The Astrophysical Journal Supplement Series, 197:36 (36pp), 2011 December doi:10.1088/0067-0049/197/2/36
C
2011. The American Astronomical Society. All rights reserved. Printed in the U.S.A.
CANDELS: THE COSMIC ASSEMBLY NEAR-INFRARED DEEP EXTRAGALACTIC LEGACY SURVEY—THE
HUBBLE SPACE TELESCOPE OBSERVATIONS, IMAGING DATA PRODUCTS, AND MOSAICS
Anton M. Koekemoer
1
, S. M. Faber
2
, Henry C. Ferguson
1
, Norman A. Grogin
1
, Dale D. Kocevski
2
, David C. Koo
2
,
Kamson Lai
2
, Jennifer M. Lotz
1
, Ray A. Lucas
1
, Elizabeth J. McGrath
2
, Sara Ogaz
1
, Abhijith Rajan
1
, Adam G. Riess
3
,
Steve A. Rodney
3
, Louis Strolger
4
, Stefano Casertano
1
, Marco Castellano
5
, Tomas Dahlen
1
, Mark Dickinson
6
,
Timothy Dolch
3
, Adriano Fontana
5
, Mauro Giavalisco
7
, Andrea Grazian
5
, Yicheng Guo
7
, Nimish P. Hathi
8
,
Kuang-Han Huang
1,3
, Arjen van der Wel
9
, Hao-Jing Yan
10
, Viviana Acquaviva
11
, David M. Alexander
12
,
Omar Almaini
13
, Matthew L. N. Ashby
14
, Marco Barden
15
, Eric F. Bell
16
,Fr
´
ed
´
eric Bournaud
17
, Thomas M. Brown
1
,
Karina I. Caputi
18
, Paolo Cassata
7
, Peter J. Challis
19
, Ranga-Ram Chary
20
, Edmond Cheung
2
, Michele Cirasuolo
18
,
Christopher J. Conselice
13
, Asantha Roshan Cooray
21
, Darren J. Croton
22
, Emanuele Daddi
17
, Romeel Dav
´
e
23
,
Duilia F. de Mello
24
, Loic de Ravel
25
, Avishai Dekel
26
, Jennifer L. Donley
1
, James S. Dunlop
25
, Aaron A. Dutton
27
,
David Elbaz
28
, Giovanni G. Fazio
14
, Alexei V. Filippenko
29
, Steven L. Finkelstein
30
, Chris Frazer
21
,
Jonathan P. Gardner
24
, Peter M. Garnavich
31
, Eric Gawiser
11
, Ruth Gruetzbauch
13
, Will G. Hartley
13
,
Boris H
¨
aussler
13
, Jessica Herrington
16
, Philip F. Hopkins
29
, Jia-Sheng Huang
32
, Saurabh W. Jha
33
, Andrew Johnson
2
,
Jeyhan S. Kartaltepe
3
, Ali A. Khostovan
21
, Robert P. Kirshner
14
, Caterina Lani
13
, Kyoung-Soo Lee
34
, Weidong Li
29
,
Piero Madau
2
, Patrick J. McCarthy
8
, Daniel H. McIntosh
35
,RossJ.McLure
36
, Conor McPartland
2
,
Bahram Mobasher
37
, Heidi Moreira
38
, Alice Mortlock
13
, Leonidas A. Moustakas
39
, Mark Mozena
2
, Kirpal Nandra
40
,
Jeffrey A. Newman
41
, Jennifer L. Nielsen
35
, Sami Niemi
1
, Kai G. Noeske
1
, Casey J. Papovich
42
, Laura Pentericci
5
,
Alexandra Pope
6
, Joel R. Primack
2
, Swara Ravindranath
43
, Naveen A. Reddy
6
, Alvio Renzini
44
, Hans-Walter Rix
9
,
Aday R. Robaina
45
, David J. Rosario
2
, Piero Rosati
9
, Sara Salimbeni
7
, Claudia Scarlata
20
, Brian Siana
20
,
Luc Simard
46
, Joseph Smidt
21
, Diana Snyder
2
, Rachel S. Somerville
1
, Hyron Spinrad
29
, Amber N. Straughn
24
,
Olivia Telford
47
, Harry I. Teplitz
20
, Jonathan R. Trump
2
, Carlos Vargas
38
, Carolin Villforth
1
, Cory R. Wagner
35
,
Pat Wandro
2
, Risa H. Wechsler
48
,BenjaminJ.Weiner
23
, Tommy Wiklind
1
, Vivienne Wild
36
, Grant Wilson
7
,
Stijn Wuyts
14
,andMinS.Yun
7
1
Space Telescope Science Institute, 3700 San Martin Dr., Baltimore, MD, 21218, USA
2
University of California Observatories/Lick Observatory, University of California, Santa Cruz, CA 95064, USA
3
Department of Physics and Astronomy, The Johns Hopkins University, Baltimore, MD, USA
4
Department of Physics and Astronomy, Western Kentucky University, Bowling Green, KY, USA
5
