arXiv:astro-ph/0312427v2 19 Dec 2003
Nature, 426, 810-812 (2003)
A gravitationally lensed quasar with quadruple images separated
by 14.62 arcseconds
Naohisa Inada1, Masamune Oguri1, Bartosz Pindor2, Joseph F. Hennawi2,
Kuenley Chiu3, Wei Zheng3, Shin-Ichi Ichikawa4, Michael D. Gregg5,6,
Robert H. Becker5,6, Yasushi Suto1, Michael A. Strauss2, Edwin L. Turner2,
Charles R. Keeton7, James Annis8, Francisco J. Castander9, Daniel J. Eisenstein10,
Joshua A. Frieman7,8, Masataka Fukugita11, James E. Gunn2, David E. Johnston7,
Stephen M. Kent8, Robert C. Nichol12, Gordon T. Richards2, Hans-Walter Rix13,
Erin Scott Sheldon7, Neta A. Bahcall2, J. Brinkmann14,ˇZeljko Ivezi´ c2,
Don Q. Lamb7, Timothy A. McKay15, Donald P. Schneider16& Donald G. York7,17
1Department of Physics, School of Science, University of Tokyo, 113-0033, Japan
2Princeton University Observatory, Peyton Hall, Princeton, NJ 08544, USA
3Department of Physics and Astronomy, Johns Hopkins University, 3701 San Martin Drive, Baltimore,
MD 21218, USA
4National Astronomical Observatory, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
5Department of Physics, University of California at Davis, 1 Shields Avenue, Davis, CA 95616, USA
6Institute of Geophysics and Planetary Physics, Lawrence Livermore National Laboratory, L-413,
7000 East Aveneu, Livermore, CA 94550, USA
7Department of Astronomy and Astrophysics, University of Chicago, 5640 South Ellis Avenue, Chicago,
IL 60637, USA
8Fermi National Accelerator Laboratory, P.O. Box 500, Batavia, IL 60510, USA
9Institut d’Estudis Espacials de Catalunya/CSIC, Gran Capita 2-4, 08034 Barcelona, Spain
10Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721, USA
11Institute for Cosmic Ray Research, University of Tokyo, 5-1-5 Kashiwa, Kashiwa City,
Chiba 277-8582, Japan
12Department of Physics, Carnegie Mellon University, Pittsburgh, PA 15213, USA
13Max-Planck Institute for Astronomy, K¨ onigstuhl 17, D-69117 Heidelberg, Germany
14Apache Point Observatory, P.O. Box 59, Sunspot, NM88349, USA
15Department of Physics, University of Michigan, 500 East University Avenue, Ann Arbor, MI 48109, USA
16Department of Astronomy and Astrophysics, Pennsylvania State University, 525 Davey Laboratory,
University Park, PA 16802, USA
17Enrico Fermi Institute, University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637, USA
Gravitational lensing is a powerful tool for the study of the distribution of
dark matter in the Universe. The cold-dark-matter model of the formation of
large-scale structures predicts1−6the existence of quasars gravitationally lensed
by concentrations of dark matter7so massive that the quasar images would
be split by over 7 arcsec.Numerous searches8−11for large-separation lensed
quasars have, however, been unsuccessful. All of the roughly 70 lensed quasars
known12, including the first lensed quasar discovered13, have smaller separations
that can be explained in terms of galaxy-scale concentrations of baryonic mat-
ter. Although gravitationally lensed galaxies14with large separations are known,
quasars are more useful cosmological probes because of the simplicity of the
resulting lens systems. Here we report the discovery of a lensed quasar, SDSS
J1004+4112, which has a maximum separation between the components of 14.62
arcsec. Such a large separation means that the lensing object must be dominated
by dark matter. Our results are fully consistent with theoretical expectations3−5
based on the cold-dark-matter model.
