High Resolution Observations of the CMB Power Spectrum with ACBAR

To appear in ApJ A Preprint typeset using L TEX style emulateapj v. 14/09/00 HIGH RESOLUTION OBSERVATIONS OF THE CMB POWER SPECTRUM WITH ACBAR C.L. Kuo1,2, P.A.R. Ade3, J.J. Bock4, C. Cantalupo5, M.D. Daub 1, J. Goldstein6,7, W.L. Holzapfel 1, A.E. Lange8, M. Lueker1, M. Newcomb1, J.B. Peterson9, J. Ruhl6, M.C. Runyan8, E. Torbet7 To appear in ApJ ABSTRACT We report the first measurements of anisotropy in the cosmic microwave background (CMB) radiation with the Arcminute Cosmology Bolometer Array Receiver (Acbar). The instrument was installed on the 2.1m Viper telescope at the South Pole in January 2001; the data presented here are the product of observations up to and including July 2002. The two deep fields presented here, have had offsets removed by subtracting lead and trail observations and cover approximately 24 deg2 of sky selected for low dust contrast. These results represent the highest signal to noise observations of CMB anisotropy to date; in the deepest 150 GHz band map, we reached an RMS of 8.0 µK per 5′ beam. The 3 degree extent of the maps, and small beamsize of the experiment allow the∼ measurement of the CMB anisotropy power spectrum over the range ℓ = 150 3000 with resolution of ∆ℓ = 150. The contributions of galactic dust and radio sources to the observed− anisotropy are negligible and are removed in the analysis. The resulting power spectrum is found to be consistent with the primary anisotropy expected in a concordance ΛCDM Universe. Subject headings: cosmic microwave background — cosmology: observations 1. introduction constraints on cosmological parameters. For example, the scale of the damping can be used to constrain Ω and Observations of the Cosmic Microwave Background M Ω (White 2001). Recent observations with the Cosmic (CMB) provide an unique probe of the Universe at the B epoch of matter and radiation decoupling. The physics Background Imager (CBI) have provided a first look at the damping tail of the CMB and found it to be consistent with of the early Universe can be described in terms of models that yield precise predictions for cosmological observables. models motivated by observations on larger angular scales Within the context of these models, observations the CMB (Sievers et al. 2002). On angular scales of a few arcmin- utes, deep pointings with the CBI detect power in excess of anisotropy power can be used to place constraints on the values of key cosmological parameters (White et al. 1994; that expected from primary CMB anisotropy (Mason et al. Hu & White 1996) 2002). In a companion paper to that work, this signal is interpreted as being due to the Sunyaev-Zel’dovich Effect On sub-horizon size scales, gravity driven acoustic os- cillations in the primordial plasma give rise to a series (SZE) in distant clusters of galaxies (Bond et al. 2002). Al- of harmonic peaks in the CMB angular power spectrum ternative interpretations have been proposed that explain the excess power as being due to local structure and non- (Sunyaev & Zel’dovich 1970; Bond & Efstathiou 1987; Hu et al. 1997). A number of experiments have characterized standard inflationary models (Cooray & Melchiorri 2002; the power spectrum up to and including the third acous- Griffiths et al. 2002). If the signal is due to the SZE, it’s precise characterization would provide information about tic peak (Lee et al. 2001; Netterfield et al. 2002; Halverson the growth of cluster scale structures (Holder et al. 2001; arXiv:astro-ph/0212289v1 12 Dec 2002 et al. 2002). These observations have been used to produce constraints on cosmological parameters such as the Zhang et al. 2002; Komatsu & Seljak 2002). The Arcminute Cosmology Bolometer Array Receiver total energy density and baryon density with a precision Acbar of 5 10% (Lange et al. 2001; Jaffe et al. 2001; Pryke ( ) is an instrument designed to produce detailed images of the CMB in three millimeter-wavelength bands. et al.∼ 2002;− Abroe et al. 2002). This paper is the first in a series reporting results from the On fine angular scales, the CMB power spectrum is ex- Acbar ponentially damped due to photon diffusion and the finite experiment, and describes the CMB angular power spectrum determined from the 150 GHz data. In a com- thickness of the surface of last scattering (Silk 1968; Hu panion paper, the Acbar CMB power spectrum and re- & White 1997). Observations of the CMB power spectrum in this region can be used to produce independent sults from other experiments are used to place constraints on cosmological parameters (Goldstein et al. 2002). Fur- 1 Department of Physics, University of California at Berkeley, Berkeley, CA 94720 2 Department of Astronomy, University of California at Berkeley, Berkeley, CA 94720 3 Department of Physics and Astronomy, Cardiff University, CF24 3YB Wales, UK 4 Jet Propulsion Laboratory, Pasadena, CA 91125 5 Lawerence Berkeley National Laboratory, Berkeley, CA 94720 