The Astrophysical Journal, 729:62 (16pp), 2011 March 1 doi:10.1088/0004-637X/729/1/62 C 2011. The American Astronomical Society. All rights reserved. Printed in the U.S.A. THE ATACAMA COSMOLOGY TELESCOPE: A MEASUREMENT OF THE COSMIC MICROWAVE BACKGROUND POWER SPECTRUM AT 148 AND 218 GHz FROM THE 2008 SOUTHERN SURVEY Sudeep Das1,2,3, Tobias A. Marriage3,31, Peter A. R. Ade4, Paula Aguirre5, Mandana Amiri6, John W. Appel2, L. Felipe Barrientos5, Elia S. Battistelli6,7, John R. Bond8, Ben Brown9, Bryce Burger6, Jay Chervenak10, Mark J. Devlin11, Simon R. Dicker11, W. Bertrand Doriese12, Joanna Dunkley2,3,13, Rolando Dunner¨ 5, Thomas Essinger-Hileman2, Ryan P. Fisher2, Joseph W. Fowler2,12,AmirHajian2,3,8, Mark Halpern6, Matthew Hasselfield6, Carlos Hernandez-Monteagudo´ 14, Gene C. Hilton12, Matt Hilton15,16, Adam D. Hincks2, Renee´ Hlozek13, Kevin M. Huffenberger17,DavidH.Hughes18,JohnP.Hughes19, Leopoldo Infante5,KentD.Irwin12, Jean Baptiste Juin5, Madhuri Kaul11, Jeff Klein11, Arthur Kosowsky9,JudyM.Lau2,20,21, Michele Limon2,11,22, Yen-Ting Lin3,5,23, Robert H. Lupton3, Danica Marsden11, Krista Martocci2,24, Phil Mauskopf4, Felipe Menanteau19, Kavilan Moodley15,16, Harvey Moseley10, Calvin B. Netterfield25, Michael D. Niemack2,12, Michael R. Nolta8, Lyman A. Page2, Lucas Parker2, Bruce Partridge26, Beth Reid2,27, Neelima Sehgal20, Blake D. Sherwin2, Jon Sievers8, David N. Spergel3, Suzanne T. Staggs2, Daniel S. Swetz11,12, Eric R. Switzer2,24, Robert Thornton11,28, Hy Trac29,30, Carole Tucker4, Ryan Warne15, Ed Wollack10, and Yue Zhao2 1 Berkeley Center for Cosmological Physics, LBL and Department of Physics, University of California, Berkeley, CA 94720, USA 2 Joseph Henry Laboratories of Physics, Jadwin Hall, Princeton University, Princeton, NJ 08544, USA 3 Department of Astrophysical Sciences, Peyton Hall, Princeton University, Princeton, NJ 08544, USA 4 School of Physics and Astronomy, Cardiff University, The Parade, Cardiff, Wales CF24 3AA, UK 5 Departamento de Astronom´ıa y Astrof´ısica, Facultad de F´ısica, Pontific´ıa Universidad Catolica,´ Casilla 306, Santiago 22, Chile 6 Department of Physics and Astronomy, University of British Columbia, Vancouver, BC V6T 1Z4, Canada 7 Department of Physics, University of Rome “La Sapienza,” Piazzale Aldo Moro 5, I-00185 Rome, Italy 8 Canadian Institute for Theoretical Astrophysics, University of Toronto, Toronto, ON M5S 3H8, Canada 9 Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, PA 15260, USA 10 Code 553/665, NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA 11 Department of Physics and Astronomy, University of Pennsylvania, 209 South 33rd Street, Philadelphia, PA 19104, USA 12 NIST Quantum Devices Group, 325 Broadway Mailcode 817.03, Boulder, CO 80305, USA 13 Department of Astrophysics, Oxford University, Oxford OX1 3RH, UK 14 Max-Planck-Institut fur¨ Astrophysik, Postfach 1317, D-85741 Garching bei Munchen,¨ Germany 15 Astrophysics and Cosmology Research Unit, School of Mathematical Sciences, University of KwaZulu-Natal, Durban 4041, South Africa 16 Centre for High Performance Computing, CSIR Campus, 15 Lower Hope St., Rosebank, Cape Town, South Africa 17 Department of Physics, University of Miami, Coral Gables, FL 33124, USA 18 Instituto Nacional de Astrof´ısica, Optica´ y Electronica´ (INAOE), Tonantzintla, Puebla, Mexico 19 Department of Physics and Astronomy, Rutgers, The State University of New Jersey, Piscataway, NJ 08854-8019, USA 20 Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, Stanford, CA 94305-4085, USA 21 Department of Physics, Stanford University, Stanford, CA 94305-4085, USA 22 Columbia Astrophysics Laboratory, 550 W. 120th St., Mail Code 5247, New York, NY 10027, USA 23 Institute for the Physics and Mathematics of the Universe, The University of Tokyo, Kashiwa, Chiba 277-8568, Japan 24 Kavli Institute for Cosmological Physics, 5620 South Ellis Avenue, Chicago, IL 60637, USA 25 Department of Physics, University of Toronto, 60 St. George Street, Toronto, ON M5S 1A7, Canada 26 Department of Physics and Astronomy, Haverford College, Haverford, PA 19041, USA 27 Institut de Ciencies del Cosmos (ICC), University of Barcelona, Barcelona 08028, Spain 28 Department of Physics, West Chester University of Pennsylvania, West Chester, PA 19383, USA 29 Department of Physics, Carnegie Mellon University, Pittsburgh, PA 15213, USA 30 Harvard-Smithsonian Center for Astrophysics, Harvard University, Cambridge, MA 02138, USA Received 2010 September 4; accepted 2011 January 1; published 2011 February 9 ABSTRACT We present measurements of the cosmic microwave background (CMB) power spectrum made by the Atacama Cosmology Telescope at 148 GHz and 218 GHz, as well as the cross-frequency spectrum between the two channels. Our results