The Seventh Data Release of the Sloan Digital Sky Survey

TLDR: SDSS-II as mentioned in this paper is the last data set of the Sloan Digital Sky Survey and contains 357 million distinct objects, including 930,000 galaxies, 120,000 quasars, and 460,000 stars.

Abstract: This paper describes the Seventh Data Release of the Sloan Digital Sky Survey (SDSS), marking the completion of the original goals of the SDSS and the end of the phase known as SDSS-II. It includes 11663 deg^2 of imaging data, with most of the roughly 2000 deg^2 increment over the previous data release lying in regions of low Galactic latitude. The catalog contains five-band photometry for 357 million distinct objects. The survey also includes repeat photometry over 250 deg^2 along the Celestial Equator in the Southern Galactic Cap. A coaddition of these data goes roughly two magnitudes fainter than the main survey. The spectroscopy is now complete over a contiguous area of 7500 deg^2 in the Northern Galactic Cap, closing the gap that was present in previous data releases. There are over 1.6 million spectra in total, including 930,000 galaxies, 120,000 quasars, and 460,000 stars. The data release includes improved stellar photometry at low Galactic latitude. The astrometry has all been recalibrated with the second version of the USNO CCD Astrograph Catalog (UCAC-2), reducing the rms statistical errors at the bright end to 45 milli-arcseconds per coordinate. A systematic error in bright galaxy photometr is less severe than previously reported for the majority of galaxies. Finally, we describe a series of improvements to the spectroscopic reductions, including better flat-fielding and improved wavelength calibration at the blue end, better processing of objects with extremely strong narrow emission lines, and an improved determination of stellar metallicities. (Abridged)

Figures

Figure 1. Distribution on the sky of the data included in DR7 (upper panel: imaging; lower panel: spectra), shown in an Aitoff equal-area projection in J2000 Equatorial Coordinates. The Galactic plane is the sinuous line that goes through each panel. The center of each panel is at α = 120◦ ≡ 8h, and the plots cut off at δ = −25◦, below which the SDSS did not extend. The Legacy imaging survey covers the contiguous area of the Northern Galactic Cap (centered roughly at α = 200◦, δ = 30◦), as well as three stripes (each of width 2.◦5) in the Southern Galactic Cap. In addition, several stripes (indicated in blue in the imaging data) are auxiliary imaging data, while the SEGUE imaging scans are indicated in red. The green scans are additional runs as described in Finkbeiner et al. (2004). In the spectroscopy panel, the lighter regions indicate that area in the Northern Galactic Cap which is new to DR7; note that the Northern Galactic Cap is now contiguous. Red points indicate SEGUE plates and blue points indicate other non-Legacy plates (mostly as described in the DR4 paper).

Figure 2. Distribution on the sky of SEGUE (red) and other non-Legacy (blue) spectroscopic observations, here plotted in Galactic coordinates. The contiguous blue stripe across the bottom is Stripe 82, along the celestial equator. As described in the DR4 paper, Stripe 82 includes extensive spectroscopy of a number of different types of targets outside the Legacy Survey.

Table 2 Asinh Magnitude Softening Parameters for the Co-Addition

Figure 5. Template-based estimated redshifts vs. the true spectroscopic redshifts for a random sample of 30,000 galaxies with redshifts from SDSS. The estimated values calculated with the old (DR6) method have significantly larger scatter and more outliers than the ones with the new hybrid (DR7) technique. Note that the sample is dominated by red galaxies (whose photometric redshifts are intrinsically easier to measure) at z > 0.2.

Figure 3. Stripe 82, the equatorial stripe in the South Galactic Cap, has been imaged multiple times. The lower pair of curves shows the number of scans covering a given right ascension in the North and South strips that are included in the co-addition (mostly data taken through 2005). In addition, Stripe 82 has been covered many more times as part of a comprehensive survey for 0.05 < z < 0.35 supernovae, although often in conditions of poor seeing, bright moon, and/or clouds; the total numbers of scans at each right ascension in the North and South strips are indicated in the upper pair of curves. All these data have been flux calibrated, as discussed in the text, and are available (together with the co-add itself) in the stripe82 database.

Figure 10. Spectra of the object SDSS J153704.18+551550.6 = Mrk487 in DR6 and DR7. The stronger [O iii] emission line at 5020 Å was mistaken for a cosmic ray and clipped away completely in DR6, while the weaker line at 4970 Å was slightly affected. With the improved algorithm in DR7, the lines are not clipped.

Full text

The Astrophysical Journal Supplement Series, 182:543–558, 2009 June doi:10.1088/0067-0049/182/2/543
C

