Analysis of Strong Lens Candidates from Early Dark Energy Survey
Data
Brian Nord, Liz Buckley-Geer, Huan Lin, Tom Diehl, Hallie
Gaitsch, (Fermi National Accelerator Laboratory) Strong Lensing
Working Group (Dark Energy Survey)
www.darkenergydetectives.org
Over five observing seasons, which started in August 2013, the
Dark Energy Survey (DES) will carry out a wide field survey of 5000
square degrees of the Southern Galactic Cap. As much of the
wide-field area has not yet been systematically surveyed, we expect
to discover many new strongly lensed galaxies and quasars. DES has
identified 24 strong lens candidate objects (galaxy- and galaxy
clusters-scale) in data from the Science Verification season and
has performed spectroscopic follow-up on a subset of these
candidates as part of a Gemini Large and Long program. We present
the current state of progress on the photometric and spectroscopic
analysis of the lens candidate systems. To obtain precise lens and
source positions and to verify the candidate system as a lensing
system, we must obtain spectroscopic redshifts. In order to model
the lens potential to the required level of precision, we also
requirehigh-resolution imaging, both available at the Gemini South
facility.
Abstract
To select lenses with arc-like features we use a combination of
automated arc-finders, catalog searches and visual scans. We carry
out these searches on the annual DES data release. The first target
list of 24 candidates comes from the Science Verification season,
which was undertaken during the 2012/2013 observing season and is
about 300 square degrees. Using the upgraded GMOS spectrographs at
Gemini South, we have begun spectroscopic observations through the
Gemini Large and Long program, awarded to PI Liz Buckley-Geer to
follow-up DES strong lens candidate systems. In future searches, we
will employ automated arc-finders and photometry-based (e.g., blue
objects around red objects) finders.
The new Gemini Large and Long Program offers an opportunity for
a multi-year follow-up effort of strong lens candidates. In total
our observing proposal received 266 hours through 2016B.
During visual scanning of early DES data, we identified ~1000
candidates, which were then culled down (via photometry and further
visual inspection) to 24 candidates that wed seek to observe during
semesters 2014B and 2015A. The candidates shown in this
presentation are from October observing run of six nights at Gemini
South (Cerro Pachon). For the Gemini observing runs, we have one
candidate lens system and about 50 targets for photometric redshift
calibration.
Large and Long ProgramObservations for DES take place at the
Cerro Tololo Inter-American Observatory (CTIO, Fig. 6) in the dry
Chilean Andes. The Dark Energy Camera (DECam), built by the DES
Collaboration (led by Fermilab), is housed in the Blanco telescope
(Fig. 7). Its highly red-sensitive CCDs allow it to see farther
into the Universe than previous sky surveys. At the end of the
five-year mission, DES expects to have observed over 300 million
galaxies and 10 thousand supernovae, all in the effort to discover
more about the accelerating expansion of space-time.
Dark Energy Survey
Example Gemini Processing Using Candidate System 3 (Fig. 4)This
figure set shows how observations become two-dimensional
spectroscopic images and then one-dimensional spectra. After
automated or visual scanning of images (Fig. 2, 4C), slit masks are
created for each field around a target of interest (Fig. 4A,
B).
Dark Energy DetectivesThis is the case file (b)log of the Dark
Energy Survey. Regularly, the agents working for DES will release
to the public another case report regarding the investigation of
the accelerating expansion of the universe. Each report will
include picture or video evidence either taken with the Dark Energy
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Mass Calibration and System 24 (Fig.5)Here, we describe mass
estimates and analysis for System 24 (Fig. 5A), which is a known
cluster from the Southern Cosmology Survey (Menenteau et al.,
2010). First, we estimate the Einstein radius (E) for each object
using DS9. Next, we estimate the lensing systems mass via Eqn. 2
(below). We do this for each of the probable lens candidates in
this presentation (Fig. 3).
Gemini South Candidates (Fig. 2)
9
$: observed (13)
18Candidates(2014B)
18
13
17
1211
8 10
$ 7
$ 6
$
2
$
22
$
16
X
This panel shows the highest-priority, most-probable lens
candidates for the 2014B Gemini South Run. Inconclusive analyses
will require either more data or further reducEon. $: Observed +
Inconclusive
: Observed + Probable lensX: Observed + Probably Not a Lens
24
23
14
5
3
1
$
24
Fig. 7
Fig. 6
Candidate Lens Systems(Fig. 3) This panel presents results for
four lens candidate systems. Three systems (5, 14, 23) have
sufficient spectroscopic and morphological evidence to support
their statuses as lensing systems. One candidate (16) appears as a
lens system morphologically, but the spectroscopic data show that
the putative source is actually three separate sources.For each
system, we show the image, the spectral data (with highlighted line
features), the estimated system redshifts and the spectroscopic
target (yellow arrow).
Collaborators
7
Fig. 8
Science Goals
One of the main objectives of the strong lensing science program
in DES is to derive constraints on dark energy. The two major
components of this part of the program will be exploring (1) lenses
with background sources at multiple redshifts and (2) lensed
quasars. In addition to cosmology, we will use the cluster-scale
lens sample to study dark matter mass profile, along with the large
sample of sources at varying redshifts to study of galaxy evolution
and substructure.
Mass CalibrationIn addition to cosmology we will also be able to
use the substantial cluster-scale lens sample to study dark matter
mass profiles. Cluster-scale lenses are particularly useful because
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
and the concentration, of an NFW model of the cluster mass density
profile, over a wider range of radii than would be possible with
either method alone. 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 the cluster mass (Becker et al.
