Search

Context. Interest in the use of the Chajnantor area for millimeter and submillimeter astronomy is increasing because of its excellent atmospheric conditions. Knowing the general site annual variability in precipitable water vapor (PWV) can contribute to the planning of new observatories in the area. Aims. We seek to create a 20-year atmospheric database (1997−2017) for the Chajnantor area in northern Chile using a single common physical unit, PWV. We plan to extract weather relations between the Chajnantor Plateau and the summit of Cerro Chajnantor to evaluate potential sensitivity improvements for telescopes fielded in the higher site. We aim to validate the use of submillimeter tippers to be used at other sites and use the PWV database to detect a potential signature for local climate change over 20 years. Methods. We revised our method to convert from submillimeter tipper opacity to PWV. We now include the ground temperature as an input parameter to the conversion scheme and, therefore, achieve a higher conversion accuracy. Reults. We found a decrease in the measured PWV at the summit of Cerro Chajnantor with respect to the plateau of 28%. In addition, we found a PWV difference of 1.9% with only 27 m of altitude difference between two sites in the Chajnantor Plateau: the Atacama Pathfinder Experiment and the Cosmic Background Imager near the Atacama Large Millimeter Array center. This difference is possibly due to local topographic conditions that favor the discrepancy in PWV. The scale height for the plateau was extracted from the measurements of the plateau and the Cerro Chajnantor summit, giving a value of 1537 m. Considering the results obtained in this work from the long-term study, we do not see evidence of PWV trends in the 20-year period of the analysis that would suggest climate change in such a timescale.

The explosion of a supernova releases almost instantaneously about 10^51 ergs of mechanical energy, an event that irreversibly changes the physical and chemical properties of large regions of their host galaxy. A supernova remnant (SNR) consists of three components: the stellar ejecta, the nebula resulting from the powerful shock waves, and sometimes a compact stellarremnant. They can radiate their energy across the whole electromagnetic spectrum, but the great majority are strongest in the radio regime. Almost 70 years after the first detection of radio emission coming from a SNR, great progress has been made in fully comprehending their physical characteristics and evolution. The radio spectra of the synchrotron emission from theSNRs has not been studied much at high frequencies (≥ few GHz), hence the motivation behind this research.In this thesis we analyse data from the Cosmic Background Imager (CBI) at 31 GHz. These data were modified and calibrated using the CBI data reduction pipeline. The culled data was then imaged and deconvolved using the CLEAN algorithm. It was decided that we would observe two SNRs, namely G20.0 − 0.2 and G312.5 − 3.0. These two SNRs have different morphologies, as well as spectral properties. To estimate the flux density of the SNRs we used an aperture photometry technique. Using this technique the flux densities of G20.0 − 0.2 and G312.5 − 3.0 at 31 GHz were found to be 3.56 ± 0.2 Jy and 0.49 ± 0.07 Jy, respectively. We estimated various uncertainties, and by taking multi-frequency maps from literature it was possible to fit a power-law spectra and estimate the spectral indices. The value of the spectral indices for G20.0 − 0.2 after fitting a power law was α = −0.36 ± 0.05 and α = −0.8 ± 0.1 after fitting a spectral break at ∼5 GHz. For G312.5 − 3.0 it was estimated to be α = −0.51 ± 0.07. Our results show that the spectrum of G20.0 − 0.2 was flat at low frequencies, similar to Crab SNR, and showing the steepening at high frequency. While the spectrum for G312.5 − 3.0 has a typical value which agrees with most of the SNRs’ spectra.

We describe an upgrade to the Cosmic Background Imager instrument to increase its surface brightness sensitivity at small angular scales. The upgrade consisted of replacing the thirteen 0.9-m antennas with 1.4-m antennas incorporating a novel combination of design features, which provided excellent sidelobe and spillover performance for low manufacturing cost. Off-the-shelf spun primaries were used, and the secondary mirrors were oversized and shaped relative to a standard Cassegrain in order to provide an optimum compromise between aperture efficiency and low spillover lobes. Low-order distortions in the primary mirrors were compensated for by custom machining of the secondary mirrors. The secondaries were supported on a transparent dielectric foam cone to minimize scattering. The antennas were tested in the complete instrument, and the beam shape and spillover noise contributions were as expected. We demonstrate the performance of the telescope and the inter-calibration with the previous system using observations of the Sunyaev-Zel'dovich effect in the cluster Abell 1689. The enhanced instrument has been used to study the cosmic microwave background, the Sunyaev-Zel'dovich effect and diffuse Galactic emission.

