The Discovery of Anomalous Microwave Emission

Hindawi Publishing Corporation Advances in Astronomy Volume 2013, Article ID 352407, 6 pages http://dx.doi.org/10.1155/2013/352407 Review Article The Discovery of Anomalous Microwave Emission Erik M. Leitch1 and A. C. R. Readhead2 1 Department of Astronomy, University of Chicago, Chicago, IL 60637, USA 2 Department of Astronomy, California Institute of Technology, Pasadena, CA 91125, USA Correspondence should be addressed to Erik M. Leitch; [email protected] Received 21 November 2012; Accepted 14 January 2013 Academic Editor: Clive Dickinson Copyright © 2013 E. M. Leitch and A. C. R. Readhead. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. We discuss the first detection of anomalous microwave emission, in the Owens Valley RING5M experiment, and its interpretation in the context of the ground-based cosmic microwave background (CMB) experiments of the early 1990s. The RING5M experiment was one of the first attempts to constrain the anisotropy power on sub-horizon scales, by observing a set of 7 -size fields around the North Celestial Pole (NCP). Fields were selected close to the NCP to allow continuous integration from the Owens Valley site. The experiment detected significant emission at both 14.5 GHz and 30 GHz, consistent with a mixture of CMB and aflat- spectrum foreground component, which we termed anomalous, as it could be explained neither by thermal dust emission, nor by standard models for synchrotron or free-free emission. A significant spatial correlation was found between the extracted foreground component and structure in the IRAS 100 m maps. While microwave emission from spinning dust may be the most natural explanation for this correlation, spinning dust is unlikely to account for all of the anomalous emission seen in the RING5M data. 1. Introduction (with the next generation cameras like SPTpol, BICEPII, the Keck Array, PolarBear, and ACTpol already in operation). From the perspective of the 21st century cosmology, it can be By contrast, the early 1990s had just witnessed the first hard to imagine how primitive the state of our knowledge was ever detection of CMB anisotropy on super-horizon scales by a short twenty years ago and how rapidly the landscape was the COBE satellite [15]. A small number of Antarctic ground- changing at the time. Today, ground-based experiments like based experiments were trying to detect any indication of a the South Pole Telescope (SPT) and the Atacama Cosmology rise toward the first Doppler peak and where that peak might Telescope (ACT) have measured the high-ℓ power spectrum with enough resolution to detect the first nine Doppler peaks lie (e.g., ACME [16], Python [17], MAX [18], IAB [19]). It is (SPT [1, 2], ACT [3]) and enough sensitivity to detect the indicative of the state of the field that model power spectra background of SZ power from unresolved galaxy clusters [4]. were routinely displayed in log space, since the only feature The combination of ground, balloon-borne, and space-based anyone hoped to detect at the time was the rise in power at ℓ missions have already determined fundamental cosmological intermediate . parameters to uncertainties of a few percent (c.f. DASI [5], The Owens Valley RING5M experiment was one ofa ACBAR [6], Boomerang [7], WMAP [8]), and new data small complement of experiments designed to probe the fromPlanckarepoisedtorefinethesefurther.TheE-mode CMB anisotropy spectrum at arcminute scales; these scales polarization of the CMB, whose detection was unthinkable wereassumedtobesub-horizon,butthathadyettobe twenty years ago, is now routinely measured by ground-based demonstrated. At the time of its inception, only upper limits experiments (first detected by DASI [9, 10], with progressive had been achieved by a small handful of experiments (at improvements in resolution and sensitivity by CBI [11], QUaD 12 by Tucker et al. [20]andat2 by the OVRO NCP [12], BICEPI [13], and QUIET [14]), while ever more sensitive experiment [21]). Collectively these instruments constituted limits on the B-mode power spectrum are beginning to place the deepest probes of the microwave spectrum to date, and interesting constraints on the tensor-to-scalar ratio [13] by contrast with the large-scale experiments, the resolution 2 Advances in Astronomy (∘) (∘) 10 1 0.1 10 1 0.1 100 100 80 80 RING5M K K 60 60 1/2 1/2 /2] /2] 40 40 ( + 1) ( + 1) [ [ NCP 20 20 NCP 0 0 1 10 100 1000 1 10 100 1000 COBE MAX COBE Python Sask Python IAB FIRS ARGO CAT Tenerife MAX OVRO SP (a) (b) Figure 1: (Reproduced from [22])(a)ThestateofCMBanisotropydetectionsin1993.ThepointsarefromCOBE[15], Python [17], MAX [18] and IAB [19].AlsoshownistheupperlimitfromtheOVRONCPexperiment[21]. (b) The state of the field in early 1998. Shown are COBE [36], FIRS [37], Tenerife [38], SP94 [39], Python [40], ARGO [41], MAX [42], Saskatoon [24], CAT [25], and the OVRO RING5M [23]. (The solid line is a CDM model with 0 =30, Ω0 =1and Ω = 0.05, and is also indicative