For bright quasars (i-band magnitude i < 19), the probability of gravitational lensing15
is only about 0.1%; the majority of these lenses have small separations, due to a single massive
galaxy. The fraction of large-separation lensed quasars is predicted to be 0.01% or less3−5;
thus it is not surprising that none have been found to date8−11. In order to find such objects,
we need samples of tens of thousands of quasars, such as generated by the Sloan Digital Sky
Survey16,17(SDSS). The SDSS is conducting both a photometric survey18−22using five broad
optical bands22(u, g, r, i and z) and a spectroscopic survey23of 10,000 square degrees of the
sky centered approximately on the North Galactic Pole, using a dedicated wide-field 2.5-m
telescope at the Apache Point Observatory.
We searched for large-separation lensed quasars in a sample of ∼29,500 spectroscopically-
confirmed SDSS quasars24at redshifts z of 0.6 − 2.3, a sample roughly three times larger
than those used in previous searches. Even with this large sample, the expected number of
large-separation lensed quasars is of the order of unity. In the field around each quasar in
the sample, we searched for stellar objects with colours differing by less than 0.1 from those
of the quasar, with separations between 7.′′0 and 60.′′0 and with flux greater than one-tenth
that of the quasar. SDSS J1004+4112 was identified as a ‘quadruple’ large-separation lensed
quasar candidate using these criteria. Only one of the four components (component B, see
below) has an SDSS spectrum (the SDSS hardware23does not allow pairs of objects sepa-
rated by less than 55′′to be observed on a single plate), and therefore, we obtained spectra of
all four components using the Keck I telescope at the W. M. Keck Observatory. The results
are shown in Fig. 1. All four components indeed show quasar-like features, with all emission
lines giving a consistent redshift z = 1.734±0.002; the velocity differences of the quasar
components are ∼ 100 km s−1, comparable to the observational uncertainty. Although it
is not obvious from Fig. 1, there are C IV absorption line systems at z = 1.732 in each
of the four quasar spectra; this is an absorption system associated with the quasar itself,
further supporting the lensing hypothesis: the four quasar images are from the same physical
source. The differences in their spectra may be explained by the modest time-variability of
the source quasar over ∼1 year, the expected gravitational lensing time-delay25among those
Additional strong support for the lensing hypothesis comes from the identification of
the galaxy cluster responsible for the large-separation lensing. From the observed image
separations (the maximum separation is 14.′′62, between images B and C), we infer that the
lensing object should have a velocity dispersion in excess of 600 km s−1. Thus the lensing
object cannot be a single galaxy, but must be rather a group or cluster of galaxies that
has a sufficiently concentrated distribution of dark matter. To identify the lensing object,
we obtained deep optical images of the system using the Subaru telescope of the National
Astronomical Observatory, Japan. The result is shown in Fig. 2. A number of galaxies are
clearly detected around component G, suggesting that it is the most luminous galaxy of the
cluster. We obtained a spectrum of component G using the Keck I telescope. The spectrum
shows a number of absorption features characteristic of a early-type galaxy at a redshift of
z = 0.6799 ± 0.0001. We also obtained spectra of two faint galaxies immediately to the
south-west of component G (Fig. 2) using the Faint Object Camera and Spectrograph26
of the Subaru telescope. The redshifts of these two faint galaxies are z = 0.6751 ± 0.0001,
strongly suggesting a cluster of galaxies at z∼0.68 centred on component G. Clusters are
dominated by elliptical galaxies, which all have very similar spectral energy distributions.
Many of the faint galaxies in Fig. 2 (∼40 galaxies around component G) have colours
that are similar to that of component G. The colours are consistent with the expected colours
of elliptical galaxies at z∼0.68 (g − r∼1.8 and r − i∼1.1). In addition, there is an X-ray
source in this direction detected by the ROSAT All-Sky Survey27(0.236 counts per second
in a 473-s exposure). The emission, however, comes most probably from the quasar, because
the detected X-ray flux is too strong for typical clusters of galaxies at z = 0.68. Finally, we
note two possible arclets (highly distorted images of background galaxies due to gravitational
lensing) in Fig. 2 (marked as ‘arc?’) close to component D. If future observations confirm
that the arclets are indeed lensed background galaxies, they will provide strong additional
constraints on the total mass distribution of the lensing cluster.