The diffuse cm-wave IR-correlated signal, the "anomalous" CMB foreground, is thought to arise in the dust in cirrus clouds. We present Cosmic Background Imager (CBI) cm-wave data of two translucent clouds, {\zeta} Oph and LDN 1780 with the aim of characterising the anomalous emission in the translucent cloud environment. In {\zeta} Oph, the measured brightness at 31 GHz is 2.4{\sigma} higher than an extrapolation from 5 GHz measurements assuming a free-free spectrum on 8 arcmin scales. The SED of this cloud on angular scales of 1{\odot} is dominated by free-free emission in the cm-range. In LDN 1780 we detected a 3 {\sigma} excess in the SED on angular scales of 1{\odot} that can be fitted using a spinning dust model. In this cloud, there is a spatial correlation between the CBI data and IR images, which trace dust. The correlation is better with near-IR templates (IRAS 12 and 25 {\mu}m) than with IRAS 100 {\mu}m, which suggests a very small grain origin for the emission at 31 GHz. We calculated the 31 GHz emissivities in both clouds. They are similar and have intermediate values between that of cirrus clouds and dark clouds. Nevertheless, we found an indication of an inverse relationship between emissivity and column density, which further supports the VSGs origin for the cm-emission since the proportion of big relative to small grains is smaller in diffuse clouds.

An anomalous radio continuum component at cm-wavelengths has been observed in various sources, including dark clouds. This continuum component represents a new property of the ISM. In this work we focus on one particular dark cloud, the bright reflection nebula M 78. The main goal of this work is to invetigate cm-wave continuum emission in a prominent molecular cloud, nearby and with complementary observational data. We acquired Cosmic Background Imager (CBI) visibility data of M 78 at 31 GHz with an angular resolution of $\sim 5.8\arcmin$ and CBI2 data at an angular resolution of $\sim 4.2\arcmin$. A morphological analysis was undertaken to search for possible correlations with templates that trace different emission mechanisms. Using data from WMAP and the Rhodes/HartRAO 2326 MHz survey we constructed the spectral energy distribution (SED) of M 78 in a $45\arcmin$ circular aperture. We used results from the literature to constrain the physical conditions and the stellar content. The 5 GHz -- 31 GHz spectral index in flux density ($\alpha = 1.89\pm 0.15$) is significantly different from optically thin free-free values. We also find closer morphological agreement with IR dust tracers than with free-free sources. Dust-correlated cm-wave emission that is not due to free-free is significant at small scales ($\sim 8\arcmin$). However, a free-free background dominates at cm-wavelengths on large scales ($\sim 1$ deg). We correct for this uniform background by differencing against a set of reference fields. The differenced SED of M 78 shows excess emission at 10-70 GHz over free-free and a modified blackbody, at $3.4\sigma$. The excess is matched by the spinning dust model from Draine and Lazarian (1998).

Lynds dark cloud LDN1622 represents one of the best examples of anomalous dust emission, possibly originating from small spinning dust grains. We present Cosmic Background Imager (CBI) 31-GHz data of LDN1621, a diffuse dark cloud to the north of LDN1622 in a region known as Orion East. A broken ring-like structure with diameter ≈20 arcmin of diffuse emission is detected at 31 GHz, at ≈20–30 mJy beam^(−1) with an angular resolution of ≈5 arcmin. The ring-like structure is highly correlated with far-infrared (FIR) emission at 12–100 μm with correlation coefficients of r ≈ 0.7–0.8, significant at ~10σ. The FIR-correlated emission at 31 GHz therefore appears to be mostly due to radiation associated with dust. Multifrequency data are used to place constraints on other components of emission that could be contributing to the 31-GHz flux. An analysis of the GB6 survey maps at 4.85 GHz yields a 3σ upper limit on free–free emission of 7.2 mJy beam^(−1) (≾30 per cent of the observed flux) at the CBI resolution. The bulk of the 31-GHz flux therefore appears to be mostly due to dust radiation. Aperture photometry, at an angular resolution of 13 arcmin and with an aperture of diameter 30 arcmin, allowed the use of IRAS maps and the Wilkinson Microwave Anisotropy Probe 5-yr W-band map at 93.5 GHz. A single modified blackbody model was fitted to the data to estimate the contribution from thermal dust, which amounts to ~10 per cent at 31 GHz. In this model, an excess of 1.52 ± 0.66 Jy (2.3σ) is seen at 31 GHz. Correlations with the IRAS 100 μm gave a coupling coefficient of 18.1 ± 4.4 μK (MJy sr^(−1))^(−1), consistent with the values found for LDN1622.

