Five New High-Redshift Quasar Lenses from the Sloan Digital Sky Survey
ABSTRACT We report the discovery of five gravitationally lensed quasars from the Sloan Digital Sky Survey (SDSS). All five systems are selected as two-image lensed quasar candidates from a sample of high-redshift (z > 2.2) SDSS quasars. We confirmed their lensing nature with additional imaging and spectroscopic observations. The new systems are SDSS J0819+5356 (source redshift zs = 2.237, lens redshift zl = 0.294, and image separation θ = 404), SDSS J1254+2235 (zs = 3.626, θ = 156), SDSS J1258+1657 (zs = 2.702, θ = 128), SDSS J1339+1310 (zs = 2.243, θ = 169), and SDSS J1400+3134 (zs = 3.317, θ = 174). We estimate the lens redshifts of the latter four systems to be zl = 0.2-0.8 from the colors and magnitudes of the lensing galaxies. We find that the image configurations of all systems are well reproduced by standard mass models. Although these lenses will not be included in our statistical sample of zs < 2.2 lenses, they expand the number of lensed quasars which can be used for high-redshift galaxy and quasar studies.
- SourceAvailable from: ArXiv[show abstract] [hide abstract]
ABSTRACT: We present observations of a new double-image gravitational lens system, ULAS J082016.1+081216, of image separation 2.3" and high (~6) flux ratio. The system is selected from the Sloan Digital Sky Survey spectroscopic quasar list using new high-quality images from the UKIRT Deep Sky Survey (UKIDSS). The lensed quasar has a source redshift of 2.024, and we identify the lens galaxy as a faint red object of redshift 0.803+/-0.001. Three other objects from the UKIDSS survey, selected in the same way, were found not to be lens systems. Together with the earlier lens found using this method, the SDSS-UKIDSS lenses have the potential to significantly increase the number of quasar lenses found in SDSS, to extend the survey to higher flux ratios and lower separations, and to give greater completeness which is important for statistical purposes. Comment: Accepted by MNRAS. 6 pages, 5 figuresMonthly Notices of the Royal Astronomical Society 06/2009; · 5.52 Impact Factor
arXiv:0809.0912v2 [astro-ph] 9 Feb 2009
Five New High-Redshift Quasar Lenses from the Sloan Digital
Naohisa Inada,1Masamune Oguri,2Min-Su Shin,3Issha Kayo,4,5Michael A. Strauss,3
Tomoki Morokuma,6Donald P. Schneider,7Robert H. Becker,8,9Neta A. Bahcall,3and
Donald G. York10,11
We report the discovery of five gravitationally lensed quasars from the Sloan
Digital Sky Survey (SDSS). All five systems are selected as two-image lensed
quasar candidates from a sample of high-redshift (z > 2.2) SDSS quasars. We
confirmed their lensing nature with additional imaging and spectroscopic obser-
vations. The new systems are SDSS J0819+5356 (source redshift zs= 2.237, lens
redshift zl = 0.294, and image separation θ = 4.′′04), SDSS J1254+2235 (zs=
3.626, θ = 1.′′56), SDSS J1258+1657 (zs= 2.702, θ = 1.′′28), SDSS J1339+1310
(zs = 2.243, θ = 1.′′69), and SDSS J1400+3134 (zs = 3.317, θ = 1.′′74). We
estimate the lens redshifts of the latter four systems to be zl= 0.2−0.8 from the
colors and magnitudes of the lensing galaxies. We find that the image configu-
rations of all systems are well reproduced by standard mass models. Although
1Cosmic Radiation Laboratory, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan.
2Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, 2575 Sand Hill Road,
Menlo Park, CA 94025, USA.
3Princeton University Observatory, Peyton Hall, Princeton, NJ 08544, USA.
4Department of Physics and Astrophysics, Nagoya University, Chikusa-ku, Nagoya 464-8602, Japan.
5Institute for the Physics and Mathematics of the Universe, University of Tokyo, 5-1-5 Kashiwanoha,
Chiba 277-8582, Japan.
