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Title: DISCOVERING BRIGHT QUASARS AT INTERMEDIATE REDSHIFTS BASED ON OPTICAL/NEAR-INFRARED COLORS

Abstract

The identification of quasars at intermediate redshifts (2.2 < z < 3.5) has been inefficient in most previous quasar surveys since the optical colors of quasars are similar to those of stars. The near-IR K-band excess technique has been suggested to overcome this difficulty. Our recent study also proposed to use optical/near-IR colors for selecting z < 4 quasars. To verify the effectiveness of this method, we selected a list of 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 nine-band SDSS-UKIDSS data. We observed 43 targets with the BFOSC instrument on the 2.16 m optical telescope at Xinglong station of the National Astronomical Observatory of China in the spring of 2012. We spectroscopically identified 36 targets as quasars with redshifts between 2.1 and 3.4. The high success rate of discovering these quasars in the SDSS spectroscopic surveyed area further demonstrates the robustness of both the Y – K/g – z selection criterion and the photometric redshift estimation technique. We also usedmore » the above criterion to investigate the possible stellar contamination rate among the quasar candidates of SDSS DR6, and found that the rate is much higher when selecting 3 < z < 3.5 quasar candidates than when selecting lower redshift candidates (z < 2.2). The significant improvement in the photometric redshift estimation when using the nine-band SDSS-UKIDSS data over the five-band SDSS data is demonstrated and a catalog of 7727 unidentified quasar candidates in SDSS DR6 selected with optical/near-IR colors and having photometric redshifts between 2.2 and 3.5 is provided. We also tested the Y – K/g – z selection criterion with the recently released SDSS-III/DR9 quasar catalog and found that 96.2% of 17,999 DR9 quasars with UKIDSS Y- and K-band data satisfy our criterion. With some available samples of red quasars and type II quasars, we find that 88% and 96.5% of these objects can be selected by the Y – K/g – z criterion, respectively, which supports our claim that using the Y – K/g – z criterion efficiently selects both unobscured and obscured quasars. We discuss the implications of our results on the ongoing and upcoming large optical and near-IR sky surveys.« less

Authors:
; ; ; ;  [1]
  1. Department of Astronomy, Peking University, Beijing 100871 (China)
Publication Date:
OSTI Identifier:
22273331
Resource Type:
Journal Article
Journal Name:
Astronomical Journal (New York, N.Y. Online)
Additional Journal Information:
Journal Volume: 146; Journal Issue: 4; Other Information: Country of input: International Atomic Energy Agency (IAEA); Journal ID: ISSN 1538-3881
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ASTRONOMY; ASTROPHYSICS; COLOR; GALAXIES; NEAR INFRARED RADIATION; PHOTOMETRY; QUASARS; RED SHIFT; STARS; TELESCOPES

Citation Formats

Wu, Xue-Bing, Zuo, Wenwen, Yang, Jinyi, Yang, Qian, and Wang, Feige. DISCOVERING BRIGHT QUASARS AT INTERMEDIATE REDSHIFTS BASED ON OPTICAL/NEAR-INFRARED COLORS. United States: N. p., 2013. Web. doi:10.1088/0004-6256/146/4/100.
Wu, Xue-Bing, Zuo, Wenwen, Yang, Jinyi, Yang, Qian, & Wang, Feige. DISCOVERING BRIGHT QUASARS AT INTERMEDIATE REDSHIFTS BASED ON OPTICAL/NEAR-INFRARED COLORS. United States. https://doi.org/10.1088/0004-6256/146/4/100
Wu, Xue-Bing, Zuo, Wenwen, Yang, Jinyi, Yang, Qian, and Wang, Feige. 2013. "DISCOVERING BRIGHT QUASARS AT INTERMEDIATE REDSHIFTS BASED ON OPTICAL/NEAR-INFRARED COLORS". United States. https://doi.org/10.1088/0004-6256/146/4/100.
@article{osti_22273331,
title = {DISCOVERING BRIGHT QUASARS AT INTERMEDIATE REDSHIFTS BASED ON OPTICAL/NEAR-INFRARED COLORS},
author = {Wu, Xue-Bing and Zuo, Wenwen and Yang, Jinyi and Yang, Qian and Wang, Feige},
abstractNote = {The identification of quasars at intermediate redshifts (2.2 < z < 3.5) has been inefficient in most previous quasar surveys since the optical colors of quasars are similar to those of stars. The near-IR K-band excess technique has been suggested to overcome this difficulty. Our recent study also proposed to use optical/near-IR colors for selecting z < 4 quasars. To verify the effectiveness of this method, we selected a list of 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 nine-band SDSS-UKIDSS data. We observed 43 targets with the BFOSC instrument on the 2.16 m optical telescope at Xinglong station of the National Astronomical Observatory of China in the spring of 2012. We spectroscopically identified 36 targets as quasars with redshifts between 2.1 and 3.4. The high success rate of discovering these quasars in the SDSS spectroscopic surveyed area further 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 stellar contamination rate among the quasar candidates of SDSS DR6, and found that the rate is much higher when selecting 3 < z < 3.5 quasar candidates than when selecting lower redshift candidates (z < 2.2). The significant improvement in the photometric redshift estimation when using the nine-band SDSS-UKIDSS data over the five-band SDSS data is demonstrated and a catalog of 7727 unidentified quasar candidates in SDSS DR6 selected with optical/near-IR colors and having photometric redshifts between 2.2 and 3.5 is provided. We also tested the Y – K/g – z selection criterion with the recently released SDSS-III/DR9 quasar catalog and found that 96.2% of 17,999 DR9 quasars with UKIDSS Y- and K-band data satisfy our criterion. With some available samples of red quasars and type II quasars, we find that 88% and 96.5% of these objects can be selected by the Y – K/g – z criterion, respectively, which supports our claim that using the Y – K/g – z criterion efficiently selects both unobscured and obscured quasars. We discuss the implications of our results on the ongoing and upcoming large optical and near-IR sky surveys.},
doi = {10.1088/0004-6256/146/4/100},
url = {https://www.osti.gov/biblio/22273331}, journal = {Astronomical Journal (New York, N.Y. Online)},
issn = {1538-3881},
number = 4,
volume = 146,
place = {United States},
year = {Tue Oct 01 00:00:00 EDT 2013},
month = {Tue Oct 01 00:00:00 EDT 2013}
}