We estimate the impact of weak lensing by strongly nonlinear cosmological structures on the cosmic microwave background. Accurate calculation of large l multipoles requires N-body simulations and ray-tracing schemes with both high spatial and temporal resolution. To this end, we have developed a new code that combines a gravitational Adaptive Particle-Particle, Particle-Mesh solver with a weak-lensing evaluation routine. The lensing deviations are evaluated while structure evolves during the simulation so that all evolution steps—rather than just a few outputs—are used in the lensing computations. The new code also includes a ray-tracing procedure that avoids periodicity effects in a universe that is modeled as a three-dimensional torus in the standard way. Results from our new simulations are compared with previous ones based on Particle-Mesh simulations. We also systematically investigate the impact of box volume, resolution, and ray-tracing directions on the variance of the computed power spectra. We find that a box size of 512 h –1 Mpc is sufficient to provide a robust estimate of the weak-lensing angular power spectrum in the l-interval (2000-7000). For a reaslistic cosmological model, the power [l(l + 1)C l/2π]1/2 takes on values of a few μK in this interval, which suggests that a future detection is feasible and may explain the excess power at high l in the Berkeley-Illinois-Maryland Association and Cosmic Background Imager observations.

We have mapped four regions of the southern Galactic plane at 31 GHz with the Cosmic Background Imager. From the maps, we have extracted the flux densities for six of the brightest \hii regions in the southern sky and compared them with multi-frequency data from the literature. The fitted spectral index for each source was found to be close to the theoretical value expected for optically thin free-free emission, thus confirming that the majority of flux at 31 GHz is due to free-free emission from ionised gas with an electron temperature of $\approx 7000-8000$ K. We also found that, for all six sources, the 31 GHz flux density was slightly higher than the predicted value from data in the literature. This excess emission could be due to spinning dust or another emission mechanism. Comparisons with $100 \mu$m data indicate an average dust emissivity of $3.3\pm1.7 \mu$K (MJy/sr)$^{-1}$, or a 95 per cent confidence limit of $, Comment: Accepted in MNRAS. 12 pages, 10 figures, 5 tables

We present new measurements of the power spectra of the E-mode of CMB polarization, the temperature T, the cross-correlation of E and T, and upper limits on the B-mode from 2.5 years of dedicated Cosmic Background Imager (CBI) observations. Both raw maps and optimal signal images in the uv-plane and real space show strong detections of the E-mode (11.7 sigma for the EE power spectrum overall) and no detection of the B-mode. The power spectra are used to constrain parameters of the flat tilted adiabatic Lambda-CDM models: those determined from EE and TE bandpowers agree with those from TT, a powerful consistency check. There is little tolerance for shifting polarization peaks from the TT-forecast locations, as measured by the angular sound crossing scale theta = 100 ell_s = 1.03 +/- 0.02 from EE and TE cf. 1.044 +/- 0.005 with the TT data included. The scope for extra out-of-phase peaks from subdominant isocurvature modes is also curtailed. The EE and TE measurements of CBI, DASI and BOOMERANG are mutually consistent, and, taken together rather than singly, give enhanced leverage for these tests., Comment: 15 pages, 9 figures, submitted to ApJ -- Accepted version. The fine-bin spectrum, covariance matrix, and window functions are now available on the web (suitable for use in COSMOMC) at: http://www.astro.caltech.edu/~tjp/CBI/data2006/index.html The pipeline in the previous version inadvertently omitted one antenna, so the new spectrum contains ~15% more data. We emphasize that previous results were in no way biased, and that the (small) changes to the spectrum solely reflect the inclusion of the additional data. Numbers and figures in the paper have been updated correspondingly. All maps now have color bars