INAF, Osservatorio Astronomico di Roma, Via Frascati 33, I-00040, Monteporzio, Italy
6
National Optical Astronomy Observatories, 950 North Cherry Avenue, Tucson, AZ 85719, USA
7
Department of Astronomy, University of Massachusetts, Amherst, MA 01003, USA
8
Observatories of the Carnegie Institution for Science, 813 Santa Barbara Street, Pasadena, CA 91101, USA
9
European Southern Observatory, Karl-Schwarzschild-Strasse 2, D-85748 Garching, Germany
10
The Ohio State University Research Foundation, OH, USA
11
Department of Physics and Astronomy, Rutgers, the State University of New Jersey, Piscataway, NJ 08854, USA
12
Department of Physics, Durham University, Durham DH1 3LE, UK
13
University of Nottingham, University Park, Nottingham, NG7 2RD, United Kingdom
14
Max-Planck-Institut f
¨
ur extraterrestrische Physik, Giessenbachstrasse 1, D-85748 Garching bei M
¨
unchen, Germany
15
Institute of Astro- and Particle Physics, University of Innsbruck, Innsbruck, Austria
16
Department of Astronomy, University of Michigan, 500 Church St., Ann Arbor, MI 48109, USA
17
Commissariat
`
al’
´
Energie Atomique, & Laboratoire AIM Paris-Saclay, 91191 Gif-sur-Yvette, France
18
SUPA, Institute for Astronomy, University of Edinburgh, Royal Observatory, Edinburgh EH9 3HJ, UK
19
Harvard College Observatory, Cambridge, MA, USA
20
California Institute of Technology, Pasadena, CA, USA
21
Department of Physics and Astronomy, University of California, Irvine, CA, USA
22
Centre for Astrophysics and Supercomputing, Swinburne University of Technology, PO Box 218, Hawthorn, VIC 3122, Australia
23
Steward Observatory, University of Arizona, 933 N. Cherry St., Tucson, AZ 85721
24
NASA’s Goddard Space Flight Center, Laboratory for Observational Cosmology, 8800 Greenbelt Rd., Greenbelt, MD 20771
25
Institute for Astronomy, University of Edinburgh, Royal Observatory, Edinburgh EH9 3HJ, UK
26
Racah Institute of Physics, The Hebrew University of Jerusalem, Jerusalem 91904, Israel
27
University of Victoria, Victoria, Australia
28
CEA-Saclay/DSM/DAPNIA/Service d
˚
uAstrophysique, 91191 Gif-sur-Yvette Cedex, France
29
Department of Astronomy, University of California, Berkeley, CA, USA
30
Texas A&M Research Foundation, 3578 TAMU College Station, TX 77843, USA
31
Department of Physics, University of Notre Dame, Notre Dame, IN, USA
32
Smithsonian Institution Astrophysical Observatory, 60 Garden Street, Cambridge, MA, USA
33
Department of Physics and Astronomy, Rutgers, the State University of New Jersey, Piscataway, NJ 08854, USA
34
Yale Center for Astronomy & Astrophysics, New Haven, CT, USA
35
Department of Physics, University of Missouri-Kansas City, MO, USA
36
Royal Observatory Edinburgh, Edinburgh, EH9 3HJ, UK
37
Department of Physics and Astronomy, UC Riverside, 900 University Ave, Riverside, CA 92521, USA
38
Department of Physics and Astronomy, Rutgers, the State University of New Jersey, Piscataway, NJ 08854, USA
39
Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
40
Imperial College of Science, London, UK
41
University of Pittsburgh, Department of Physics and Astronomy and PITT-PAC, Pittsburgh, PA, USA
42
Texas A&M Research Foundation, 3578 TAMU College Station, TX 77843, USA
1
The Astrophysical Journal Supplement Series, 197:36 (36pp), 2011 December Koekemoer et al.