6 Department of Physics, Case Western Reserve University, Cleveland, OH 44106 7 Department of Physics, University of California, Santa Barbara, CA 93106 8 Department of Physics, Math, and Astronomy, California Institute of Technology, Pasadena, CA 91125 9 Department of Physics, Carnegie Mellon University, Pittsburgh, PA 15213 1 2 Kuo et al. ther papers describing the Acbar instrument (Runyan ements. Behind each beam defining horn there is a filter et al. 2002b), SZE cluster searching (Runyan et al. 2002a), stack that defines the passbands and blocks high frequency and pointed SZE observations (Gomez et al. 2002) are in leaks. The band centers are 150, 220, and 280GHz with preparation. associated bandwidths of 30, 30, and 50 GHz, respectively. We present a brief overview of the telescope, receiver, In the 2002 observations, each of the 8 150 GHz channels and site in 2. Our observing technique, including data typically achieved a noise equivalent CMB temperature of editing and calibration§ are described in 3. The analysis of 340 µK√s. Details of the instrument construction and the data is presented in 4, including several§ developments performance∼ are presented by Runyan et al. (2002b). in the treatment of high§ sensitivity ground based CMB ob- The South Pole station is located at an altitude of servations. The main results of the paper are presented in 2900m and experiences ambient temperatures ranging 5. In 6, potential sources of foreground emission and from∼ 30◦C to 80◦C. In the best winter weather, the per- their§ treatment§ in the analysis are discussed. Tests for ceptible− water− vapor has been measured to be 0.2 mm systematic error in the power spectrum are discussed in (Chamberlin 2001). The high altitude, dry air,∼ and lack 7. In section 8, we present our conclusions. of diurnal variations result in a transparent and extremely § § stable atmosphere (Lay & Halverson 2000; Peterson et al. 2002). During the winters of 2001 and 2002, the typical acbar instrument 2. atmospheric optical depth for the Acbar 150 GHz chan- The Arcminute Cosmology Bolometer Array Receiver nels was measured to be approximately 3%. The entire (Acbar) is a 16 element 235mK bolometer array that has southern celestial hemisphere is available year round al- been used to image the sky in three millimeter-wavelength lowing very deep integrations. The combination of these bands. The instrument was designed to make use of unique features and the established infrastructure for re- the Viper telescope at the South Pole to produce multi- search make the South Pole a nearly ideal site for ground frequency maps of the CMB with high sensitivity and high based observations of the CMB. angular resolution ( 4 5′). During an observation, the telescope tracks the position The Viper telescope∼ − is a 2.1 m off-axis aplanatic Gre- of the observed field. Due to the proximity of the telescope gorian telescope designed specifically for observations of to the geographic South Pole, the telescope needs to ro- CMB anisotropy. A servo controlled chopping tertiary tate in azimuth with only small changes in elevation. The mirror is used to modulate the optical signal reaching the beams of the array follow a constant velocity 3◦ triangle Acbar receiver. The secondary mirror produces an im- wave in azimuth with a speed fast enough that the atmo- age of the primary mirror at the tertiary and therefore the sphere is essentially stationary, and slow enough that the motion of the chopping mirror produces minimal changes beams take several time constants to move across a point in the illumination pattern on the primary. With this sys- source on the sky. Subject to these constraints, the chop tem, it is possible to modulate the 16 Acbar beams 3◦ frequency was chosen to be 0.7Hz in 2001 and 0.3 Hz in in azimuth in a fraction of a second without introducing 2002. excessive modulated telescope emission or vibration. The The combination of large chop ( 3◦) and small beam primary is surrounded by an additional 0.5m skirt that sizes ( 4 5′) make Acbar sensitive∼ to a wide range of reflects primary spillover to the sky. To minimize possi- angular∼ scales− (150 <ℓ< 3000), with high ℓ-space res- ble ground pickup, a reflective conical ground shield com- olution (∆ℓ 150). Another unique feature of Acbar pletely surrounds the telescope, blocking emission from is its multi-frequency∼ coverage, which has the potential elevations below 20◦. to discriminate between sources of signal and foreground Between 1998-1999,∼ Viper was equipped with the single- confusion. The CMB power spectrum we present in this element HEMT-based CORONA receiver operating at work is derived from the 150 GHz channel data. In 2001, 40 GHz. These observations produced a detection of the Acbar was configured with four 150 GHz detectors; this first acoustic peak in the CMB power spectrum (Peterson number was increased to eight for the 2002 observations.

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