Studies of the diffuse Galactic radio emission are interesting both for better understanding the physical conditions in our Galaxy and for minimising the contamination in cosmological measurements. Motivated by this we present Cosmic Background Imager 31 GHz observations of the Galactic regions NGC 6357, NGC 6334, W51 and W40 at $\sim$4$'$.5 resolution and conduct an investigation of the spectral emission process in the regions at 4$'$.5 and 1$^{\circ}$ resolution. We find that most of the emission in the regions is due to optically thin free-free. For 2 sub-regions of NGC 6334 and for a sub-region of W51 though, at 4$'$.5 resolution and at 31 GHz we detect less emission than expected from extrapolation of radio data at lower frequencies assuming a spectral index of $-$0.12 for optically thin free-free emission, at 3.3$\sigma$, 3.7$\sigma$ and 6.5$\sigma$ respectively. We also detect excess emission in a sub-region of NCG 6334 at 6.4$\sigma$, after ruling out any possible contribution from Ultra Compact HII (UCHII) regions. At 1$^{\circ}$ resolution we detect a spinning dust component in the Spectral Energy Distribution (SED) of W40 that accounts for 18$\pm$7 % of the total flux density in the region at the peak frequency of 37 GHz. Comparison with 100 ${\rm \mu m}$ data indicate an average dust emissivity for the sub-regions of $0.5\pm4.4$ $\mu$K(MJy sr$^{-1}$)$^{-1}$. Finally we translate the excess emission in the regions to an Anomalous Microwave Emission (AME) emissivity relative to the optical depth at 250 ${\rm \mu m }$. We find that this form of emissivity is independent of the AME significance and has a value somewhere in the order of 10$^4$ Jy., Comment: 13 pages, 4 figures

Interferometry has long been used in radio astronomy to enable imaging of astronomical sources with angular resolutions exceeding the diffraction limit of a single aperture. In the past decade, interferometry of the CMB has been carried out with instruments such as the CBI, DASI and VSA which exploited the inherent instrumental stability and simplicity of ell-space analysis of interferometer data. The practice of interferometric polarimetry has been particularly well-developed in the radio astronomical community and DASI and CBI were able to measure the polarization of the CMB over the multipole range 200 < l < 1500. In this talk, I discuss the theory of interferometry and the mathematics of CMB interferometric polarimetry, using the recent polarization observations of the Cosmic Background Imager (CBI) as an example. Topics will also include description of the data pipeline, handling of contaminating signals and sources, and the construction of optimal maps. I will conclude with the possibility of future CMB interferometers with kilo-element arrays and mega-pixel imaging.

We present coincident observations of the Cosmic Microwave Background (CMB) from the Very Small Array (VSA) and Cosmic Background Imager (CBI) telescopes. The consistency of the full datasets is tested in the map plane and the Fourier plane, prior to the usual compression of CMB data into flat bandpowers. Of the three mosaics observed by each group, two are found to be in excellent agreement. In the third mosaic, there is a 2σ discrepancy between the correlation of the data and the level expected from Monte Carlo simulations. This is shown to be consistent with increased phase calibration errors on VSA data during summer observations. We also consider the parameter estimation method of each group. The key difference is the use of the variance window function in place of the bandpower window function, an approximation used by the VSA group. A re-evaluation of the VSA parameter estimates, using bandpower windows, shows that the two methods yield consistent results.

We discuss the nature of the possible high-l excess in the Cosmic Microwave Background (CMB) anisotropy power spectrum observed by the Cosmic Background Imager (CBI). We probe the angular structure of the excess in the CBI deep fields and investigate whether it could be due to the scattering of CMB photons by hot electrons within clusters, the Sunyaev-Zeldovich (SZ) effect. We estimate the density fluctuation parameters for amplitude, sigma_8, and shape, Gamma, from CMB primary anisotropy data and other cosmological data. We use the results of two separate hydrodynamical codes for Lambda-CDM cosmologies, consistent with the allowed sigma_8 and Gamma values, to quantify the expected contribution from the SZ effect to the bandpowers of the CBI experiment and pass simulated SZ effect maps through our CBI analysis pipeline. The result is very sensitive to the value of sigma_8, and is roughly consistent with the observed power if sigma_8 ~ 1. We conclude that the CBI anomaly could be a result of the SZ effect for the class of Lambda-CDM concordance models if sigma_8 is in the upper range of values allowed by current CMB and Large Scale Structure (LSS) data., Comment: Accepted by The Astrophysical Journal; 17 pages including 12 color figures. v2 matches accepted version. Additional information at http://www.astro.caltech.edu/~tjp/CBI/

