J/AJ/146/100 New 2.2<z<3.5 quasars from SDSS and UKIDSS (Wu+, 2013)
Discovering bright quasars at intermediate redshifts based on
optical/near-infrared colors.
Wu X.-B., Zuo W., Yang J., Yang Q., Wang F.
<Astron. J., 146, 100 (2013)>
=2013AJ....146..100W 2013AJ....146..100W
ADC_Keywords: Photometry, SDSS ; Redshifts ; QSOs ; Photometry, infrared
Keywords: galaxies: active - galaxies: high-redshift - quasars: emission lines -
quasars: general
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 2.16m 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 17999 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.
Description:
In March to May of 2012, we made spectroscopic observations over eight
nights of 43 bright quasar candidates (Table2) with photometric
redshifts at 2.2<zph<3.5 using the Baryon Acoustic Oscillations (BAO)
Faint Object Spectrograph and Camera (BFOSC) of the 2.16m optical
telescope at the Xinglong station of the National Astronomical
Observatories, Chinese Academy of Sciences (NAOC). A low resolution
grism with the dispersion of 198Å/mm, wavelength coverage from 3850
to 7000Å, and a spectral resolution of 2.97Å was used.
We selected the quasar targets from the SDSS DR6 1 million quasar
candidate catalog of Richards et al. (2009, cat J/ApJS/180/67) and
used both the SDSS and UKIDSS photometric data for further selection
and photometric redshift estimates to achieve a high success rate in
identifying 2.2<z<3.5 quasars (Table4).
File Summary:
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FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
table2.dat 119 43 Parameters and observation details of 43 quasar
candidates
table4.dat 85 7727 A catalog of 7727 SDSS-UKIDSS quasar candidates
at 2.2≤zph≤3.5 selected from Richards et
al. (2009, cat J/ApJS/180/67)
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See also:
VII/269 : SDSS Quasar Catalog, DR9Q (Paris+, 2012)
II/314 : UKIDSS-DR8 LAS, GCS and DXS Surveys (Lawrence+ 2012)
VII/260 : The SDSS-DR7 quasar catalog (Schneider+, 2010)
VII/241 : The 2dF QSO Redshift Survey (Croom+ 2004)
J/ApJS/199/3 : The quasars MMT-BOSS pilot survey (Ross+, 2012
J/AJ/144/49 : Quasars from SDSS-DR7, WISE and UKIDSS surveys (Wu+, 2012)
J/MNRAS/424/2876 : Large area KX quasar catalogue (Maddox+, 2012)
J/ApJ/753/125 : NIR spectroscopy follow-up of 60 SDSS-DR7 QSOs (Shen+, 2012)
J/ApJS/194/45 : QSO properties from SDSS-DR7 (Shen+, 2011)
J/AJ/141/105 : NIR-matched quasar catalog (Peth+, 2011)
J/AJ/142/78 : New 2.2<z<3 quasars from SDSS and UKIDSS (Wu+, 2011)
J/MNRAS/406/1583 : Quasar from SDSS and UKIDSS (Wu+, 2010)
J/ApJ/692/758 : BAL QSOs in SDSS-DR5 (Gibson+, 2009)
J/ApJS/180/67 : Photometric selection of SDSS quasars. II. (Richards+, 2009)
J/MNRAS/386/1605 : Luminous K-band selected QSOs from UKIDSS (Maddox+, 2008)
J/AJ/136/2373 : Type 2 quasars from SDSS (Reyes+, 2008)
J/ApJS/165/1 : BAL QSOs from SDSS DR3 (Trump+, 2006)
J/AJ/131/2788 : Redshifts in the SFQS survey (Jiang+, 2006)
J/AJ/131/2766 : Quasar luminosity function from SDSS-DR3 (Richards+, 2006)
Byte-by-byte Description of file: table2.dat
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Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 18 A18 --- SDSS SDSS quasar candidate name (JHHMMSS.ss+DDMMSS.s)
20- 30 A11 "Y:M:D" Date Date of observation
32- 35 I4 s Exp Exposure time
37- 41 F5.2 mag umag SDSS u-band AB magnitude
43- 47 F5.2 mag gmag SDSS g-band AB magnitude
49- 53 F5.2 mag rmag SDSS r-band AB magnitude
55- 59 F5.2 mag imag SDSS i-band AB magnitude
61- 65 F5.2 mag zmag SDSS z-band AB magnitude
67- 71 F5.2 mag Ymag UKIDSS Y band Vega magnitude
73- 77 F5.2 mag Jmag UKIDSS J band Vega magnitude
79- 83 F5.2 mag Hmag UKIDSS H band Vega magnitude
85- 89 F5.2 mag Kmag UKIDSS K band Vega magnitude
91- 95 F5.3 --- zph1 Richards et al. (2009, cat J/ApJS/180/67)
photometric redshift (G1)
97-101 F5.3 --- zph2 [2.225/3.55] This work photometric redshift (G2)
103-113 A11 --- Object Identification result (1)
115-119 F5.3 --- zsp ? Spectral redshift from our observations
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Note (1): F star, low S/N, quasar, quasar(BAL)=Broad Absorption Line quasars.
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Byte-by-byte Description of file: table4.dat
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Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 18 A18 --- SDSS SDSS quasar candidate name (JHHMMSS.ss+DDMMSS.s)
20- 25 F6.3 --- zph1 Richards et al. (2009, cat J/ApJS/180/67)
photometric redshift (G1)
27- 31 F5.3 --- zph2 [2.225/3.45] This work photometric redshift (G2)
33- 37 F5.2 mag umag SDSS u-band AB magnitude
39- 43 F5.2 mag gmag SDSS g-band AB magnitude
45- 49 F5.2 mag rmag SDSS r-band AB magnitude
51- 55 F5.2 mag imag SDSS i-band AB magnitude
57- 61 F5.2 mag zmag SDSS z-band AB magnitude
63- 67 F5.2 mag Ymag UKIDSS Y band Vega magnitude
69- 73 F5.2 mag Jmag UKIDSS J band Vega magnitude
75- 79 F5.2 mag Hmag UKIDSS H band Vega magnitude
81- 85 F5.2 mag Kmag UKIDSS K band Vega magnitude
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Global notes:
Note (G1): Obtained from the five-band SDSS data.
Note (G2): Obtained from the nine-band SDSS-UKIDSS data.
History:
From electronic version of the journal
(End) Greg Schwarz [AAS], Sylvain Guehenneux [CDS] 15-Jul-2014