2009. The American Astronomical Society. All rights reserved. Printed in the U.S.A.
THE SEVENTH DATA RELEASE OF THE SLOAN DIGITAL SKY SURVEY
Kevork N. Abazajian
1
, Jennifer K. Adelman-McCarthy
2
, Marcel A. Ag
¨
ueros
3,102
, Sahar S. Allam
2,4
,
Carlos Allende Prieto
5
, Deokkeun An
6,7
, Kurt S. J. Anderson
8,9
, Scott F. Anderson
10
, James Annis
2
, Neta
A. Bahcall
11
,C.A.L.Bailer-Jones
12
, J. C. Barentine
13
, Bruce A. Bassett
14,15
, Andrew C. Becker
10
, Timothy
C. Beers
16
, Eric F. Bell
12
, Vasily Belokurov
17
, Andreas A. Berlind
18
, Eileen F. Berman
2
, Mariangela Bernardi
19
,
Steven J. Bickerton
11
, Dmitry Bizyaev
8
, John P. Blakeslee
20
, Michael R. Blanton
21
, John J. Bochanski
10,22
, William
N. Boroski
2
, Howard J. Brewington
8
, Jarle Brinchmann
23,24
, J. Brinkmann
8
, Robert J. Brunner
25
,Tam
´
as Budav
´
ari
26
,
Larry N. Carey
10
, Samuel Carliles
26
, Michael A. Carr
11
, Francisco J. Castander
27
, David Cinabro
28
, A. J. Connolly
10
,
Istv
´
an Csabai
29
, Carlos E. Cunha
30
, Paul C. Czarapata
2
, James R. A. Davenport
31
, Ernst de Haas
32
, Ben Dilday
33,34,35
,
Mamoru Doi
36,37
, Daniel J. Eisenstein
38
, Michael L. Evans
10
,N.W.Evans
17
, Xiaohui Fan
38
, Scott D. Friedman
39
,
Joshua A. Frieman
2,34,40
, Masataka Fukugita
41
, Boris T. G
¨
ansicke
42
, Evalyn Gates
34
, Bruce Gillespie
26
,G.Gilmore
17
,
Belinda Gonzalez
2
, Carlos F. Gonzalez
2
, Eva K. Grebel
43
, James E. Gunn
11
, Zsuzsanna Gy
¨
ory
29
, Patrick B. Hall
44
,
Paul Harding
45
, Frederick H. Harris
46
, Michael Harvanek
47
, Suzanne L. Hawley
10
, Jeffrey J. E. Hayes
48
, Timothy
M. Heckman
26
, John S. Hendry
2
, Gregory S. Hennessy
49
, Robert B. Hindsley
50
, J. Hoblitt
51
, Craig J. Hogan
2
, David
W. Hogg
21
, Jon A. Holtzman
9
, Joseph B. Hyde
19
, Shin-ichi Ichikawa
52
, Takashi Ichikawa
53
, Myungshin Im
54
,
ˇ
Zeljko Ivezi
´
c
10
, Sebastian Jester
12
, Linhua Jiang
38
, Jennifer A. Johnson
6
, Anders M. Jorgensen
55
, Mario Juri
´
c
56
,
Stephen M. Kent
2
,R.Kessler
34
,S.J.Kleinman
57
,G.R.Knapp
11
, Kohki Konishi
41,58
, Richard G. Kron
2,40
,
Jurek Krzesinski
8,59
, Nikolay Kuropatkin
2
, Hubert Lampeitl
60
, Svetlana Lebedeva
2
, Myung Gyoon Lee
54
, Young
Sun Lee
16
, R. French Leger
10
,S
´
ebastien L
´
epine
61
, Nolan Li
26
, Marcos Lima
19,33,34
,HuanLin
2
,DanielC.Long
8
, Craig
P. Loomis
11
, Jon Loveday
62
, Robert H. Lupton
11
, Eugene Magnier
51
, Olena Malanushenko
8
, Viktor Malanushenko
8
,
Rachel Mandelbaum
56,103
, Bruce Margon
63
, John P. Marriner
2
, David Mart
´
ınez-Delgado
64
, Takahiko Matsubara
65
,
Peregrine M. McGehee
7
, Timothy A. McKay
30
, Avery Meiksin
66
, Heather L. Morrison
45
, Fergal Mullally
11
, Jeffrey
A. Munn
46
, Tara Murphy
66,67
, Thomas Nash
2
, Ada Nebot
68
, Eric H. Neilsen, Jr.
2
, Heidi Jo Newberg
69
, Peter
R. Newman
8,70
, Robert C. Nichol
60
, Tom Nicinski
2,71
, Maria Nieto-Santisteban
26
, Atsuko Nitta
57
,
Sadanori Okamura
72
, Daniel J. Oravetz
8
, Jeremiah P. Ostriker
11
, Russell Owen
10
, Nikhil Padmanabhan
73,103
,
Kaike Pan
8
, Changbom Park
74
, George Pauls
11
, John Peoples Jr.
2
, Will J. Percival
60
, Jeffrey R. Pier
46
, Adrian
C. Pope
51,75
, Dimitri Pourbaix
11,76
, Paul A. Price
51
, Norbert Purger
29
, Thomas Quinn
10
, M. Jordan Raddick
26
, Paola
Re Fiorentin
12,77
, Gordon T. Richards
78
, Michael W. Richmond
79
, Adam G. Riess
26
, Hans-Walter Rix
12
, Constance
M. Rockosi
80
, Masao Sako
19,81
, David J. Schlegel
73
, Donald P. Schneider
82
, Ralf-Dieter Scholz
68
, Matthias
R. Schreiber
83
, Axel D. Schwope
68
,Uro
ˇ
s Seljak
73,84,85
, Branimir Sesar
10
, Erin Sheldon
21,86
, Kazu Shimasaku
72
,
Valena C. Sibley
2
, A. E. Simmons
8
, Thirupathi Sivarani
16,87
, J. Allyn Smith
88
, Martin C. Smith
17
, Vernesa Smol
ˇ
ci
´
c
89
,
Stephanie A. Snedden
8
, Albert Stebbins
2
, Matthias Steinmetz
68
, Chris Stoughton
2
, Michael A. Strauss
11
,
Mark SubbaRao
40,90
, Yasushi Suto
58
, Alexander S. Szalay
26
, Istv
´
an Szapudi
51
, Paula Szkody
10
, Masayuki Tanaka
91
,
Max Tegmark
92
, Luis F. A. Teodoro
93
, Aniruddha R. Thakar
26
, Christy A. Tremonti
12
, Douglas L. Tucker
2
,
Alan Uomoto
94
, Daniel E. Vanden Berk
82,95
, Jan Vandenberg
26
,S.Vidrih
43
, Michael S. Vogeley
78
,
Wolfgang Voges
96
, Nicole P. Vogt
9
, Yogesh Wadadekar
11,97
, Shannon Watters
8,98
, David H. Weinberg
6
, Andrew
A. West
22
, Simon D. M. White
99
, Brian C. Wilhite
100
, Alainna C. Wonders
26
, Brian Yanny
2
,D.R.Yocum
2
, Donald
G. York
40,101
, Idit Zehavi
45
, Stefano Zibetti
12
, and Daniel B. Zucker
17
1
Department of Physics, University of Maryland, College Park, MD 20742, USA
2
Fermi National Accelerator Laboratory, P.O. Box 500, Batavia, IL 60510, USA
3
Columbia Astrophysics Laboratory, 550 West 120th Street, New York, NY 10027, USA
4
Department of Physics and Astronomy, University of Wyoming, Laramie, WY 82071, USA
5
Mullard Space Science Laboratory, University College London, Holmbury Sl Mary, Surrey, RH5 6NT, UK
6
Department of Astronomy, Ohio State University, 140 West 18th Avenue, Columbus, OH 43210, USA
7
IPAC, MS 220-6, California Institute of Technology, Pasadena, CA 91125, USA
8
Apache Point Observatory, P.O. Box 59, Sunspot, NM 88349, USA
9
Department of Astronomy, MSC 4500, New Mexico State University, P.O. Box 30001, Las Cruces, NM 88003, USA
10
Department of Astronomy, University of Washington, Box 351580, Seattle, WA 98195, USA
11
Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA
12
Max-Planck-Institut f
¨
ur Astronomie, K
¨
onigstuhl 17, D-69117 Heidelberg, Germany
13
McDonald Observatory and Department of Astronomy, The University of Texas, 1 University Station, C1400, Austin, TX 78712-0259, USA
14
South African Astronomical Observatory, Observatory, Cape Town, South Africa
15
University of Cape Town, Rondebosch, Cape Town, South Africa
16
Department of Physics & Astrophysics, CSCE: Center for the Study of Cosmic Evolution, and JINA: Joint Institute for Nuclear Astrophysics, Michigan State
University, E. Lansing, MI 48824, USA
17
Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge, CB3 0HA, UK
18
Department of Physics and Astronomy, Vanderbilt University, Nashville, TN 37235, USA
19
Department of Physics and Astronomy, University of Pennsylvania, 209 South 33rd Street, Philadelphia, PA 19104, USA
20
Herzberg Institute of Astrophysics, National Research Council of Canada, 5071 West Saanich Road, Victoria, B. C., V9E 2E7, Canada
21
Center for Cosmology and Particle Physics, Department of Physics, New York University, 4 Washington Place, New York, NY 10003, USA
22
MIT Kavli Institute for Astrophysics and Space Research, 77 Massachusetts Avenue, Cambridge, MA 02139, USA
23
Leiden Observatory, Leiden University, P.O. Box 9513, 2300 RA Leiden, The Netherlands
543