We present Ho results from Cosmic Background Imager (CBI) observations of the Sunyaev-Zel'dovich Effect (SZE) in 7 galaxy clusters, A85, A399, A401, A478, A754, A1651, and A2597. These observations are part of a program to study a complete, volume-limited sample of low-redshift (z

We investigate the nature of the diffuse Galactic emission in the Wilkinson Microwave Anisotropy Probe (WMAP) temperature anisotropy data. Substantial dust-correlated emission is observed at all WMAP frequencies, far exceeding the expected thermal dust emission in the lowest frequency channels (23, 33, 41 GHz). The WMAP team (Bennett et al.) interpret this emission as dust-correlated synchrotron radiation, attributing the correlation to the natural association of relativistic electrons produced by SNae with massive star formation in dusty clouds, and deriving an upper limit of 5% on the contribution of Draine & Lazarian spinning dust at K-band (23 GHz). We pursue an alternative interpretation that much, perhaps most, of the dust-correlated emission at these frequencies is indeed spinning dust, and explore the spectral dependence on environment by considering a few specific objects as well as the full sky average. Models similar to Draine & Lazarian spinning dust provide a good fit to the full-sky data. The full-sky fit also requires a significant component with free-free spectrum uncorrelated with \Halpha, possibly hot (~million K) gas within 30 degrees of the Galactic center.

Mounting evidence from $x$-ray observations reveals that bound objects should receive some form of energy in the past injected from non-gravitaional sources. We report that an instantaneous heating scheme, for which gases in dense regions were subjected to a temperature jump of 1keV at $z=2$ whereas those in rarified regions remained intact, can produce bound objects obeying the observed mass-temperature and luminosity-temperature relations. Such preheating lowers the peak Sunyaev-Zeldovich (SZ) power by a factor of 2 and exacerbates the need for the normalization of matter fluctuations $\sigma_8$ to assume an extreme high value $(\sim 1.1)$ for the SZ signals to account for the excess anisotropy on 5-arcminute scale detected by the Cosmic Background Imager in the cosmic microwave background radiation.

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 deg^2 of sky selected for low dust contrast. These results represent the highest signal to noise observations of CMB anisotropy to date; in the deepest 150GHz band map, we reached an RMS of 8.0\mu K 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 \ell = 150-3000 with resolution of \Delta \ell=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 \Lambda CDM Universe.

We report measurements of anisotropy in the cosmic microwave background radiation over the multipole range l ~ 200 - 3500 with the Cosmic Background Imager based on deep observations of three fields. These results confirm the drop in power with increasing l first reported in earlier measurements with this instrument, and extend the observations of this decline in power out to l \~ 2000. The decline in power is consistent with the predicted damping of primary anisotropies. At larger multipoles, l = 2000 - 3500, the power is 3.1 sigma greater than standard models for intrinsic microwave background anisotropy in this multipole range, and 3.5 sigma greater than zero. This excess power is not consistent with expected levels of residual radio source contamination but, for sigma_8 >~ 1, is consistent with predicted levels due to a secondary Sunyaev-Zeldovich anisotropy. Further observations are necessary to confirm the level of this excess and, if confirmed, determine its origin., Accepted by The Astrophysical Journal; 15 pages including 7 color figures. Additional information at http://www.astro.caltech.edu/~tjp/CBI/

We describe an algorithm for the extraction of the angular power spectrum of an intensity field, such as the cosmic microwave background (CMB), from interferometer data. This new method, based on the gridding of interferometer visibilities in the aperture plane followed by a maximum likelihood solution for bandpowers, is much faster than direct likelihood analysis of the visibilities, and deals with foreground radio sources, multiple pointings, and differencing. The gridded aperture-plane estimators are also used to construct Wiener-filtered images using the signal and noise covariance matrices used in the likelihood analysis. Results are shown for simulated data. The method has been used to determine the power spectrum of the cosmic microwave background from observations with the Cosmic Background Imager, and the results are given in companion papers.

This paper is devoted to the methods of the determination of cosmological parameters and distortions of ionization history from recent cosmic microwave background (CMB) observations. We show that the more complex models of kinetics of recombination with a few "missing" parameters describing the recombination process provide better agreement between the measured and the expected characteristics of the CMB anisotropy. In particular, we consider external sources of resonance and ionizing radiation and a model with strong clustering of the baryonic component. These factors can constrain the estimates of the cosmological parameters usually discussed. We also demonstrate that the measurements of the CMB polarization can improve these estimates and, for the precision expected from the Planck mission, allow the discrimination of a wide class of models.

