Discovering bright quasars at intermediate redshifts based on the optical/near-IR colors
arXiv:1207.0204 · doi:10.1088/0004-6256/146/4/100
Abstract
Identifications of quasars at intermediate redshifts (2.2<z<3.5) are inefficient in most previous quasar surveys as their optical colors are similar to those of stars. The near-IR K-band excess technique has been suggested to overcome this difficulty. Our study also proposed to use optical/near-IR colors for selecting z<4 quasars. To this method, we selected 105 unidentified bright targets with i<18.5 from the quasar candidates of SDSS DR6 with both SDSS ugriz optical and UKIDSS YJHK near-IR photometric data, which satisfy our proposed Y-K/g-z criterion and have photometric redshifts between 2.2 and 3.5 estimated from the 9-band SDSS-UKIDSS data. 43 of them were observed with the 2.16m telescope of NAOC in 2012. 36 of them were identified as quasars at 2.1<z<3.4. High success rate of discovering these quasars in the SDSS spectroscopic surveyed area demonstrates the robustness of both the Y-K/g-z selection criterion and the photometric redshift estimation technique. We also used the above criterion to investigate the possible star contamination rate to the quasar candidates of SDSS DR6, and found that it is much higher in selecting 3<z<3.5 quasar candidates than selecting lower redshift ones (z<2.2). The significant improvement in the photometric redshift estimation by using the 9-band SDSS-UKIDSS data than using the 5-band SDSS data is demonstrated and a catalog of 7,727 unidentified quasar candidates with photometric redshifts between 2.2 and 3.5 is provided. We also tested the Y-K/g-z selection criterion with the SDSS-III/DR9 quasar catalog, and found 96.2% of 17,999 DR9 quasars with UKIDSS Y and K-band data satisfy our criterion. With some samples of red and type II quasars, we found that 88% and 96.5% of them can be selected by the Y-K/g-z criterion respectively, which supports that using the Y-K/g-z criterion we can efficiently select both unobscured and obscured quasars. (abridged)
32 pages, 7 figures, 4 tables, accepted to AJ
References in corpus (20)
- The UKIRT Infrared Deep Sky Survey (UKIDSS)
- A Catalog of Quasar Properties from SDSS DR7
- Biases in Virial Black Hole Masses: An SDSS Perspective
- The WFCAM Science Archive
- The Sloan Digital Sky Survey Quasar Catalog IV. Fifth Data Release
- A Catalog of Broad Absorption Line Quasars in Sloan Digital Sky Survey Data Release 5
- The Sloan Digital Sky Survey quasar catalog: ninth data release
- Fe II Emission in 14 Low-Redshift Quasars: I - Observations
- Characterizing the Mid-IR Extragalactic Sky with WISE and SDSS
- FIRST-2MASS Red Quasars: Transitional Objects Emerging from the Dust
- The FIRST-2MASS Red Quasar Survey
- Quasar Selection Based on Photometric Variability
- Luminous K-band Selected Quasars from UKIDSS
- The correlations between optical variability and physical parameters of quasars in SDSS Stripe 82
- A Spectroscopic Survey of Faint Quasars in the SDSS Deep Stripe: I. Preliminary Results from the Co-added Catalog
- A Pilot Survey for KX QSOs in the UKIDSS Ultra Deep Survey Field
- The large area KX quasar catalogue: I. Analysis of the photometric redshift selection and the complete quasar catalogue
- Near Infrared Photometric properties of 130,000 Quasars: An SDSS-UKIDSS matched catalog
- The Optical and Near-Infrared Properties of 2837 Quasars in the UKIRT Infrared Deep Sky Survey (UKIDSS)
- Exploring the Variable Sky with the Sloan Digital Sky Survey