6National Astronomical Observatory, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan.
7Department of Astronomy and Astrophysics, The Pennsylvania State University, 525 Davey Laboratory,
University Park, PA 16802, USA.
8IGPP-LLNL, L-413, 7000 East Avenue, Livermore, CA 94550, USA.
9Department of Physics, University of California at Davis, 1 Shields Avenue, Davis, CA 95616, USA.
10Department of Astronomy and Astrophysics, The University of Chicago, 5640 South Ellis Avenue,
Chicago, IL 60637, USA.
11Enrico Fermi Institute, The University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637, USA.
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these lenses will not be included in our statistical sample of zs< 2.2 lenses, they
expand the number of lensed quasars which can be used for high-redshift galaxy
and quasar studies.
Subject headings: gravitational lensing — quasars: individual (SDSS J081959.79+535624.3,
SDSS J125418.95+223536.5, SDSS J125819.24+165717.6, SDSS J133907.13+131039.6,
Gravitationally lensed quasars are unique astronomical and cosmological tools, as de-
scribed in the review of Kochanek (2006). We can study the mass distributions of lensing
objects from individual mass modeling, as well as the substructures in lensing objects (e.g.,
Kochanek 1991; Mao & Schneider 1998). We can also investigate their interstellar media
from dust extinctions (e.g., Falco et al. 1999; Mu˜ noz et al. 2004) or absorption lines appear-
ing in spectra of multiple quasar images (e.g., Curran et al. 2007). The statistics of lensed
quasars and the measurement of time delays between lensed images are useful tools to con-
strain cosmological parameters (e.g., Refsdal 1964; Turner 1990; Fukugita et al. 1990). In
addition, lensed quasars sometimes provide opportunities to study the central structures
of quasar host galaxies in detail through microlensing events (e.g., Richards et al. 2004;
Poindexter et al. 2008).
Motivated by these ideas, astronomers have searched for lensed quasars using various
methods and wavebands. Roughly 100 lensed quasars have been identified to date (Kochanek
2006). A number of homogeneously selected samples have been constructed (e.g., Maoz et al.
1993), allowing statistical studies to be done. For example, the Cosmic Lens All Sky Survey
(CLASS; Myers et al. 2003; Browne et al. 2003) has created a sample of 22 lensed objects
selected from ∼16,000 radio sources. This sample has been used to obtain a variety of
cosmological and astrophysical results (e.g., Rusin & Tegmark 2001; Mitchell et al. 2005;
Chae et al. 2006).
The Sloan Digital Sky Survey (SDSS; York et al. 2000) has discovered ∼ 80,000 spectro-
scopically identified quasars (Schneider et al. 2007). We are conducting a survey of lensed
quasars selected from the large dataset of the SDSS. The survey, the SDSS Quasar Lens
Search (SQLS; Oguri et al. 2006, 2008a; Inada et al. 2008) has discovered more than 30
lensed quasars (e.g., Kayo et al. 2007; Oguri et al. 2008b, and references therein), making it
the current largest lensed quasar survey. The SQLS also recovered nine previously known
lensed quasars included in the SDSS footprint (Walsh et al. 1979; Weymann et al. 1980;
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Surdej et al. 1987; Bade et al. 1997; Oscoz et al. 1997; Schechter et al. 1998; Myers et al.
1999; Morgan et al. 2001; Magain et al. 1988). The first statistical sample of 11 SQLS lenses
(Inada et al. 2008) was constructed from the SDSS Data Release 3 quasar catalog (4188
deg2; Schneider et al. 2005), and used to constrain dark energy (Oguri et al. 2008a).