43
Inter-University Center for Astronomy & Astrophysics, Post Bag - 4, Ganeshkhind, Pune, Maharashtra - 411007, India
44
Osservatorio Astronomico di Padova, Padova, Italy
45
Institut de Ciencies del Cosmos, ICC-UB, IEEC, Marti i Franques 1, 08028, Barcelona, Spain
46
Dominion Astrophysical Observatory, BC, Canada
47
Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, PA, USA
48
Physics Department, Kavli Institute for Particle Astrophysics and Cosmology, and SLAC National Accelerator Laboratory, Stanford, CA, 94305, USA
Received 2011 May 12; accepted 2011 November 8; published 2011 December 6
ABSTRACT
This paper describes the Hubble Space Telescope imaging data products and data reduction procedures for the
Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS). This survey is designed to
document the evolution of galaxies and black holes at z ≈ 1.5–8, and to study Type Ia supernovae at z>1.5.
Five premier multi-wavelength sky regions are selected, each with extensive multi-wavelength observations. The
primary CANDELS data consist of imaging obtained in the Wide Field Camera 3 infrared channel (WFC3/IR) and
the WFC3 ultraviolet/optical channel, along with the Advanced Camera for Surveys (ACS). The CANDELS/Deep
survey covers ∼125 arcmin
2
within GOODS-N and GOODS-S, while the remainder consists of the CANDELS/
Wide survey, achieving a total of ∼800 arcmin
2
across GOODS and three additional fields (Extended Groth Strip,
COSMOS, and Ultra-Deep Survey). We summarize the observational aspects of the survey as motivated by the
scientific goals and present a detailed description of the data reduction procedures and products from the survey.
Our data reduction methods utilize the most up-to-date calibration files and image combination procedures. We have
paid special attention to correcting a range of instrumental effects, including charge transfer efficiency degradation
for ACS, removal of electronic bias-striping present in ACS data after Servicing Mission 4, and persistence effects
and other artifacts in WFC3/IR. For each field, we release mosaics for individual epochs and eventual mosaics
containing data from all epochs combined, to facilitate photometric variability studies and the deepest possible
photometry. A more detailed overview of the science goals and observational design of the survey are presented in
a companion paper.
Key words: cosmology: observations – galaxies: high-redshift
Online-only material: color figures
1. INTRODUCTION
In this paper, we describe the Hubble Space Telescope (HST)
imaging and mosaic data products from the Cosmic Assembly
Near-infrared Deep Extragalactic Legacy Survey (CANDELS),
a 902-orbit Multi-Cycle Treasury (MCT) program aimed at
documenting the evolution of galaxies and black holes from
redshift z ≈ 1.5 to 8, characterizingType Ia supernovae (SNe Ia)
at z>1.5 to better constrain the nature of dark energy, and
probing galaxy evolution into the epoch of reionization. The
CANDELS program uses the Wide Field Camera 3 infrared
channel (WFC3/IR) as its prime instrument, as well as the
WFC3 ultraviolet/optical channel (WFC3/UVIS), and obtains
parallel observations with the Advanced Camera for Surveys
(ACS). It is executing across Cycles 18, 19, and 20, and
resulted from the combination of two approved proposals that
were submitted in response to the special Hubble MCT Call
for Proposals in 2009, which provided for large programs
to address unique and broad science themes that could not
be accommodated within the standard annual time-allocation
process.
The structure of the survey includes essential elements of
two MCT programs that were submitted separately: one, led by
H.C.F., involved studying the full area of the GOODS-North
and GOODS-South fields (Giavalisco et al. 2004) to a uniform
depth, including also ultraviolet (UV) imaging, and carrying
out an extensive search for high-redshift SNe Ia; the other pro-
gram, led by S.M.F., aimed at studying half the GOODS-N
and GOODS-S areas to a greater depth, together with
wider/shallower imaging of the Extended Groth Strip (EGS;
Davis et al. 2007; J. A. Newman et al. 2011, in prepara-
tion), COSMOS (Scoville et al. 2007; Koekemoer et al. 2007),
and the UKIDSS Ultra-Deep Survey (UDS; Lawrence et al.
2007; Cirasuolo et al. 2007), while also permitting a search for
SNe Ia.