of the state of late 20th century cosmology). of the RING5M instruments presented one of the first oppor- Figure 1) demonstrated that the power had dropped signifi- tunities for probing microwave emission from specific galac- cantly at ∼2 scales. The RING5M experiment was, therefore, tic features. It is therefore not surprising, in retrospect, that designedtooperateat7 –22 scales, where the peak of the these observations resulted in the first detection of anomalous power spectrum might lie in an Ω<1universe, but which microwave emission from the Galaxy, as we discuss in should nonetheless be detectable even in an Ω=1cosmology. Section 3. Like our counterparts in the southern hemisphere, we were In the following section, we review details of the RING5M driven to observe near the celestial pole; in our case, the experiment design relevant to understanding the data. In North Celestial Pole (NCP) was the only part of the northern Section 3, we present the evidence for anomalous emission in hemisphere sky available for round-the-clock observations. the RING5M data and discuss its interpretation in the context The experiment consisted of two independent telescopes of multifrequency observations of the NCP in Section 4. operating at widely separated frequencies. A 30 GHz channel Finally, in Section 5 we consider the relation of the anomalous was provided by a dual-feed receiver installed on a 5.5-meter emission detected near the NCP to the dust correlated telescope at the Owens Valley Radio Observatory (OVRO), components seen in degree-scale CMB experiments. with a beam of approximately 7 FWHM. To provide lever- age against potential foreground contamination, a second 14.5 GHz receiver was constructed on the OVRO 40-meter 2. The RING5M Experiment telescope, with optics designed to underilluminate the dish, so that matched beams were produced at both frequencies. A Figure 1, reproduced from [22], shows the state of anisotropy Dicke switch provided fast azimuthal switching between two detections when the RING5M experiment was constructed. positions on the sky separated by 22 , while a second, slower Taken together, results from the early Antarctic experiments level of differencing was achieved by slewing the telescope to were somewhat suggestive of a rise in power at scales alternate the beams on the target field, producing an effective ∘ approaching ∼1 , while the NCP upper limit (also shown in beam pattern indicated in Figure 2.Inall,36fieldswere Advances in Astronomy 3 2 Dust 500 500 0 CMB K) ( 0 0 Δ −2 Free-free Synchrotron −500 −500 −4 0 6 12 18 24 0 50 100 150 200 RING5M field (hours) Δsky (K) ℛKa, = 85.41, = 17.78 ℛKu, = 238.06, = 28.28 (a) (b) Figure 2: (a) Source-subtracted Ka (30 GHz) and Ku-band (14.5 GHz) data plotted to equal brightness temperature scale. Both represent sky amplitudes, convolved with the double-switched beam pattern of the RING5M observations, indicated at the bottom left of the figure. Approximately half of the RING5M fields show CMB-like signals (equal brightness temperature at both frequencies), while the other half show temperature signals with a steep spectral dependence. (b) The likelihood of the spectral index of the RING5M data, assuming that a single process is responsible for the signals at both frequencies. Clearly pure CMB (=0) is ruled out with high significance. observedandspacedevenlyinaringaroundtheNCP.The structure in 325 MHz maps of the NCP regions from the fields were observed only during transit, so that common Westerbork Northern Sky Survey (WENSS, [26]), we were mode contamination from the ground would be removed by able to place a lower bound of > −2.2 on the spectral index thedoubleswitching. of a single foreground, making synchrotron emission an As detailed in [23], over three years of observation, a untenable model for the 14.5 GHz signals, unless the fields variety of null tests demonstrated high signal-to-noise detec- happen to be associated with an active region where the tion of structure in the RING5M data at both frequencies, normally steep synchrotron spectrum is kept unusually flat consistent from year-to-year. These data were ultimately by the injection of high-energy electrons, for example, a used to place a sensitive new constraint on the small-scale supernova remnant that has undergone recent repowering (ℓ ∼ 600)CMBanisotropy[23] that remains in excellent (see for example [27]). The lack of any correlation with the agreement with modern measurements (see Figure 1). By WENSS synchrotron maps, however, makes synchrotron of 1997, the Saskatoon experiment had released data that started any variety an unlikely explanation. to resolve some of the scatter at low-ℓ into a more convincing Moreover, although the spectral index was consistent picture of a primary Doppler peak [24], and the CAT with free-free emission, the amplitude of the signals was experiment had also released its preliminary results, which not.

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