The Serendipitous Observation of a Gravitationally Lensed Galaxy at z = 0.9057 from the Blanco Cosmology Survey: The Elliot Arc
ABSTRACT We report on the serendipitous discovery in the Blanco Cosmology Survey (BCS) imaging data of a z = 0.9057 galaxy that is being strongly lensed by a massive galaxy cluster at a redshift of z = 0.3838. The lens (BCS J2352–5452) was discovered while examining i- and z-band images being acquired in 2006 October during a BCS observing run. Follow-up spectroscopic observations with the Gemini Multi-Object Spectrograph instrument on the Gemini-South 8 m telescope confirmed the lensing nature of this system. Using weak-plus-strong lensing, velocity dispersion, cluster richness N 200, and fitting to a Navarro-Frenk-White (NFW) cluster mass density profile, we have made three independent estimates of the mass M 200 which are all very consistent with each other. The combination of the results from the three methods gives M 200 = (5.1 ± 1.3) × 1014 M ☉, which is fully consistent with the individual measurements. The final NFW concentration c 200 from the combined fit is c 200 = 5.4+1.4 – 1.1. We have compared our measurements of M 200 and c 200 with predictions for (1) clusters from ΛCDM simulations, (2) lensing-selected clusters from simulations, and (3) a real sample of cluster lenses. We find that we are most compatible with the predictions for ΛCDM simulations for lensing clusters, and we see no evidence based on this one system for an increased concentration compared to ΛCDM. Finally, using the flux measured from the [O II]3727 line we have determined the star formation rate of the source galaxy and find it to be rather modest given the assumed lens magnification.
arXiv:1108.4681v1 [astro-ph.CO] 23 Aug 2011
The serendipitous observation of a gravitationally lensed galaxy
at z = 0.9057 from the Blanco Cosmology Survey: The Elliot Arc
E. J. Buckley-Geer,1H. Lin,1E. R. Drabek,1,19S. S. Allam,1D. L. Tucker,1R. Armstrong,2
W. A. Barkhouse,3E. Bertin,4M. Brodwin,5,6S. Desai,15J. A. Frieman,1,7S. M. Hansen,8
F. W. High,7J. J. Mohr,9,10,11Y.-T. Lin,12,13C.-C. Ngeow,14,15A. Rest,16R. C. Smith,17
J. Song,18A. Zenteno,9,10
1Center for Particle Astrophysics, Fermi National Accelerator Laboratory, P.O. Box 500, Batavia, IL
2National Center for Supercomputing Applications,University of Illinois, 1205 West Clark Street, Ur-
banan, IL 61801
3Department of Physics & Astrophysics,University of North Dakota, Grand Forks, ND 58202
4Institut d’Astrophysique de Paris, UMR 7095 CNRS, Universit´ e Pierre et Marie Curie, 98 bis boulevard
Arago, F-75014 Paris, France
5Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138
6W. M. Keck Postdoctoral Fellow at the Harvard-Smithsonian Center for Astrophysics
7University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637
8National Science Foundation Astronomy & Astrophysics Postdoctoral Fellow, University of California
Observatories & Department of Astronomy, University of California, Santa Cruz, CA 95064
9Department of Physics, Ludwig-Maximilians-Universit¨ at, Scheinerstr. 1, 81679 M¨ unchen, Germany
10Excellence Cluster Universe, Boltzmannstr. 2, 85748 Garching, Germany
11Max-Planck-Institut f¨ ur extraterrestrische Physik, Giessenbachstr. 85748 Garching, Germany
12Institute for Physics and Mathematics of the Universe, University of Tokyo, 5-1-5 Kashiwa-no-ha,
Kashiwa-shi, Chiba 277- 8568, Japan
13Institute of Astronomy & Astrophysics, Academia Sinica, Taipei, Taiwan
14Graduate Institute of Astronomy, National Central University, No. 300 Jonghda Rd, Jhongli City 32001
15Department of Astronomy, University of Illinois, 1002 West Green Street, Urbana, IL 61801
16Space Telescope Science Institute, 3700 San Martin Dr., Baltimore, MD 21218
17Cerro Tololo Inter-American Observatory, National Optical Astronomy Observatory, La Serena, Chile
18Department of Physics, University of Michigan, 450 Church St. Ann Arbor, MI 48109
19School of Physics, University of Exeter, Stocker Road, Exeter EX4 4QL, United Kingdom
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We report on the serendipitous discovery in the Blanco Cosmology Survey
(BCS) imaging data of a z = 0.9057 galaxy that is being strongly lensed by a
massive galaxy cluster at a redshift of z = 0.3838. The lens (BCS J2352-5452)
was discovered while examining i- and z-band images being acquired in October
2006 during a BCS observing run. Follow-up spectroscopic observations with
the GMOS instrument on the Gemini South 8m telescope confirmed the lensing
nature of this system. Using weak plus strong lensing, velocity dispersion, cluster
richness N200, and fitting to an NFW cluster mass density profile, we have made
three independent estimates of the mass M200which are all very consistent with
each other. The combination of the results from the three methods gives M200=
(5.1±1.3)×1014M⊙, which is fully consistent with the individual measurements.