The combined CANDELS program obtains observations
across all five fields, as well as including the SN Ia follow-
up program, the UV imaging, and the multi-tier Deep+Wide
observing strategy. Most of the observations use WFC3/IR as
prime and ACS/WFC in parallel, and substituting UV imaging
with WFC3/UVIS for parts of the orbit in Hubble’s continuous
viewing zone (CVZ) that are too bright for WFC3/IR imaging.
More detailed information is presented at our CANDELS Web
site, http://candels.ucolick.org, and by Grogin et al. (2011),
which provides a thorough overview of the science goals and
observing strategy.
The structure of this paper is as follows. A brief outline
of the major science goals is given in Section 2 to place the
data products in context, a description of the fields is provided
in Section 3, the observations are described in Section 4,the
data products are presented in Section 5, and we conclude in
Section 6.
2. SCIENCE GOALS
We summarize here the CANDELS science goals in the
context of how they relate to the Hubble data products. We
refer to Grogin et al. (2011) for a more detailed description
of the science goals, which include studies of galaxies in the
reionization era (“cosmic dawn”), the growth and morphological
transformation of galaxies during the era of peak star formation
and active galactic nucleus (AGN) activity (“cosmic high
noon”), and measurements of high-redshift SNe Ia to constrain
dark energy and measure supernova rate evolution.
2
The Astrophysical Journal Supplement Series, 197:36 (36pp), 2011 December Koekemoer et al.
Cosmic Dawn. Quasar observations (Fan et al. 2006) and
the Wilkinson Microwave Anisotropy Probe (Page et al. 2007;
Spergel et al. 2007) indicate that the intergalactic medium was
reionized between t
0
= 0.5 and 1 Gyr. The intergalactic medium
was seeded with metals to Z ≈ 4 × 10
−4
within the first billion
years, and the energy released by the stars that produced these
metals appears sufficient to reionize the intergalactic medium
(Songaila 2001; Ryan-Weber et al. 2006). The bright end of
the UV luminosity function (UVLF) of star-forming galaxies
evolves rapidly at 4 <z<7 (e.g., Dickinson et al. 2004;
Bouwens et al. 2007, 2008, 2010), but the UV flux from these
galaxies appears insufficient to explain reionization without
extrapolations—an important puzzle to solve. Current z ≈ 8
luminosity functions are based on only a handful of objects,
mostly with L
UV
L
∗
. The CANDELS data in the z
8
50, Y
105
,
J
125
, and H
160
filters can provide measurements of the bright end
of the UVLF at z ≈ 7–8 and also permit robust Lyman-break
galaxy (LBG) color selection at z=5.8, 6.6, and 8.0, for L
∗
for z = 7(J
125
≈ 27 mag) and 1.5 L
∗
for z = 8(H
160
≈ 27 mag),
as well as fainter LBGs at higher redshifts. This can constrain
extinction via UV spectral slopes and improve measurements of
their evolution.
Furthermore, the evolution of faint AGNs at z 6–7 can
be directly probed by cross-correlating the drop-out samples
with the deep X-ray data in these fields (e.g., Fan et al. 2003;
Koekemoer et al. 2004; Aird et al. 2008; Brusa et al. 2009)
for which the depth in the near-IR data is crucial. Moreover,
photometric redshifts and non-LBG color criteria can help reveal
whether there are non-star-forming galaxies lurking at these
redshifts (Mobasher et al. 2005; Wiklind et al. 2008; Chary
et al. 2007; Dunlop et al. 2007), thereby driving the photometric
requirements of the CANDELS data products. The CANDELS
data also allow fluctuations in the extragalactic background light
to be probed, potentially constraining the properties of the first
generations of stars (Cooray et al. 2004; Fernandez et al. 2010),
as well as enabling the use of clustering statistics to constrain
the properties of dark matter halos (Conroy et al. 2006;Lee
et al. 2006, 2009), which drives the need to produce contiguous
mosaics across each of the CANDELS fields.