The SQLS restricts the statistical lens sample to zs < 2.2 because we cannot make
a well-defined quasar sample for homogeneous lens surveys at higher redshifts. The SDSS
quasars at zs> 2.2 are required to be point sources (see Richards et al. 2002), and therefore
they have a strong bias against the homogeneous lens candidate selection (Oguri et al. 2006;
Inada et al. 2008). However, the SQLS candidate finding algorithm can easily be extended to
locate higher redshift lensed quasars (Inada et al. 2008). Such high-redshift lensed quasars
can be used as astronomical and cosmological tools to study (high-redshift) lensing galaxies
(e.g., Kochanek et al. 2000) and constrain the Hubble constant (e.g., Oguri 2007). They are
also useful for detailed studies of (lensed) high-redshift quasars. In this paper, we report the
discoveries of five lensed quasars with high source redshifts (zs= 2.237–3.626). They were
selected as lensed quasar candidates from the SDSS data, and were confirmed as lenses with
the observations at the University of Hawaii 2.2-meter telescope (UH88), the Astrophysi-
cal Research Consortium 3.5-meter telescope (ARC 3.5m), and the 3.58-meter Telescopio
Nazionale Galileo (TNG 3.6m). All five candidates are confirmed to be double-image lensed
quasars, with image separations of 1.′′28–4.′′04.
The structure of this paper is as follows. Brief descriptions of the SDSS data and our
lens candidate selection algorithm are presented in § 2. We present the results of imaging and
spectroscopic observations to confirm the lensing hypotheses for the five objects and estimate
the redshifts of the lensing galaxies in § 3. We model the five lensed quasars in § 4 and summa-
rize our results in §5. We use a standard cosmological model with matter density ΩM= 0.27,
cosmological constant ΩΛ= 0.73, and Hubble constant h = H0/100kmsec−1Mpc−1= 0.71
(e.g., Spergel et al. 2003) throughout this paper.
2.SDSS Data and Candidate Selection
SDSS J0819+5356 was selected as a lens candidate in the SDSS-I, and the other four
lenses were selected as lens candidates in the SDSS-II Sloan Legacy Survey. The SDSS con-
sists of a photometric (Gunn et al. 1998) and a spectroscopic survey, and has mapped ap-
proximately 10,000 square degrees primarily in a region centered on the North Galactic Cap,
through the SDSS-I and the subsequent SDSS-II Legacy Survey. The survey was conducted
with a dedicated wide-field 2.5-m telescope (Gunn et al. 2006) at the Apache Point Observa-
tory in New Mexico, USA. The photometric survey uses five broad-band optical filters (ugriz,
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Fukugita et al. 1996). The spectroscopic survey is carried out with a multi-fiber spectrograph
covering 3800˚ A to 9200˚ A with a resolution of R ∼ 1800. The data in each imaging observa-
tion are processed by the photometric pipeline (Lupton et al. 2001), and then the target se-
lection pipelines (Eisenstein et al. 2001; Richards et al. 2002; Strauss et al. 2002) find quasar
and galaxy candidates; the candidates are tiled in each plate according to the algorithm of
Blanton et al. (2003). The SDSS produces very homogeneous data with an astrometric accu-
racy better than about 0.′′1 rms per coordinate (Pier et al. 2003) and photometric zeropoint
accuracy better than about 0.02 magnitude over the entire survey area (Hogg et al. 2001;
Smith et al. 2002; Ivezi´ c et al. 2004; Tucker et al. 2006; Padmanabhan et al. 2008). The
SDSS is continuously making its data public (Stoughton et al. 2002; Abazajian et al. 2003,
2004, 2005; Adelman-McCarthy 2006, 2007, 2008). The final release (Date Release Seven)
was made on 2008 October 31.
The lensed quasar candidate selection algorithm of the SQLS (Oguri et al. 2006; Inada et al.
2008) is composed of two parts. One is “morphological selection”, which selects candidates
as extended quasars using the difference between the shapes of each quasar and the Point
Spread Function (PSF) in each field. The other method is “color selection”, which finds
quasars with objects (usually fainter) within ?20′′, whose colors are similar to the quasars.