The lensing interpretation is further supported by a theoretical model of SDSS J1004+4112.
We fitted the positions of the four quasar components with a singular isothermal ellipsoid
(SIE) plus external shear model using lens modeling software28. The best-fit model is il-
lustrated in Fig. 3. The positions and relative brightnesses of all components agree well
with the lens model predictions. The centre of the lensing mass is offset from the centre of
component G by about 10 kpc at the cluster redshift, but brightest galaxy of a cluster is not
always found exactly at the centre of the potential well of that cluster.
The identical redshifts (z = 1.734) and the spectral energy distributions of the four
lensed components, the existence of a lensing cluster of galaxies (z = 0.68), and the presence
of possible arclets confirm the hypothesis that the quasar is lensed by this cluster. Further-
more, a theoretical lensing model involving the cluster and external shear simultaneously
accounts for the observed geometry of the system and the relative brightness of the images.
The present work represents the discovery of a long-predicted but previously undetected
population of large-separation lensed quasars.
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Acknowledgments Funding for the creation and distribution of the SDSS Archive has
been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the Na-
tional Aeronautics and Space Administration, the National Science Foundation, the U.S.
Department of Energy, the Japanese Monbukagakusho, and the Max Planck Society. The
SDSS Web site is http://www.sdss.org/.
The SDSS is managed by the Astrophysical Research Consortium (ARC) for the Partic-
ipating Institutions. The Participating Institutions are The University of Chicago, Fermilab,
the Institute for Advanced Study, the Japan Participation Group, The Johns Hopkins Uni-
versity, Los Alamos National Laboratory, the Max-Planck-Institute for Astronomy (MPIA),
the Max-Planck-Institute for Astrophysics (MPA), New Mexico State University, University
of Pittsburgh, Princeton University, the United States Naval Observatory, and the University
This paper is based in part on data collected at the Subaru Telescope, which is oper-
ated by the National Astronomical Observatory of Japan, W. M. Keck Observatory, which
is operated as a scientific partnership among the California Institute of Technology, the
University of California, and the National Aeronautics and Space Administration, and the
Apache Point Observatory (APO) 3.5-meter telescope, which is owned and operated by the
Astrophysical Research Consortium.
Part of this work was performed under the auspices of the U.S. Department of Energy
at the University of California Lawrence Livermore National Laboratory.
Competing interests statement The authors declare that they have no competing finan-
Correspondence and requests for materials should be addressed to N.I.
ASTROMETRY AND PHOTOMETRY FOR SDSS J1004+4112
10 04 34.794
10 04 34.910
10 04 33.823
10 04 34.056
10 04 34.170
+41 12 39.29
+41 12 42.79
+41 12 34.82
+41 12 48.95
+41 12 43.66
aR.A., right ascension; Dec., declination. RA and Dec. are given in the J2000 system. These celestial
coordinates were measured on the basis of the celestial coordinates of component B. The positional
errors of components A, C, and D (not including the absolute positional errors of component B) are
0.′′01 and that of component G is 0.′′05 per coordinate.
bg, r, i and z mean the magnitudes of each band.
cSeparation angles relative to component B
LyaC IVMg II Si IV
Ca II H&K
flux density (erg/sec/cm/cm/angstrom)
G–band Mg I
Figure 1 The Keck spectra of the four quasar components A–D and the brightest galaxy G in the
lensing cluster. See Fig. 2 for these identifications (A–D, and G). The data were taken using the
Low-Resolution Imaging Spectrometer29(LRIS) of the Keck I telescope. The exposure times were
900 s for each component. The dispersion is 1.09˚ A pixel−1. The data were reduced in a standard
method using IRAF (IRAF is the image reduction and analysis facility, distributed by the National
Optical Astronomy Observatories, which are operated by the Association of Universities for Re-
search in Astronomy, Inc., under cooperative agreement with the National Science Foundation).