Cosmic High Noon. At z ≈ 2, star formation and nuclear
activity within galaxies are at their peaks while the morpholog-
ical differentiation of galaxies is well underway. A key ques-
tion is what drives stellar mass buildup, bulge growth, and the
emergence of passive ellipticals. To resolve this requires accu-
rate mass function measurements well below M
∗
, achieved by
robust spectral energy distribution fitting at rest-frame optical
wavelengths, where the 4000 Å and Balmer breaks constrain
accurate photometric-redshift measurements, stellar population
ages, and M/L ratios. The CANDELS data are designed to
match the photometric depths of the existing Spitzer Infrared
Array Camera (IRAC) and HST ACS data, accurately determin-
ing the mass function of quiescent galaxies for M>10
9
M
at z ≈ 2 and their contribution to the global mass density. An-
other key question in galaxy growth is the relative importance
of mergers and structural instabilities. The discovery of large,
rotating, clumpy, gas-rich disks at z ≈ 2 implies that disk insta-
bilities may drive bulge formation more rapidly than previously
thought (F
¨
orster Schreiber et al. 2006, 2009; Genzel et al. 2006,
2008).
The star formation rate (SFR) also correlates strongly with
stellar mass M
, and the zero point of the SFR(M
) relation
declines steadily below z ≈ 2.5 (Daddi et al. 2007; Elbaz
et al. 2007; Noeske et al. 2007). The empirical evidence is now
reminiscent of the “clump-merging” scenario for the growth
of bulges in gas-rich disks from numerical simulations (e.g.,
Noguchi 1999; Immeli et al. 2004; Bournaud et al. 2007;
Elmegreen et al. 2008). CANDELS can provide a census of
clumps within galaxies along with their sizes and masses, and
the resulting estimates of bulge formation rates can be compared
to the timescale of clump migration driven by dynamical
friction. Through comparison with the deep X-ray catalogs,
CANDELS also provides detailed morphological information
on AGNs in this redshift range, tracking the connection between
galaxy mergers and black hole growth. Finally, the evolution
of galaxies with very low specific SFRs (passive galaxies) is
also of interest, with sources having been found out to at least
z = 2.5 (e.g., Daddi et al. 2005; Trujillo et al. 2006; Cimatti
et al. 2008; van Dokkum et al. 2008). CANDELS enables the
luminosity function of large numbers of these sources to be
directly constrained, in addition to probing their morphological
structure to faint limits in the near-IR with better resolution than
previous studies.
Deep Ultraviolet Observations. An important feature of
CANDELS is the fact that the GOODS-N field is in the HST
CVZ; thus, we use the bright day side of the orbit to observe
with WFC3/UVIS in the UV (F275W and F336W). This enables
measurements of the Lyman continuum (LyC) escape fraction
(f
esc
) from galaxies at z ≈ 2.5, identification of ∼350 LBGs
at z ≈ 2, and measurements of the SFR in low-luminosity
dwarfs which may just be “turning on” at z ≈ 1 (Babul & Rees
1992; Bullock et al. 2000). There are ∼40–50 UV-luminous
LBGs (L
UV
> 0.25 L
∗
) in this field at 2.38 <z<2.55
(half with spectroscopic redshifts), which is the optimal redshift
for constraints with the F275W filter, many of which may be
bright enough to detect if f
esc
> 0.5 (e.g., Shapley et al. 2006;
Iwata et al. 2009). Importantly, these galaxies are at redshifts
that allow Hα measurements for an independent measure of
the ionizing continuum. The resolved LyC distributions provide
tests of different mechanisms for high f
esc
including supernova
winds (Clarke & Oey 2002; Fujita et al. 2002), minor galaxy
interactions (Gnedin et al. 2008), and emission from globular
cluster formation (Ricotti 2002).
Supernova Cosmology. While SNe Ia at z 1–1.5 have
already provided startling evidence of dark energy (Riess et al.
1998, 2004, 2007; Perlmutter et al. 1999), CANDELS now
provides a direct probe of 1.5 <z<2.5 to test the nature of
SN Ia progenitors and their possible evolution (Ruiz-Lapuente
& Canal 1998; Mannucci et al. 2005; Kobayashi & Nomoto
2009; Greggio et al. 2008), which can be tested with CANDELS
since the predicted rates diverge significantly at z>1.5. In
addition, CANDELS yields SNe Ia at z 1.5 which remain
crucial tracers of the evolution in the dark energy equation-
of-state parameter w. Increasing the samples at 0.7 <z<1
in the IR reduces the uncertainties in host-galaxy extinction,
thereby testing whether dust is a factor in the declining high-
redshift SN Ia rate. CANDELS also includes follow-up WFC3
or ACS grism observations to determine the supernovae type and
redshift, and rest-frame B and V light curves for each SN (thus
limiting the K-correction errors to below the random distance
error of the SNe). We note also that related follow-up programs
to CANDELS (including the SN Ia search) obtain grism data on
these fields (e.g., GO-12099), but these are separate programs
from the CANDELS imaging survey and are not discussed in
further detail here.
