along the line of sight behind the lens (e.g.,
SDSSJ0946+1006, which shows multiple
concentric rings) the relationship between
the distances and the lens mass contains
information about the dark energy density. However, these are rare: only 10 are expected in the entire DES footprint (Gavazzi
et al., 2008). Also, time-delay measurements
of variable-luminosity objects, like lensed
quasars, can allow for measurements of the
Hubble constant (Refsdal et al., 1964).
We see a variety of morphologies in the first
galaxy- and galaxy cluster-scale lenses discovered in early DES data sets, shown in Figure 1; the lensed sources range in redshift,
0.80 < z < 3.2. The STRong-lensing Insights
into Dark Energy Survey (STRIDES; Treu et al.,
2015) program aims to discover and follow
up new time-delay lenses in DES data. Under these auspices, we have also discovered
and confirmed two lensed quasars at z ~ 1.6
and ~ 2.4 (Agnello et al., 2015). Although
these discoveries were made using Magellan/Baade, our Gemini Large and Long Program (GLLP) is providing the capability for
future confirmations.
Detective Work
DES is a deep-sky survey that covers 5,000
square degrees (sq. deg.) of the southern
Galactic Cap in five optical filter bands (g, r,
i, z, and Y). The main instrument for DES is
the Dark Energy Camera (DECam), a widefield (3 sq. deg.) camera mounted on the
Blanco 4-meter telescope at the Cerro Tololo
Inter-American Observatory in the Chilean
Andes (Flaugher et al., 2015). The survey has
finished three out of the planned five years.
The Science Verification (SV) season took
place after commissioning in late 2012 before the official science survey began. The SV
data cover 250 sq. deg. (< 5% of the full area)
and provide the imaging data for this work.
Searching through this area of sky is the
July 2016
first challenge in finding lenses. A team of
~ 20 DES scientists visually scanned the SV
sky area, looking primarily for morphological features — multiple images, arcs, and
full (Einstein) rings. We first performed a
non-targeted search of the entire SV area,
without focusing on any particular fields
or objects. We then undertook a targeted
search in the fields of galaxy clusters in the
DES footprint. The redMaPPer cluster-finder
(Rykoff et al., 2014) provided optically selected clusters. Overlapping fields of South
Pole Telescope (SPT) data provided clusters
selected with the Sunyaev-Zel’dovich effect
(Bleem et al., 2015).
The resulting list of candidates was then refined by a group of three expert scanners,
who reduced the total number of highly
ranked candidates to 53.
We also predicted the number of lenses we
could find in DES by comparing our list to
a different sample of highly ranked candidates/confirmed lenses found in the Canada-France-Hawai‘i Telescope Legacy Survey
(CFHTLS) Strong Lensing Legacy Survey
(S2LS; More et al., 2012) — including source
galaxies that survived a cut on the DES magnitude limit (24.5 magnitude in g-band).
There may be over 2,000 similar lenses in the
full DES area, and about 100 in the SV region.
While we accounted for the relative sky areas and depths of the two surveys, we had
no mechanism to affirm the efficiency of human visual inspection.
Confirming Lenses with
Spectroscopic Follow-up
The next puzzle piece we needed was confirmation that a source galaxy lies beyond the
putative lensing galaxy. This requires a sufficiently precise spectroscopic measurement
of the source galaxy’s redshift. Photometric
redshifts provide a measure of distance, but
they are relatively imprecise and much less
GeminiFocus
5