3 map the initial growth phase of supermassive BH evolution in the universe.
However, high-redshift quasar surveys are technically challenging for several reasons: z>3 quasars are rare -- their surface density is about 1 per deg2 at i~20; they have to be selected against a large number of galactic stars using multicolor imaging surveys which are subject to various selection biases and incompleteness, in particular, traditional optical survey selection is sensitive to dust reddening, therefore is likely to be missing quasars that are even modestly reddened. Establishment of complete samples of quasars is a main goal, indeed a prerequisite to understanding early supermassive BH growth history.
Wu et al. (2012) present new discoveries of six quasars at z>2.4, with the highest redshift at z=4.6. They used a new photometric selection method based mainly on near-infrared imaging. As shown in their previous work (e.g., Wu & Jia 2010), the near-infrared selected candidate sample is less contaminated by galactic stars, can achieve very high completeness, and is far less affected by dust extinction. Therefore, although this initial sample size is only modest, it shows promise of building up large samples based on future large near-infrared sky surveys.
The other feature to be noted about this paper is the fact that all spectroscopic identification is carried out by telescope facilities accessible to the Chinese astronomical community. Six of the quasars were found using the Lijiang 2.4m, and the highest redshift z=4.6 quasar was confirmed using the 6.5m MMT as part of the Chinese Telescope Access Program (TAP). Work presented here should be viewed as a precursor study of the large scale quasar surveys to be carried out by the Guo Shou Jing - LAMOST telescope in the coming years (e.g., Zhao et al. 2012), which is expected to produce a few hundred thousands quasars, including tens of thousands at z>3.