clearly show the second through the seventh acoustic peaks in the CMB power spectrum. The measurements of these higher-order peaks provide an additional test of the ΛCDM cosmological model. At >3000, we detect power in excess of the primary anisotropy spectrum of the CMB. At lower multipoles 500 <<3000, we find evidence for gravitational lensing of the CMB in the power spectrum at the 2.8σ level. We also detect a low level of Galactic dust in our maps, which demonstrates that we can recover known faint, diffuse signals. Key words: cosmic background radiation – cosmology: observations Online-only material: color figures 1. INTRODUCTION 31 Current address: Department of Physics and Astronomy, The Johns Hopkins Accurate measurements of the arcminute scale tempera- University, 3400 N. Charles St., Baltimore, MD 21218-2686, USA. ture anisotropies in the millimeter-wave sky are uncovering a 1 The Astrophysical Journal, 729:62 (16pp), 2011 March 1 Das et al. complex, yet revealing picture (Lueker et al. 2010; Fowler et al. in Swetz et al. (2010), a 148 GHz point-source catalog is 2010, hereafter F10). On intermediate scales (500 3000), presented in Marriage et al. (2010b, hereafter M10), and a the primordial acoustic features imprinted on the cosmic mi- 148 GHz SZ cluster catalog is presented in Marriage et al. crowave background (CMB) at last scattering (z 1100) domi- (2010a). Menanteau et al. (2010) discuss the multi-wavelength nate, with subtle distortions expected from gravitational lensing follow-up of ACT clusters, while Sehgal et al. (2010a) present σ8 by intervening large-scale structure. This intermediate range constraints from SZ cluster detections. On the power spectrum of multipoles is often called the damping tail of the CMB, as side, Hajian et al. (2010) report on the calibration of ACT maps the acoustic oscillations are exponentially damped due to pho- using cross-correlations with Wilkinson Microwave Anisotropy ton diffusion (Silk 1968; Bond & Efstathiou 1984). On the Probe (WMAP) seven-year maps, and Dunkley et al. (2010) smallest scales ( 3000), emission from radio- and dust- present the constraints on cosmological parameters derived from enshrouded star-forming galaxies (SFGs), together with the the power spectrum presented here. Sunyaev–Zel’dovich (SZ) effect (Zeldovich & Sunyaev 1969), The paper is organized as follows. In Section 2, we briefly dominates over primary CMB fluctuations. review the ACT instrument and observations made so far, The damping tail measurements are an additional test of the touching upon mapmaking and beam determination techniques, predictions of the ΛCDM cosmological model, a model that is and the calibration of our data. In Section 3, we describe the an excellent fit to current CMB data (e.g., Larson et al. 2011; method used in power spectrum estimation. Simulations used Reichardt et al. 2009b; Brown et al. 2009), large-scale structure to test our pipeline are discussed in Section 4. We present our measurements (e.g., Reid et al. 2010; Percival et al. 2010), results in Section 5 and discuss tests performed to validate our supernova observations (e.g., Kessler et al. 2009; Amanullah results. The main sources of foreground contamination and et al. 2010), and a host of other astronomical observations (see, the methods used to treat them are discussed in Section 6. e.g., Spergel et al. 2007; Komatsu et al. 2011 for reviews). The We discuss the lensing contribution to the power spectrum in amplitude of the fluctuations in the damping tail is a sensitive Section 7, before concluding in Section 8. probe of matter fluctuations at k 0.1–0.25 Mpc−1—thus, precision measurements constrain the spectral index of the 2. OBSERVATIONS AND MAPMAKING primordial curvature perturbations, n , and its variation with s ACT is a 6 m off-axis Gregorian telescope (Fowler et al. scale. Because the positions of the high-order acoustic peaks are 2007) situated at an elevation of 5190 m on Cerro Toco in the sensitive to the evolution of the sound speed of the universe and Atacama desert in northern Chile. ACT has three frequency its composition, these measurements constrain the primordial bands centered at 148 GHz (2.0 mm), 218 GHz (1.4 mm), helium fraction and the number of relativistic species including and 277 GHz (1.1 mm) with angular resolutions of roughly neutrinos (see, e.g., White 2001; Bashinsky & Seljak 2004; 1.4, 1.0, and 0.9, respectively. The high-altitude site in the Trotta & Hansen 2004; Ichikawa et al. 2008a, 2008b; Komatsu arid desert is excellent for microwave observations due to et al. 2011). While the damping tail gives us leverage on low precipitable water vapor and stability of the atmosphere. early universe cosmology, the composite tail ( 3000) is The tropical