3
The Astrophysical Journal Supplement Series, 197:36 (36pp), 2011 December Koekemoer et al.
Tab le 1
Total HST WFC3 and ACS Exposure Depths in the CANDELS Fields
a
Field WFC3/IR WFC3/UVIS ACS/WFC
F105W F125W F160W F275W F336W F350LP F606W F814W F850LP
UDS (Wide) ··· 2/34/3 ··· ··· ∼0.3 1 1 ···
GOODS-S Deep 3 4 6 ··· ··· ∼12 9 1
GOODS-S Wide ··· 2/34/3 ··· ··· ∼0.3 1 1 ···
EGS (Wide) ··· 2/34/3 ··· ··· ∼0.3 1 1 ···
GOODS-N Deep 34622∼12 9 1
GOODS-N Wide ··· 2/34/3 ··· ··· ∼0.3 1 1 ···
COSMOS (Wide) ··· 2/34/3 ··· ··· ∼0.3 1 1 ···
Note.
a
Approximate exposure depth in each filter, in HST orbits (for details, see Section 4).
3. THE CANDELS FIELDS
Here we summarize the general properties of the CANDELS
fields and refer to Grogin et al. (2011) for a more detailed
description. The CANDELS survey consists of a two-tier
Deep+Wide survey designed to address the science goals dis-
cussed in Section 2 as well as providing a legacy data set on
these fields. The CANDELS Deep portion covers ∼125 arcmin
2
to ∼10-orbit depth within the GOODS-N and GOODS-S
fields (Giavalisco et al. 2004), including the E-CDFS (Rix et al.
2004) as well as the WFC3 ERS2 field
49
(Windhorst et al.
2011). The full area of the CANDELS survey covers a total
of ∼800 arcmin
2
, where the additional area includes the shal-
lower Wide portion to ∼2-orbit depth around the Deep portions
of GOODS, together with subsections of three additional fields,
namely the EGS (Davis et al. 2007; J. A. Newman et al. 2011, in
preparation), COSMOS (Scoville et al. 2007; Koekemoer et al.
2007), and the UKIDSS UDS (Lawrence et al. 2007; Cirasuolo
et al. 2007). When combined with the existing Ultra Deep Field
(UDF; Beckwith et al. 2006; Thompson et al. 2005; Oesch et al.
2007; Bouwens et al. 2010) within GOODS-S, CANDELS pro-
vides a unifying survey at three principal exposure-time depths
with roughly an order of magnitude difference between each.
Another unifying aspect of the survey is that all five CANDELS
fields are the targets of the Spitzer Extended Deep Survey
(G. G. Fazio et al. 2011, in preparation), which is a 2108 hr pro-
gram, together with a more recently approved 1200 hr follow-on
program, covering each of these regions with Spitzer IRAC
3.6 μm and 4.5 μm imaging to a total depth of 12 hr per
pointing.
Each of the five CANDELS fields has a wealth of additional
imaging and spectroscopic ancillary data from X-rays to radio
wavelengths, described in the aforementioned papers and refer-
ences therein. For the present work, we note in particular that all
of them, except the UDS, have pre-existing HST data covering
the field. In addition, all five fields have extensive pre-existing
catalogs that can serve as astrometric and photometric reference
standards as well as being combined with the catalogs from
new HST data to obtain derived measurements of source prop-
erties including photometric redshifts, stellar masses, and star
formation histories.
4. OBSERVATIONS
Here we briefly outline the general layout of the observations,
referring to Grogin et al. (2011) for a more detailed description.
The HST observations in the CANDELS fields can be sum-
marized as consisting of three complementary sets of imaging
49
HST Proposal ID 11539, p.1.: R. O’Connell
data: WFC3/IR, WFC3/UVIS, and ACS/WFC imaging expo-
sures.
50,51
In all cases, the WFC3 observations are taken as the
prime observations, while the ACS data are obtained in paral-
lel. The filter breakdown is shown in Table 1 for each of the
different fields. We also discuss two generally different sets
of observations, namely the GOODS fields, which contain the
CANDELS-Deep pointings (together with a CANDELS-Wide
“flanking field” in GOODS-S), and the other three fields (COS-
MOS, EGS, and UDS), which only consist of the CANDELS-
Wide component and thus have a somewhat different structure.