544 ABAZAJIAN ET AL. Vol. 182
24
Centro de Astrof
´
ısica da Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
25
Department of Astronomy, University of Illinois, 1002 West Green Street, Urbana, IL 61801, USA
26
Center for Astrophysical Sciences, Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218, USA
27
Institut de Ci
`
encies de l’Espai (IEEC/CSIC), Campus UAB, E-08193 Bellaterra, Barcelona, Spain
28
Department of Physics and Astronomy, Wayne State University, Detroit, MI 48202, USA
29
Department of Physics of Complex Systems, E
¨
otv
¨
os Lor
´
and University, Pf. 32, H-1518 Budapest, Hungary
30
Departments of Physics and Astronomy, University of Michigan, 450 Church Street, Ann Arbor, MI 48109, USA
31
Department of Astronomy, San Diego State University, PA 221, 5500 Campanile Drive, San Diego, CA 92182-1221, USA
32
Joseph Henry Laboratories, Princeton University, Princeton, NJ 08544, USA
33
Department of Physics, University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637, USA
34
Kavli Institute for Cosmological Physics, The University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637, USA
35
Department of Physics and Astronomy Rutgers, The State University of New Jersey 136 Frelinghuysen Road Piscataway, NJ 08854-8019, USA
36
Institute of Astronomy, Graduate School of Science, The University of Tokyo, 2-21-1 Osawa, Mitaka, 181-0015, Japan
37
Institute for the Physics and Mathematics of the Universe, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, 277-8568, Japan
38
Steward Observatory, 933 North Cherry Avenue, Tucson, AZ 85721, USA
39
Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
40
Department of Astronomy and Astrophysics, University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637, USA
41
Institute for Cosmic Ray Research, The University of Tokyo, 5-1-5 Kashiwa, Kashiwa City, Chiba 277-8582, Japan
42
Department of Physics, University of Warwick, Coventry, CV4 7AL, UK
43
Astronomisches Rechen-Institut, Zentrum f
¨
ur Astronomie, University of Heidelberg, M
¨
onchhofstrasse 12-14, D-69120 Heidelberg, Germany
44
Department of Physics & Astronomy, York University, 4700 Keele St., Toronto, ON, M3J 1P3, Canada
45
Department of Astronomy, Case Western Reserve University, Cleveland, OH 44106, USA
46
US Naval Observatory, Flagstaff Station, 10391 W. Naval Observatory Road, Flagstaff, AZ 86001-8521, USA
47
Lowell Observatory, 1400 W Mars Hill Rd, Flagstaff AZ 86001, USA
48
Heliophysics Division, Science Mission Directorate, NASA Headquarters, 300 E Street SW, Washington, DC 20546-0001, USA
49
US Naval Observatory, 3540 Massachusetts Avenue NW, Washington, DC 20392, USA
50
Code 7215, Remote Sensing Division, Naval Research Laboratory, 4555 Overlook Avenue SW, Washington, DC 20392, USA
51
Institute for Astronomy, 2680 Woodlawn Road, Honolulu, HI 96822, USA
52
National Astronomical Observatory, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
53
Astronomical Institute, Tohoku University, Aoba, Sendai 980-8578, Japan
54
Department of Physics & Astronomy, Seoul National University, Shillim-dong, San 56-1, Kwanak-gu, Seoul 151-742, Korea
55
Electrical Engineering Department, New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, NM 87801, USA
56
Institute for Advanced Study, Einstein Drive, Princeton, NJ 08540, USA
57
Gemini Observatory, 670 N. A’ohoku Place, Hilo, HI 96720, USA
58
Department of Physics and Research Center for the Early Universe, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo,
Tokyo 113-0033, Japan
59
Obserwatorium Astronomiczne na Suhorze, Akademia Pedogogiczna w Krakowie, ulica Podchora
˙
zych 2, PL-30-084 Krac
´
ow, Poland
60
Institute of Cosmology and Gravitation (ICG), Mercantile House, Hampshire Terrace, University of Portsmouth, Portsmouth, PO1 2EG, UK
61
Department of Astrophysics, American Museum of Natural History, Central Park West at 79th Street, New York, NY 10024, USA
62
Astronomy Centre, University of Sussex, Falmer, Brighton, BN1 9QH, UK
63
Department of Astronomy & Astrophysics, University of California, Santa Cruz, CA 95064, USA
64
Instituto de Astrof
´
ısica de Canarias, E38205 La Laguna, Tenerife, Spain
65
Department of Physics and Astrophysics, Nagoya University, Chikusa, Nagoya 464-8602, Japan
66
SUPA, Institute for Astronomy, Royal Observatory, University of Edinburgh, Blackford Hill, Edinburgh, EH9 3HJ, UK
67
Sydney Institute of Astronomy, The University of Sydney, NSW 2006, Australia
68
Astrophysical Institute Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
69
Department of Physics, Applied Physics, and Astronomy, Rensselaer Polytechnic Institute, 110 Eighth Street, Troy, NY 12180, USA
70
322 Fulham Palace Road, London, SW6 6HS, UK
71
CMC Electronics Aurora, 84 N. Dugan Rd. Sugar Grove, IL 60554, USA
72
Department of Astronomy and Research Center for the Early Universe, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo
113-0033, Japan
73
Lawrence Berkeley National Laboratory, One Cyclotron Road, Berkeley, CA 94720, USA
74
Korea Institute for Advanced Study, 87 Hoegiro, Dongdaemun-Gu, Seoul 130-722, Korea
75
Los Alamos National Laboratory, P.O. Box 1663, Los Alamos, NM 87545, USA
76
FNRS Institut d’Astronomie et d’Astrophysique, Universit
´
e Libre de Bruxelles, CP 226, Boulevard du Triomphe, B-1050 Bruxelles, Belgium
77
Department of Physics, University of Ljubljana, Jadranska 19, 1000 Ljubljana, Slovenia
78
Department of Physics, Drexel University, 3141 Chestnut Street, Philadelphia, PA 19104, USA
79
Department of Physics, Rochester Institute of Technology, 84 Lomb Memorial Drive, Rochester, NY 14623-5603, USA
80
UCO/Lick Observatory, University of California, Santa Cruz, CA 95064, USA
81
Kavli Institute for Particle Astrophysics & Cosmology, Stanford University, P.O. Box 20450, MS29, Stanford, CA 94309, USA
82
Department of Astronomy and Astrophysics, 525 Davey Laboratory, Pennsylvania State University, University Park, PA 16802, USA
83
Universidad de Valparaiso, Departamento de Fisica y Astronomia, Valparaiso, Chile
84
Physics Department, University of California, Berkeley, CA 94720, USA
85
Institute for Theoretical Physics, University of Zurich, Zurich 8057, Switzerland
86
Bldg 510 Brookhaven National Laboratory Upton, NY 11973, USA
87
Department of Astronomy, University of Florida, Bryant Space Science Center, Gainesville, FL 32611-2055, USA
88
Department of Physics and Astronomy, Austin Peay State University, P.O. Box 4608, Clarksville, TN 37040, USA
89
California Institute of Technology, 1200 East California Blvd, Pasadena, CA 91125, USA
90
Adler Planetarium and Astronomy Museum, 1300 Lake Shore Drive, Chicago, IL 60605, USA
91
European Southern Observatory, Karl-Schwarzschild-Str. 2, D-85748 Garching bei M
¨
unchen, Germany
92
Department of Physics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
93
Astronomy and Astrophysics Group, Department of Physics and Astronomy, Kelvin Building, University of Glasgow, Glasgow, G12 8QQ, Scotland, UK
94
Observatories of the Carnegie Institution of Washington, 813 Santa Barbara Street, Pasadena, CA 91101, USA
95
Department of Physics, Saint Vincent College, 300 Fraser Purchase Road, Latrobe, PA 15650, USA
96
Max-Planck-Institut f
¨
ur extraterrestrische Physik, Giessenbachstrasse 1, D-85741 Garching, Germany