The final NFW concentration c200from the combined fit is c200= 5.4+1.4
compared our measurements of M200 and c200 with predictions for (a) clusters
from ΛCDM simulations, (b) lensing selected clusters from simulations, and (c)
a real sample of cluster lenses. We find that we are most compatible with the
predictions for ΛCDM simulations for lensing clusters, and we see no evidence
based on this one system for an increased concentration compared to ΛCDM.
Finally, using the flux measured from the [OII]3727 line we have determined the
star formation rate (SFR) of the source galaxy and find it to be rather modest
given the assumed lens magnification.
−1.1. We have
Subject headings: gravitational lensing: strong — gravitational lensing: weak —
Strong gravitational lenses offer unique opportunities to study cosmology, dark mat-
ter, galactic structure, and galaxy evolution.
namely the lenses themselves, that are selected based on total mass rather than luminos-
ity or surface brightness. The majority of lenses discovered in the past decade were found
through dedicated surveys using a variety of techniques. For example, the Sloan Digital
Sky Survey (SDSS) data have been used to effectively select lens candidates from rich clus-
ters (Hennawi et al. 2008) through intermediate scale clusters (Allam et al. 2007; Lin et al.
2009) to individual galaxies (Bolton et al. 2008; Willis et al. 2006). Other searches using the
CFHTLS (Cabanac et al. 2007) and COSMOS fields (Faure et al. 2008; Jackson et al. 2008)
have yielded 40 and 70 lens candidates respectively. These searches cover the range of giant
arcs with Einstein radii θEIN> 10′′all the way to small arcs produced by single lens galaxies
They also provide a sample of galaxies,
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with θEIN< 3′′.
In this paper we report on the serendipitous discovery of a strongly lensed z = 0.9057
galaxy in the Blanco Cosmology Survey (BCS) imaging data. The lens is a rich cluster
containing a prominent central brightest cluster galaxy (BCG) and has a redshift of z =
0.3838. Cluster-scale lenses are particularly useful as they allow us to study the effects of
strong lensing in the core of the cluster and weak lensing in the outer regions. Strong lensing
provides constraints on the mass contained within the Einstein radius of the arcs whereas
weak lensing provides information on the mass profiles in the outer reaches of the cluster.
Combining the two measurements allows us to make tighter constraints on the mass M200and
the concentration c200, of an NFW (Navarro, Frenk, & White 1995) model of the cluster mass
density profile, over a wider range of radii than would be possible with either method alone
(Natarajan et al. 1998, 2002; Brada˘ c et al. 2006, 2008a,b; Diego et al. 2007; Limousin et al.
2007; Hicks et al. 2007; Deb et al. 2008; Merten et al. 2009; Oguri et al. 2009). In addition,
if one has spectroscopic redshifts for the member galaxies one can determine the cluster
velocity dispersion, assuming the cluster is virialized, and hence obtain an independent
estimate for M200(Becker et al. 2007). Finally one can also derive an M200estimate from
the maxBCG cluster richness N200(Hansen et al. 2005; Johnston et al. 2007). These three
different methods, strong plus weak lensing, cluster velocity dispersion, and optical richness,
provide independent estimates of M200 (M200 is defined as the mass within a sphere of
overdensity 200 times the critical density at the redshift z) and can then be combined to
obtain improved constraints on M200 and c200. Measurements of the concentration from
strong lensing clusters is of particular interest as recent publications suggest that they may
be more concentrated than one would expect from ΛCDM models (Broadhurst & Barkana
2008; Oguri & Blandford 2009).
The paper is organized as follows. In § 2 we describe the Blanco Cosmology Survey.
Then in § 3 we discuss the initial discovery and the spectroscopic follow-up that led to
confirmation of the system as a gravitational lens, the data reduction, the properties of the
cluster, the extraction of the redshifts, and finally the measurement of the cluster velocity
dispersion and estimate of the cluster mass. In § 4 we summarize the strong lensing features
of the system. In § 5 we describe the weak lensing measurements. In § 6 we present the results
of combining of the strong and weak lensing results and the final mass constraints derived
from combining the lensing results with the velocity dispersion and richness measurements.
We describe the source galaxy star formation rate measurements in § 7 and finally in § 8 we
conclude. We assume a flat cosmology with ΩM= 0.3, ΩΛ= 0.7, and H0= 70 km s−1Mpc−1,
unless otherwise noted.