4.1. Filters and Exposure Times
We first describe the filter choice and exposure structure of
the three Wide fields (COSMOS, EGS, and UDS), each of which
is covered using a mosaic grid of tiles and repeated over two
epochs. During every epoch, each tile is observed for one orbit
(∼2000 s), divided into two exposures in F125W (at a depth
of ∼1/3 orbit) and two exposures in F160W (at a depth of
∼2/3 orbit), together with parallel exposures using ACS/WFC
in F606W and F814W. However, some WFC3/IR tiles near one
end of the Wide mosaics are not covered by ACS parallels so a
short 434 s WFC3/UVIS F350LP exposure is inserted, creating
a total of five WFC3 exposures per orbit for these tiles, and five
ACS/WFC exposures in parallel.
The GOODS-N and GOODS-S fields contain the Deep
portions of the CANDELS survey, with a total depth of at
least 4 orbits in both WFC3/IR F125W and F160W and
3 orbits in F105W, spread across 10 epochs. Each single-
orbit pointing, for each epoch, contains four WFC3/IR expo-
sures (two F125W and two F160W), and one WFC3/UVIS
(F350LP). In parallel, we also obtain five ACS/WFC expo-
sures, where the primary requirement is to obtain at least
32,000 s depth in F814W. Once this requirement is met,
the next ACS/WFC priorities are ∼2500 s of F850LP, fol-
lowed by ∼5000 s in F606W, and finally any remaining
depth is placed back into F814W. Since the GOODS-N
field is in the CVZ, some portions of the orbit are too bright
for observations using WFC3/IR, so WFC3/UVIS exposures
are substituted using the F275W and F336W filters. In these
cases the ACS parallels retain their structure as described for
the remainder of the Deep observations.
Finally, the GOODS-S Deep portion has a wider “flanking
field,” similar to the Wide fields in its filter choice and exposure
time requirements. This is divided into two epochs, achieving
a depth of ∼1/3 orbit in F125W and ∼2/3 orbit in F160W per
50
For current details on ACS see http://www.stsci.edu/hst/acs
51
For current details on WFC3 see http://www.stsci.edu/hst/wfc3
4
The Astrophysical Journal Supplement Series, 197:36 (36pp), 2011 December Koekemoer et al.
Tab le 2
CANDELS ACS and WFC3 Filter Zero Points
a
Instrument/Camera Filter Zero Point (ABmag)
ACS/WFC F606W 26.49
ACS/WFC F814W 25.94
ACS/WFC F850LP 24.84
WFC3/UVIS F275W 24.14
WFC3/UVIS F336W 24.64
WFC3/UVIS F350LP 26.94
WFC3/IR F105W 26.27
WFC3/IR F125W 26.25
WFC3/IR F160W 25.96
Note.
a
For current filter zero-point information, please see the fol-
lowing Web sites: http://www.stsci.edu/hst/acs/analysis/zeropoints;
http://www.stsci.edu/hst/wfc3/phot_zp_lbn
epoch, also using short WFC3/UVIS F350LP exposures where
necessary, and with ACS/WFC F606W and F814W exposures
in parallel. For the ACS/WFC exposures falling outside the
Deep area, the exposure time requirements are first to obtain
∼2500 s in F814W, followed by ∼2500 s in F850LP, and
distribute any remaining time according to the same priorities
as for the Wide mosaics. The GOODS-N “flanking field” is
similar in design, with a slightly different layout. Note that,
since the ACS/WFC field of view is larger than WFC3/IR, the
ACS parallel pointings overlap considerably and their effective
exposure time on overlapping pointings can exceed the nominal
two-orbit observing time, due to the dense packing of pointings
for contiguous WFC3/IR coverage.
In Table 2, we list the current zero points (in the AB
magnitude system) for all the filters used in the CANDELS
survey observations. We note that these are subject to change
and we provide links to the instrument Web sites where the most
up-to-date zero points can be obtained in future. The primary
uncertainties associated with these are related to the spectral
characteristics of the standard stars that are used by staff at
STScI to carry out the photometric zero-point calibrations, as
well as changes in the instrument and filters over time. Generally
these are accurate to better than ∼1%–2%, and we present later
in Section 5.9 a quantitative validation of this level of accuracy
using the photometry from our CANDELS data, compared with
photometric data from ground-based imaging.