No. 2, 2009 SEVENTH DATA RELEASE OF THE SLOAN DIGITAL SKY SURVEY 545
97
National Centre for Radio Astrophysics, Tata Institute of Fundamental Research, Post Bag 3, Ganeshkhind, Pune 411007, India
98
Advanced Technology and Research Center, Institute for Astronomy, 34 Ohia Ku St., Pukalani, HI 96768, USA
99
Max-Planck-Institut f
¨
ur Astrophysik, Postfach 1, D-85748 Garching, Germany
100
Department of Physics, Elmhurst College, 190 Prospect Ave., Elmhurst, IL 60126, USA
101
Enrico Fermi Institute, University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637, USA
Received 2008 December 2; accepted 2009 March 19; published 2009 May 18
ABSTRACT
This paper describes the Seventh Data Release of the Sloan Digital Sky Survey (SDSS), marking the completion
of the original goals of the SDSS and the end of the phase known as SDSS-II. It includes 11,663 deg
2
of imaging
data, with most of the ∼2000 deg
2
increment over the previous data release lying in regions of low Galactic
latitude. The catalog contains five-band photometry for 357 million distinct objects. The survey also includes
repeat photometry on a 120
◦
long, 2.
◦
5 wide stripe along the celestial equator in the Southern Galactic Cap, with
some regions covered by as many as 90 individual imaging runs. We include a co-addition of the best of these
data, going roughly 2 mag fainter than the main survey over 250 deg
2
. The survey has completed spectroscopy
over 9380 deg
2
; the spectroscopy is now complete over a large contiguous area of the Northern Galactic Cap,
closing the gap that was present in previous data releases. There are over 1.6 million spectra in total, including
930,000 galaxies, 120,000 quasars, and 460,000 stars. The data release includes improved stellar photometry at low
Galactic latitude. The astrometry has all been recalibrated with the second version of the USNO CCD Astrograph
Catalog, reducing the rms statistical errors at the bright end to 45 milliarcseconds per coordinate. We further quantify
a systematic error in bright galaxy photometry due to poor sky determination; this problem is less severe than
previously reported for the majority of galaxies. Finally, we describe a series of improvements to the spectroscopic
reductions, including better flat fielding and improved wavelength calibration at the blue end, better processing
of objects with extremely strong narrow emission lines, and an improved determination of stellar metallicities.
Key words: atlases – catalogs – surveys
Online-only material: color figures
1. OVERVIEW OF THE SLOAN DIGITAL SKY SURVEY
The Sloan Digital Sky Survey (SDSS; York et al. 2000)saw
first light a decade ago, with the goals of obtaining CCD imaging
in five broad bands over 10,000 deg
2
of high-latitude sky, and
spectroscopy of a million galaxies and 100,000 quasars over
this same region. With this, its seventh public data release,
these goals have been realized. The survey facilities have
also been used to carry out a comprehensive imaging and
spectroscopic survey to explore the structure, composition, and
kinematics of the Milky Way Galaxy (Sloan Extension for
Galactic Understanding and Exploration (SEGUE); Yanny et al.
2009), and a repeat imaging survey that has discovered more
than 500 spectroscopically confirmed Type Ia supernovae with
superb light curves (Frieman et al. 2008; Holtzman et al. 2008).
The SDSS uses a dedicated wide-field 2.5 m telescope (Gunn
et al. 2006) located at Apache Point Observatory (APO) near
Sacramento Peak in Southern New Mexico. The telescope uses
two instruments. The first is a wide-field imager (Gunn et al.
1998) with 24 2048 × 2048 CCDs on the focal plane with 0.