The black solid line, the red solid line, the green solid line, and the blue solid line represent the
spectra of components A, B, C, and D, respectively. The vertical gray dotted lines (3323.6˚ A, 3818.7
˚ A, 4235.1˚ A, 5218.5˚ A, and 7651.8˚ A) represent the positions of emission lines of the respective ions
redshifted to z = 1.734 of Lyα (1215.67˚ A), Si IV (1396.76˚ A), C IV (1549.06˚ A), C III] (1908.73˚ A),
and Mg II (2798.75˚ A), respectively. All emission lines are clearly at the same redshift. The orange
solid line represents the Keck spectrum of component G at the same dispersion. The exposure time
was also 900 s for component G. The vertical thinner gray dotted lines (3933.7˚ A, 3968.5˚ A, 4304.4
˚ A, and 5175.3˚ A) represent the positions of absorption lines of the respective ions redshifted to
z = 0.680 of Ca II H&K (3933.7˚ A and 3968.5˚ A), G-band (4304.4˚ A), and Mg I b-band (5175.3
˚ A), respectively. There are no data below ∼ 4900˚ A in the spectrum of component G.
Figure 2 The gri composite Subaru image of the field around SDSS J1004+4112. The data were
taken using the Subaru Prime Focus Camera30of the Subaru telescope. The magnitude limit is
i ≈ 26.0. The central 60′′square is shown in an expanded view. The four quasar components are
marked as A, B, C and D, and the bright galaxy located between the four quasar components is
marked as G. The separation between components A and D is 12.′′77, and that between components
B and C is 14.′′62. The positions (J2000) and the magnitudes of the components A–D and the
brightest galaxy (component G) between the four quasar components are summarized in Table 1.
Many faint galaxies can be seen — their positions and colours are consistent with being members
of a cluster (z = 0.68) centred on component G. Two possible arclets (marked as ‘arc?’) can also
be seen. The seeing had a full-width at half-maximum of 0.′′6.
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relative R.A. (arcsecond)
relative dec. (arcsecond)
center of lens mass
source quasar position
Figure 3 The best fit lens model prediction compared with the observation. We used a lensing
model of a singular isothermal ellipsoid (SIE) with external shear. The best-fit model has an
Einstein radius of the SIE model αe= 6.′′906 (corresponding to a velocity dispersion ∼ 700 km sec−1
at the cluster redshift), magnitude and position angle of the shear γ = 0.250 and θγ= −60.925◦
(measured East of North), and ellipticity and its position angle e = 0.498 and θe = 21.434◦
(measured East of North), with a source quasar position (∆R.A., ∆Dec.)=(−7.′′124, −0.′′574) and
a centre of lensing mass (∆R.A., ∆Dec.)=(−7.′′387, −0.′′004) relative to the centre of component A.
The black filled circles represent the observed positions of components A, B, C, D and G, and the
red filled circles represent the predicted positions of components A–D. The green solid line is the
position of the caustic in the source plane, and the blue dashed line represent the critical curve in
the image plane. The small brown filled cross is the predicted position of the source quasar, and the
small orange triangle is the predicted position of the center of the lens mass. The differences between
the observed and modelled image positions are much smaller than the observational uncertainties.
The flux ratios predicted from the model, B/A, C/A and D/A are 0.78, 0.43 and 0.22, respectively.
The total magnification of the quasar images which is predicted by the model is 56.48.
predicted flux ratios are close to the observational results; B/A= 0.69±0.04, C/A= 0.46±0.02,
and D/A= 0.25±0.01 (measured from the i band image). Microlensing by substructures and/or
reddening by the Mg II absorption line systems that are seen in each spectrum might be the cause
of the differences between the predicted flux ratios and the observations.