4.2. Mosaic Layout Design
For each of the five CANDELS fields, the goal is to cover
a contiguous area with WFC3/IR (thus, the larger ACS/WFC
parallel exposures overlap somewhatto create deeper pointings),
and to overlap as much as possible the existing relevant ancillary
data sets. Here we summarize the specific considerations for
each of the fields and how they impact the overall design of the
mosaic observations.
For the three Wide fields (COSMOS, EGS, and UDS), the
layout consists of a rectangular region, which for COSMOS and
UDS comprises a grid of 4 × 11 tiles (∼8.
6 × 23.
8), at spacing
intervals designed to allow for maximal contiguous coverage
in WFC3/IR without introducing gaps between tiles as a result
of pointing errors. The exposures are all oriented so that the
ACS/WFC parallels are offset along the long axis of the mosaic,
thereby producing a similar-sized mosaic overlapping the bulk
of the WFC3/IR mosaic, except at its ends where some tiles are
covered only by WFC3/IR or by ACS/WFC, but not both. For
the EGS field the mosaic is instead 3 × 15 tiles (∼6.
5 × 32.
5),
to optimize coverage with ancillary data.
For the GOODS-N and GOODS-S Deep regions, the layout
consists of a smaller rectangular grid of 3 × 5 tiles (∼6.
5×10.
8).
In GOODS-S the WFC3/IR pointings are placed adjacent to the
existing WFC3/IR ERS2 observations (Windhorst et al. 2011).
Since the field is observed over 10 epochs, the orientation of the
tiles rotates by ∼45
◦
–50
◦
from one epoch to the next. This also
changes the coverage of the parallel ACS/WFC observations,
creating a net effect of a larger area covered by ACS/WFC to
shallower depth, surrounding the deep central WFC3/IR data,
which therefore has a slight deficit of ACS coverage.
Finally, the shallower “flanking field” region in GOODS-S
covers ∼2 × 4 tiles, divided into two epochs, and using a
similar filter choice and exposure time strategy as the other Wide
fields. In GOODS-N the layout is similar, with differences due
to the field geometry. The pointings are oriented such that the
ACS/WFC parallels land mostly on the GOODS-S Deep region.
The GOODS-S “flanking field” region is also covered with
F105W to one-orbit depth which provides additional parallel
ACS/WFC data for the central region.
4.3. Sub-pixel Dither Pattern
Each mosaic tile is observed for one orbit during each
epoch, where the prime WFC3/IR observations consist of four
exposures. In most cases these four exposures consist of two
exposures each in F125W and F160W, except for the Y-band
visits where all four exposures are obtained in F105W. Due
to the relatively large pixel scale of the WFC3/IR detector
(0.
128 pixel
−1
at its central reference pixel), we offset the four
WFC3/IR exposures in each orbit using a four-point small-
scale dither pattern to provide half-pixel subsampling of the
point-spread function (PSF) while also mitigating the impact
of hot pixels and persistence. The strategy of dividing the
four-point dither pattern into two pointings with F125W and
two with F160W was particularly motivated by the supernova
science, where we carried out tests to ensure that good supernova
subtraction could be achieved. Since the second epoch on these
tiles also contain two exposures in F125W and F160W, we
ultimately achieve a four-point dither pattern in each filter, on
all tiles, once the data from all the epochs are combined.
The dither pattern serves two complementary purposes:
(1) provide non-integer shifts to subsample the PSF and
(2) add integer components to these shifts in order to ensure
that hot pixels and possible persistence from previous bright
sources are moved around sufficiently. In particular, persistence
is a concern, especially “self-persistence” from sources in previ-
ous CANDELS exposures executed as part of the dither pattern,
since it typically tends to be more extended than a single pixel
and is diffuse, thereby subtly impacting the photometry if it
is not mitigated. For compact sources, the expected spatial ex-
tent has been quantified as a circle ∼2.5 pixels in diameter
(WFC3/IR), and therefore constrains the minimum size of the
dither offset. On the other hand, due to the geometric distortion
of the detector, an offset of a certain number of pixels at the cen-
ter corresponds to a different number of pixels near the edge, and
for sufficiently large shifts the subsampling can vary from half-
pixel to integer several times between the center and the edge
of the detector, introducing non-uniform sub-pixel sampling.
Therefore, the desire to retain uniform half-pixel sampling
across the entire WFC3/IR detector during each epoch con-
strains the dither offsets to be as small as possible, which in
this case is the minimum size needed to avoid issues from
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