396
pixels that covers the sky in drift-scan mode in five filters in the
order riuzg (Fukugita et al. 1996). The imaging is done with the
telescope tracking great circles at the sidereal rate; the effective
exposure time per filter is 54.1 s, and 18.75 deg
2
are imaged
per hour in each of the five filters. The images are mostly taken
under good seeing conditions (the median is about 1.

4inr)on
moonless photometric nights (Hogg et al. 2001); the exceptions
are a series of repeat scans of the celestial equator in the Fall
for a supernova search (Frieman et al. 2008), as is described
in more detail in Section 3.2. The 95% completeness limits
of the images are u, g, r, i, z = 22.0, 22.2, 22.2, 21.3, 20.5,
102
NSF Astronomy and Astrophysics Postdoctoral Fellow.
103
Hubble Fellow.
respectively (Abazajian et al. 2004), although these values
depend as expected on seeing and sky brightness. The images
are processed through a series of pipelines that determine an
astrometric calibration (Pier et al. 2003) and detect and measure
the brightnesses, positions, and shapes of objects (Lupton
et al. 2001; Stoughton et al. 2002). The astrometry is good
to 45 milliarcseconds (mas) rms per coordinate at the bright
end, as described in more detail in Section 4.4. The photometry
is calibrated to an AB system (Oke & Gunn 1983), and the
zero points of the system are known to 1%–2% (Abazajian et al.
2004). The photometric calibration is done in two ways, by tying
to photometric standard stars (Smith et al. 2002) measured by a
separate 0.5 m telescope on the site (Tucker et al. 2006; Ivezi
´
c
et al. 2004), and by using the overlap between adjacent imaging
runs to tie the photometry of all the imaging observations
together, in a process called ubercalibration (Padmanabhan et al.
2008). Results of both processes are made available; with this
data release, the ubercalibration results, which are uncertain
at the ∼1% level in griz and 2% in u, are now the default
photometry made available in the data release described in this
paper.
The photometric catalogs of detected objects
are used to identify objects for spectroscopy with the second
of the instruments on the telescope: a 640-fiber-fed pair of mul-
tiobject double spectrographs, giving coverage from 3800 Å to
9200 Å at a resolution of λ/Δλ  2000. The objects chosen for
spectroscopic follow-up are selected based on photometry cor-
rected for Galactic extinction following Schlegel et al. (1998;
hereafter SFD) and include:
1. A sample of galaxies complete to a Petrosian (1976)
magnitude limit of r = 17.77 (Strauss et al. 2002).
2. Two deeper samples of luminous red ellipticals selected
in color–magnitude space to r = 19.2 and r = 19.5,