Although the selection algorithm is basically designed for quasars with zs < 2.2, we can
easily extend it to target higher redshift quasars by shifting to longer wavelength bands, as
H I absorption significantly reddens colors at wavelengths shortward of the Lyα emission
line. (Schneider et al. 1991; Fan 1999; Richards et al. 2002). For example, we can search
for quasars with zs?3.5 using information from the griz bands rather than the ugri bands
used at lower redshifts (Inada et al. 2008), and zs?4.8 using the riz bands. We selected
SDSS J0819+5356, SDSS J1258+1657, and SDSS J1400+3134 as lens candidates by mor-
phological selection with griz (or riz for SDSS J1400+3134), and SDSS J1254+2235 and
SDSS J1339+1310 by color selection with griz. SDSS J0819+5356 was not selected by color
selection despite its large image separation, because of the presence of the bright lensing
galaxy between the two stellar components. We note that SDSS J0819+5356 was also se-
lected as a possible lensed Lyα emitting galaxy by the algorithm described in Shin et al.
(2008) to identify strong galaxy-galaxy lenses.
The SDSS r-band images of the fields around each lensed quasar candidate are shown
in Figure 1. The SDSS asinh magnitudes (Lupton et al. 1999) without Galactic extinction
corrections and redshifts of the five objects are summarized in Table 1. The u-band asinh
magnitude of SDSS J1254+2235 is not given because it is undetected in the u-band. All five
candidates appear to be doubly-imaged lenses in the SDSS images, as we will confirm with
the imaging and spectroscopic follow-up observations described in the next section.
– 5 –
As described in Inada et al. (2008), our criteria to confirm the lensing hypothesis for
a candidate double-image lensed quasar are 1) the existence of a lensing object between
the two stellar (quasar) components, and 2) similar spectral energy distributions (SEDs) for
the two quasar images. All five candidates are marginally resolved in the SDSS imaging
data and spatially unresolved in the SDSS spectroscopic data (the fiber diameter is 3′′and
the minimum separation between each fiber on a single plate is ∼55′′), and therefore we
conducted optical/near-infrared imaging and spectroscopic follow-up observations to confirm
their lensing natures, using the UH88, ARC 3.5m, and TNG 3.6m telescopes.
We obtained V RI images for all five candidates and B images for SDSS J0819+5356
with the Tektronix 2048×2048 CCD camera (Tek2k, 0.′′22 pixel−1) at the UH88 telescope.
The observations were conducted on 2007 April 11, 2007 November 13, and 2008 March 6,
with typical seeing of FWHM∼0.′′8. The exposures were between 300 and 480 sec depending
on the magnitudes of the objects and the observing conditions in each night, and 800 sec for
the B band image of SDSS J0819+5356. The instruments, observing dates, and exposure
times for these observations are summarized in Tables 2 and 3.
The I-band images of all five candidates are shown in the left column of Figure 2. In
each image, we clearly detect two stellar components (denoted as A and B; A being the
brighter component) with typical separations of ∼1.′′5, except for SDSS J0819+5356, which
has a larger image separation of 4.′′04. The BV RI images for SDSS J0819+5356 clearly show
an extended object (component G) between components A and B, which we interpret as the
lensing galaxy. To see whether the other four candidates also have lensing galaxies between
the stellar components, we subtracted two PSFs from the V RI images of each candidate,
using nearby stars as PSF templates. In all 12 images, the V RI images of the four candidates,
there is extended residual flux between components A and B that we designate component
G. The I-band PSF subtracted images are shown in the lower four panels of the middle
column of Figure 2. The morphology of the lensing galaxy of SDSS J1400+3134 appears
to be unusual due to its low signal-to-noise ratio. Finally we subtracted two PSFs plus
an extended component modeled by a S´ ersic profile using GALFIT (Peng et al. 2002) from
the V RI images of the four candidates; the resulting images show virtually no residuals
(see the lower four panels of the right column in Figure 2). We also subtracted two PSFs,
and two PSFs plus a galaxy component, from the BV RI images of SDSS J0819+5356. The
results from the I-band image are shown in the top panels of the middle and right columns of