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2.The BCS Survey
The Blanco Cosmology Survey (BCS) is a 60-night NOAO imaging survey program
(2005-2008), using the Mosaic-II camera on the Blanco 4m telescope at CTIO, that has
uniformly imaged 75deg2of the sky in the SDSS griz bands in preparation for cluster finding
with the South Pole Telescope (SPT) (Vanderlinde et al. 2010) and other millimeter-wave
experiments. The depths in each band were chosen to allow the estimation of photometric
redshifts for L ≥ L∗galaxies out to a redshift of z = 1 and to detect galaxies to 0.5L∗at
5σ to these same redshifts. The survey was divided into two fields to allow efficient use of
the allotted nights between October and December. Both fields lie near δ = −55◦which
allows for overlap with the SPT. One field is centered near α = 23.5 hr and the other is at
α = 5.5 hr. In addition to the large science fields, BCS also covers 7 small fields that overlap
large spectroscopic surveys so that photometric redshifts (photo-z’s) using BCS data can be
trained and tested using a sample of over 5,000 galaxies.
3.Discovery of the lens and spectroscopic follow-up
The lens BCS J2351-5452 was discovered serendipitously while examining i- and z-band
images being acquired in October 2006 during the yearly BCS observing run. The discoverer
(EJB-G) decided to name it “The Elliot Arc” in honor of her then 8 year old nephew. Table 1
lists the observed images along with seeing conditions. Fig. 1 shows a gri color image of the
source, lens and surrounding environment (the pixel scale is 0.268′′per pixel). The source
forms a purple ring-like structure of radius ∼ 7.5′′with multiple distinct bright regions. The
lens is the BCG at the center of a large galaxy cluster. Photometric measurements estimated
the redshift of the cluster at z ∼ 0.4, using the expected g −r and r −i red sequence colors,
and also provided a photo-z for the source of z ∼ 0.7, as described below.
We obtained Gemini Multi-Object Spectrograph (GMOS) spectra of the source and a
number of the neighboring galaxies (Lin et al. 2007). We targeted the regions of the source
labeled A1-A4 in Fig. 2, and photometric properties of these bright knots are summarized
in Table 2. In addition we selected 51 more objects for a total of 55 spectra. The additional
objects were selected using their colors in order to pick out likely cluster member galaxies.
Fig. 3 shows the r − i versus i color-magnitude diagram (top plot) and the g − r vs. r − i
color-color diagram (bottom plot) of the field. The blue squares in the bottom panel of Fig. 3
show the four targeted knots in the lensed arcs. The green curve is an Scd galaxy model
(Coleman, Wu, & Weedman 1980) with the green circles indicating a photometric redshift
for the arc of z ∼ 0.7. Note this is not a detailed photo-z fit, but is just a rough estimate
meant to show that the arc is likely at a redshift higher than the cluster redshift. Highest
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target priority was given to the arc knots and to the BCG. Then cluster red sequence galaxy
targets were selected using the simple color cuts 1.55 ≤ g − r ≤ 1.9 and 0.6 ≤ r − i ≤ 0.73
(also shown in the bottom panel of Fig. 3), which approximate the more detailed final
cluster membership criteria described below in §3.2. Red sequence galaxies with i < 21.6
(3′′-diameter SExtractor aperture magnitudes) were selected, with higher priority given to
brighter galaxies with i(3′′) ≤ 21. Additional non-cluster targets lying outside the cluster
color selection box were added at lowest priority.
We used the GMOS R150 grating + the GG455 filter in order obtain spectra with about
4600 – 9000˚ A wavelength coverage. This was designed to cover the [OII] 3727 emission line
expected at ∼ 6300˚ A, given the photo-z estimate of ∼ 0.7 for the arcs as well as the Mg
absorption features at ∼ 7000˚ A (and the 4000˚ A break at ∼ 5600˚ A) for the z ∼ 0.4 cluster
We used 2 MOS masks in order to fully target these cluster galaxies (along with the
arcs) for spectroscopy. Each mask had a 3600 second exposure time split into 4 900-second
exposures for cosmic ray removal. We also took standard Cu-Ar lamp spectra for wavelength
calibrations and standard star spectra for flux calibrations. All data were taken in queue
observing mode. A summary of the observations is given in Table 1.
3.1. Data Reduction
The BCS imaging data were processed using the Dark Energy Survey data management
system (DESDM V3) which is under development at UIUC/NCSA/Fermilab (Mohr et al.
2008; Ngeow et al. 2006; Zenteno et al. 2011). The images are corrected for instrumental
effects which include crosstalk correction, pupil ghost correction, overscan correction, trim-
ming, bias subtraction, flat fielding and illumination correction. The images are then astro-
metrically calibrated and remapped for later coaddition. For photometric data, a photomet-
ric calibration is applied to the single-epoch and coadd object photometry. The AstrOmatic
software1SExtractor (Bertin & Arnouts 1996), SCAMP (Bertin 2006) and SWarp (Bertin et al.
2002) are used for cataloging, astrometric refinement and remapping for coaddition over each
image. We have used the coadded images in the griz bands for this analysis.
The spectroscopic data were processed using the standard data reduction package pro-
vided by Gemini that runs in the IRAF framework2. We used version 1.9.1. This produced