546 ABAZAJIAN ET AL. Vol. 182
respectively, which produce an approximately volume-
limited sample to z = 0.38, and a flux-limited sample
extending to z = 0.55, respectively (Eisenstein et al. 2001).
3. Flux-limited samples of quasar candidates, selected by
their nonstellar colors or FIRST (Becker et al. 1995) radio
emission to i = 19.1 in regions of color space characteristic
of z<3 quasars, and to i = 20.2 for quasars with
3 <z<5.5 (Richards et al. 2002).
4. A variety of ancillary samples, including optical counter-
parts to ROSAT-detected X-ray sources (Anderson et al.
2007).
5. Stars for spectrophotometric calibration and telluric absorp-
tion correction, as well as regions of blank sky for accurate
sky subtraction.
6. A variety of categories of stellar targets with a series of color
and magnitude cuts for measurements of radial velocity,
metallicity, surface temperature, and Galactic structure as
part of SEGUE (Yanny et al. 2009).
These targets are arranged on tiles of radius 1.
◦
49, with centers
chosen to maximize the number of targeted objects (Blanton
et al. 2003). Each tile contains 640 objects, and forms the
template for an aluminum spectroscopic plate, in which holes
are drilled to hold optical fibers that feed the spectrographs.
Spectroscopic exposures are 15 minutes long, and three or more
are taken for a given plate to reach predefined requirements
of signal-to-noise ratio (S/N), namely (S/N)
2
> 15 per 1.5 Å
pixel for stellar objects of fiber magnitude g = 20.2,r= 20.25,
and i = 19.9. For the SEGUE faint plates, the exposures are
considerably deeper, and typically consist of eight 15 minute
exposures, giving (S/N)
2
∼ 100 at the same depth (Yanny et al.
2009).
Spectra are extracted and calibrated in wavelength and flux.
The typical S/N of a galaxy near the main sample flux limit is
10 per pixel. The broadband spectrophotometric calibration is
accurate to 4% rms for point sources (Adelman-McCarthy et al.
2008), and the wavelength calibration is good to 2 km s
−1
.The
spectra are classified and redshifts determined using a pair of
pipelines (Stoughton et al. 2002; Subbarao et al. 2002), which
give consistent results 98% of the time; the discrepant objects
tend to be of very low S/N, or very unusual objects, such
as extreme broad absorption line quasars, superposed sources,
and so on. The vast majority of the spectra of galaxies and
quasars yield reliable redshifts; the failure rate is of order 1%
for galaxies and slightly larger for quasars. The stellar targets
are further processed by a separate pipeline (Lee et al. 2008a,
2008b; Allende Prieto et al. 2008a) which determines surface
temperatures, metallicities, and gravities.
The resulting catalogs are stored and distributed via a database
accessible on the web (the Catalog Archive Server (CAS);
104
Thakar et al. 2008), and the images and flat files are available in
bulk through the Data Archive Server (DAS).
105
The SDSS saw first light in 1998 May and started routine
operations in 2000 April. It was originally funded for five years
of operations, but had not completed its core goals of imaging
and spectroscopy of a large contiguous area of the Northern
Galactic Cap by 2005. The surveywasextended for an additional
three years, with the additional goals of the SEGUE and the
supernova surveys mentioned above. The extended program
is known as SDSS-II, and the component of SDSS-II that
104
http://cas.sdss.org/astro
105
http://das.sdss.org
represents the completion of SDSS-I is known as the Legacy
Survey. SDSS-II observations were completed in 2008 July.
The SDSS data have been made public in a series of yearly
data releases (Stoughton et al. 2002; Abazajian et al. 2003, 2004,
2005; Adelman-McCarthy et al. 2006, 2007, 2008; hereafter
the EDR, DR1, DR2, DR3, DR4, DR5, and DR6 papers,
respectively). The most recent of these papers described the
Sixth Data Release (DR6), which included data taken through
2006 July. The present paper describes the Seventh Data Release
(DR7), including data taken through the end of SDSS-II in
2008 July, and thus represents two additional years of data. The
data releases are cumulative; DR7 includes all data included
in the previous releases as well. In Section 2, we describe the
footprint of this survey; most importantly, we have completed
our goals of
1. contiguous imaging and spectroscopy over 7500 deg
2
of
the Northern Galactic Cap (the Legacy Survey);
2. imaging and spectroscopy of stellar sources over an addi-
tional 3500 deg
2
at lower Galactic latitudes to study the
structure of the Milky Way; and
3. repeat imaging of >250 deg
2
on the celestial equator
in the Fall months to discover Type Ia supernovae with
0.1 <z<0.4.
In Section 3, we describe the repeat scans on the celestial
equator, including a co-addition of the images to reach about
2 mag deeper than the main survey. In Section 4, we present
improvements in the processing of the imaging data, includ-
ing improved stellar photometry at low Galactic latitudes, an
astrometric recalibration, and improvements in our photomet-
ric redshift algorithms for galaxies. The DR6 paper described
a problem with the photometry of bright galaxies; we explore
this further in Section 5. In Section 6, we discuss improvements
in the spectroscopic processing of the data. The DR6 paper de-
scribed improvements in the wavelength and spectrophotomet-
ric calibration; we have implemented further refinements which
are important in the determination of accurate stellar parameters
from the spectra.
We conclude in Section 7 with a discussion of the future of
the SDSS project.
2. SURVEY FOOTPRINT
Table 1 summarizes the contents of DR7, giving the imaging
and spectroscopic sky coverage and number of objects. The
imaging footprint has increased by roughly 22% since DR6
(most of it outside the contiguous area of the North Galactic
Cap), and the number of spectra has increased by 29%.
The imaging for the Legacy Survey was substantially com-
plete with DR6. In DR7, we include imaging of a few small gaps
that were missed in the contiguous region of the North Galactic
Cap, and repeat observations of a few regions of the sky which
had particularly poor seeing in previous data releases. The total
footprint has increased by less than 10 deg
2
in total. The Legacy
imaging footprint is visible as the large contiguous gray area on
the left side of the upper panel of Figure 1, together with the
three gray stripes visible on the right side. The principal aug-
mentation of the imaging data in DR7 is the stripes which are
part of the SEGUE survey. They are indicated in red in the figure
and increase the SDSS imaging footprint by roughly 2000 deg
2
over DR6. Note that many of these cross the Galactic plane
(indicated by the sinuous line crossing the figure). Unlike DR6,
the union of the Legacy and SEGUE data are now available in
a single database in CAS in DR7.