2007). These different methods, strong plus weak lensing and
cluster velocity dispersion, provide independent estimates of the
cluster mass and can then be combined to obtain improved
constraints on the mass and concentration (e.g Buckley-Geer et al.
2011).
Galaxy EvolutionStrongly lensed galaxies are particularly useful
for the study of high- redshift galaxies due to the magnification
of the apparent galaxy flux. Therefore, lensed galaxies are prime
candidates for detailed studies, since they can be investigated
using only a fraction of the telescope time that would be needed to
study comparably distant but unmagnified galaxies.
Gravitational Time Delay
Lenses with multiple sources are known to exist at both the
galaxy (Gavazzi et al. 2008) scale and the cluster scale (e.g.,
Jullo et al. 2010 and references therein). The lens equation (see
below) depends on the cosmological parameters via the angular
diameter distances Ds and Dls (the observer-source and lens-source
distances respectively) If we have two strongly lensed sources at
two redshifts zs1 and zs2 that are observed as multiple image
families in the same in the same cluster lens at redshift zl we can
define a family ratio. This ratio is independent of the Hubble
constant (H0) and so this makes it complementary to other probes
which depend on H0.
Lensed TransientsThis idea goes back many years (Refsdal 1964)
but has only really started to yield precision results due to a
substantial amount of work in controlling the systematic errors
(e.g., Suyu et al. 2013). In order to measure the time delay
distance (see Fig. 1) for each lensed quasar we need the following
elements, 1) redshifts of both the lens and the source, 2) deep
high resolution images of the lensed quasar host galaxy to model
the gravitational potential of the lens, 3) precise time delays, 4)
the stellar velocity dispersion of the lens and 5) multi-band
imaging of the field of the lens and redshifts of nearby companions
to characterize the environment along the line of sight.
ReferencesBayliss, M. B., et al. 2011, ApJS, 193, 8Becker, M.R.
et al. 2007, ApJ, 669, 905 Buckley-Geer, E.J. et al. 2011, ApJ,
742, 48Collett, T. E., et al. 2012, MNRAS, 424, 2864 Cunha, C., et
al. 2012, MNRAS, 423, 909Gavazzi, R., et al. 2008, ApJ, 677,
1046Gilmore J., & Natarajan, P. 2009, MNRAS, 396, 354 Golse,
G., et al 2002, a, 387, 788 Jullo E., et al. 2010, Science, 329,
924 More, A., et al. 2012, ApJ, 749, 38Oguri & Marshall 2010,
MNRAS, 405, 2579 Refsdal, S., 1964, MNRAS, 128, 307 Stark, D. P.,
et al. 2013, MNRAS, 436, 1040 Suyu, S., 2012, MNRAS, 426, 868Suyu,
S., et al. 2013, ApJ, 766, 70Weinberg, D., et al. 2013 Physics
Reports, 530, 87
D: 2-dimensional spectral image for sources. These are the
smudges you are looking for.
C: corresponding DES imageB: Zoomed-in mask A: Full-frame field
image and Gemini Mask
5
14
16
X
Lens @ z = 0.7M ~ 4 x 1013
Obj 1: continuum and linesObj 2: continuum, no linesObj 3: no
continuum
Lens @ z=0.7M ~ 3 x 1013
= 8230 [OII]z = 1.2
~ 5200 (Ly)z = 2.72
= 5963 (O3727); = 7934/8011 [OIII] = 4959/5007
(zl, zs1, zs2;M ,, w) =Dls(zs1)
Ds(zs1)
Ds(zs2)
Dls(zs2)
absorption line pair: = 8160 (Ca H+K)z = 1.06
23
line: ~ 4550 (Ly)z = 3.28M ~ 2 x 1013
Note: here we measure the lens, not the source.
Fig. 1
Eqn. 1
~ 8000 [OII]z = 0.53
For each candidate lens system, we target the lens and/or
sources (red arrows in Fig 4B, C, D). The targets for System 3 are
the putative sources (blue arcs). Targets are imaged through slits
in the mask (yellow vertical lines in Fig 4A, B) along the
dispersion direction (white box in Fig 4A). The smudges in the DES
image (Fig 4C) appear in the same shape and position in the 2D
spectra (Fig 4D, red arrows).
The same wavelength position of the images in the 2D and 1D
spectra show that both arcs are due to the same source object.
After basic image reduction (flat-fielding, bias removal, etc.),
IRAF is used to complete spectroscopic reduction: primarily, these
are 1) identification and calibration of spectral lines, 2)
transformation of pixels to wavelengths and 3) conversion to a 1D
spectrum (Fig 4D).
E: 1-dimensional spectrum
@darkenergydetec@TheDESurvey@briandnord
Visual Scan
For System 24, we use measurements from multiple arcs to
constrain the mass density profile of the lensing cluster. A single
isothermal sphere is ruled out, because the ratio of the observed
arc Einstein radii do not agree with that predicted by an SIS
profile for any choice of vacuum energy density.
An NFW (Navarro-Frenk-White) profile fits the data for a range
of cosmologies and clusters masses. The 3D mass density profile
scales as 1/(radiusn+1). For a range of vacuum energy densities,
the profile index is weakly constrained (Fig 5C). This example
demonstrates the power of lensing to constrain cluster masses and
cosmologies.
C: Mass Profile vs. Dark Energy
A: DES Image B: 1-dimensional spectrum
M =
E
0.9mas
2 DOLDLS
DOS10 kpc
Msol
Eqn. 2