No. 2, 2009 SEVENTH DATA RELEASE OF THE SLOAN DIGITAL SKY SURVEY 547
Tab le 1
Coverage and Contents of DR7
Imaging
Imaging area in CAS 11,663 deg
2
Imaging catalog in CAS 357 million unique objects
Legacy footprint area 8423 deg
2
(7646 deg
2
in North Galactic Cap)
Legacy imaging catalog 230 million unique objects
585 million entries (including duplicates)
SEGUE footprint area, available in DAS
a
3500 deg
2
(more than double DR6)
SEGUE footprint area, available in CAS 3240 deg
2
SEGUE imaging catalog 127 million unique objects
M31, Perseus, Sagittarius scan area ∼46 deg
2
Southern Equatorial Stripe with >70 repeat scans ∼250 deg
2
Commissioning (“Orion”) data 832 deg
2
Spectroscopy
Spectroscopic footprint area 9380 deg
2
Legacy 8032 deg
2
SEGUE 1348 deg
2
Total number of plate observations (640 fibers each) 2564
Legacy Survey plates 1802
SEGUE and special plates 676
Repeat observations of plates 86
Total number of spectra
b
1,630,960
Galaxies 929,555
Quasars 121,363
Stars 464,261
Sky 97,398
Unclassifiable 28,383
Spectra after removing skies and duplicates 1,440,961
Notes.
a
Includes regions of high stellar density, where the photometry is likely to be poor. See the text for details. This area also
includes some regions of overlap.
b
Spectral classifications from the spectro1d code; numbers include duplicates.
These data have been recalibrated using ubercalibration (Pad-
manabhan et al. 2008) using the overlap between adjacent scans;
the resulting photometry is now the default photometry found
in the CAS. We also make available the original photometry
calibrated by the auxiliary Photometric Telescope (Tucker et al.
2006). The ubercalibration solution was regenerated using all
the imaging data, but the changes are tiny from the ubercali-
bration results published in DR6: 0.001 mag rms in griz and
0.003 mag in u. The ubercalibrated photometry zero points are
defined to be the same as that measured from the Photometric
Telescope.
The green and blue patches indicate supplementary imaging
stripes, which contain scans over M31 or in its halo, through
the center of the Perseus cluster of galaxies, over the low-
latitude globular cluster M71, near the South Galactic Pole,
along the orbit of the Sagittarius Tidal Stream, and through
the star-forming regions of Orion (Finkbeiner et al. 2004). In
addition, there are a number of scans at angles perpendicular, or
at an oblique angle, to the regular Legacy or SEGUE imaging
stripes. These scans are used in the ubercalibration procedure to
tie the zero points of the stripes together and to determine the
flat fields.
The lower panel in Figure 1 shows the coverage of spec-
troscopy in DR7; the light gray area shows the increment in the
Legacy Survey over DR6. Most importantly, the gap cutting the
North Galactic Cap in two pieces in previous data releases has
been closed; we now have complete spectroscopy of our princi-
pal galaxy and quasar targets over a contiguous area of roughly
7500 deg
2
. An additional dozen plates were observed to fill holes
in the nominally contiguous regions in DR6. Adding in the three
stripes in the Southern Galactic Cap, the Legacy spectroscopy
footprint is 8032 deg
2
, a 26% increment over DR6.
In addition, spectroscopy was carried out using a series of
target selection algorithms designed to find stars of a wide
variety of types as part of the SEGUE project (DR6 paper;
Yanny et al. 2009). These targets were drawn from both the
SEGUE and Legacy imaging, and are shown in red in the lower
panel of Figure 1. As some of these are lost in the density of
Legacy spectra, we show the distribution of SEGUE and other
non-Legacy spectra in Galactic coordinates in Figure 2.
Finally, as described in Yanny et al. (2009), we carried out
spectroscopy of stars in 12 open and globularclusters to calibrate
the measurements of stellar parameters in SEGUE (Lee et al.
2008a, 2008b). Many of these clusters are sufficiently close that
the giant branches are brighter than the photometric saturation
limit of SDSS, so the targets for these plates were selected from
the literature. Indeed, the spectrographs would saturate as well
with our standard 15 minute exposures, so these observations
had individual exposure times as short as 1 or 2 minutes. Without
proper flux calibrators or exposure of bright sky lines to set the
zero point of the wavelength scale, the spectrophotometry and
wavelength calibration of the spectra on these plates are often
quite inferior to that of the main survey, and these plates are
available only in the DAS, not the CAS.
As described in more detail below, the 2.
◦
5 stripe centered
on the celestial equator was imaged multiple times through-

Frequently asked questions

Q1. What are the contributions in "C: " ?

This paper describes the Seventh Data Release of the Sloan Digital Sky Survey ( SDSS ), marking the completion of the original goals of the SDSS and the end of the phase known as SDSS-II. The authors further quantify a systematic error in bright galaxy photometry due to poor sky determination ; this problem is less severe than previously reported for the majority of galaxies. Finally, the authors describe a series of improvements to the spectroscopic reductions, including better flat fielding and improved wavelength calibration at the blue end, better processing of objects with extremely strong narrow emission lines, and an improved determination of stellar metallicities. 

Q2. How many spectra have been obtained for the SSPP?

In addition, high-resolution spectra have been obtained for about 100 field stars included in the SDSS, and used to expand the SSPP checks over a wider parameter space (Allende Prieto et al. 2008a). 

Q3. What is the effect of the flat field on the spectrograph?

If it shifts during an exposure, it will not be properly corrected by the flat field, causing significant distortion of blue absorption lines in stellar spectra, and systematically affecting estimates of metallicities and surface temperatures. 

Q4. Why is the pipeline known to fail at lower latitudes?

at lower latitudes, when the density of stars brighter than r = 21 grows above 5000 deg−2, the pipeline is known to fail, as it is unable to find sufficiently isolated stars to measure an accurate PSF, and the deblender does poorly with overly crowded images. 

Q5. What is the effect of the dichroic coating on the instrument response?

The thickness of the dichroic coating is believed to be sensitive to the ambient humidity, and moisture which enters the system during plate changes affects the instrument response, shifting the interference pattern in wavelength in unpredictableways on timescales comparable to the 900 s exposure time. 

Q6. Why is the wavelength solution poorly constrained?

for a small fraction of plates, the arcs are weak (perhaps because the arc lamps themselves were faulty at that time, or because the petals which reflect the arc lamp light were not properly deployed), and the wavelength solution is poorly constrained. 

Q7. What is the PSF model used to describe?

It uses an analytical model based on Gaussians to describe the basic PSF shape, with parameters which may vary across the field of the image to follow the PSF variations. 

Q8. How long did the spectrographs take to saturate?

the spectrographs would saturate as well with their standard 15 minute exposures, so these observations had individual exposure times as short as 1 or 2 minutes. 

Q9. How did the code work in crowded regions?

In crowded regions, one cannot find sufficiently isolated stars to measure counts through such a large aperture, and in practice, the code corrected PSF magnitudes to an aperture photometry radius of 3.′′00 instead, whenever any part of a given run dipped below |b| = 8◦. 

Q10. What conditions were used to take the images of Stripe 82?

In Fall 2005, 2006, and 2007, 219 additional imaging runs were taken on Stripe 82 as part of the SDSS supernova survey (Frieman et al. 2008), often under less optimal conditions: poor seeing, bright moonlight, and/or nonphotometric conditions. 

Q11. What is the reason why the CAS is marked with a problem?

Only 0.3% of all fields in the CAS are marked with one of these problems (the majority of which are due to focus problems); these flags should be consulted when examining the reliability of the photometry in a given area of sky. 

Q12. How can a spectroscopic wavelength be distorted when there is substantial moonlight?

when there is substantial moonlight in the sky spectrum, a fit to what is assumed to be an isolated emission line can be significantly biased, systematically skewing the wavelength solution at the blue end by as much as 20 km s−1. 

Q13. How did Mandelbaum et al. (2008) find that summing the p(?

Cunha et al. (2008) showed that summing the p(z) for a sample of galaxies yields a better estimation of their true redshift distribution than that of the individual photometric redshifts. 

Q14. How was the algorithm applied to the stars in the catalog?

In order to allow users to analyze completeness and efficiency of SEGUE stellar target selection samples, the latest (v4.6) version of the algorithms (Yanny et al. 2009) was applied to all stellar objects in the imaging catalog which had g < 21 or z < 21, over the entire sky. 

Q15. How was the PSPhot photometry used to make the stars on the same system?

PSPhot photometry was forced to agree with these large-aperture magnitudes for bright stars; this was done in practice by determining, for each CCD in the imaging camera for each run, the average aperture correction needed to put the two on the same system, using stars at Galactic latitude |b| > 15◦, where crowding effects are less severe. 

Q16. How is the flux of a simulated galaxies underestimated?

On average, however, the flux is underestimated by approximately 0.2 mag at r = 12.5 and <0.1 mag at r = 15 for simulated galaxies with an Sérsic index of 1. 

Q17. What is the rms error of the redshift estimation for the reference set?

The rms error of the redshift estimation for the reference set decreases from 0.044 in DR6 to 0.025 in DR7 with this improved algorithm (Figure 5). 

Q18. Why did Bramich et al. solve for the offsets?

Bramich et al. solved for photometric offsets both parallel and perpendicular to the scan direction in data from a given CCD; the authors found that the term perpendicular to the scan direction added little, and the authors did not include it here.