We present preliminary results of an analysis into the effects of primordial voids on the cosmic microwave background (CMB). We show that an inflationary bubble model of void formation predicts excess power in the CMB angular power spectrum that peaks between 2000 < l < 3000. Therefore, voids that exist on or close to the last scattering surface at the epoch of decoupling can contribute significantly to the apparent rise in power on these scales recently detected by the Cosmic Background Imager (CBI)., 5 pages, 3 figures. MNRAS accepted version

The detection of an excess of emission at microwave frequencies with respect to the predicted free-free emission has been reported for several Galactic HII regions. Here, we investigate the case of RCW 49, for which the Cosmic Background Imager tentatively (~ 3 sigma) detected Anomalous Microwave Emission at 31 GHz on angular scales of 7'. Using the Australia Telescope Compact Array, we carried out a multi-frequency (5 GHz, 19 GHz and 34 GHz) continuum study of the region, complemented by observations of the H109$\alpha$ radio recombination line. The analysis shows that: 1) the spatial correlation between the microwave and IR emission persists on angular scales from 3.4' to 0.4", although the degree of the correlation slightly decreases at higher frequencies and on smaller angular scales, 2) the spectral indices between 1.4 and 5 GHz are globally in agreement with optically thin free-free emission, however, ~ 30% of these are positive and much greater than -0.1, consistently with a stellar wind scenario, 3) no major evidence for inverted free-free radiation is found, indicating that this is likely not the cause of the Anomalous Emission in RCW 49. Although our results cannot rule out the spinning dust hypothesis to explain the tentative detection of Anomalous Microwave emission in RCW 49, they emphasize the complexity of astronomical sources very well known and studied such as HII regions, and suggest that, at least in these objects, the reported excess of emission might be ascribed to alternative mechanisms such as stellar winds and shocks., Comment: Accepted for publication in ApJ. 11 pages, 8 figures

We report the detection of polarized anisotropy in the Cosmic Microwave Background radiation with the Degree Angular Scale Interferometer (DASI), located at the Amundsen-Scott South Pole research station. Observations in all four Stokes parameters were obtained within two 3.4 FWHM fields separated by one hour in Right Ascension. The fields were selected from the subset of fields observed with DASI in 2000 in which no point sources were detected and are located in regions of low Galactic synchrotron and dust emission. The temperature angular power spectrum is consistent with previous measurements and its measured frequency spectral index is -0.01 (-0.16 -- 0.14 at 68% confidence), where 0 corresponds to a 2.73 K Planck spectrum. The power spectrum of the detected polarization is consistent with theoretical predictions based on the interpretation of CMB anisotropy as arising from primordial scalar adiabatic fluctuations. Specifically, E-mode polarization is detected at high confidence (4.9 sigma). Assuming a shape for the power spectrum consistent with previous temperature measurements, the level found for the E-mode polarization is 0.80 (0.56 -- 1.10), where the predicted level given previous temperature data is 0.9 -- 1.1. At 95% confidence, an upper limit of 0.59 is set to the level of B-mode polarization with the same shape and normalization as the E-mode spectrum. The TE correlation of the temperature and E-mode polarization is detected at 95% confidence, and also found to be consistent with predictions. These results provide strong validation of the underlying theoretical framework for the origin of CMB anisotropy and lend confidence to the values of the cosmological parameters that have been derived from CMB measurements., 20 pages, 6 figures

The angular resolution of the new Cosmic Background Imager is fine enough to reveal, for the first time, the primordial seeds of galaxy clusters.

We present measurements of anisotropy in the Cosmic Microwave Background (CMB) from the first season of observations with the Degree Angular Scale Interferometer (DASI). The instrument was deployed at the South Pole in the austral summer 1999--2000, and made observations throughout the following austral winter. We have measured the angular power spectrum of the CMB in the range 100

We present the first results of observations of the intrinsic anisotropy of the cosmic microwave background radiation with the Cosmic Background Imager from a site at 5080 m altitude in northern Chile. Our observations show a sharp decrease in C_l in the range l=400 - 1500. The broadband amplitudes we have measured are deltaT(band) = 58.7 (-6.3, +7.7) microK for l = 603 (-166, +180) and 29.7 (-4.2, +4.8) microK for l = 1190 (-224, +261), where these are half-power widths in l. Such a decrease in power at high l is one of the fundamental predictions of the standard cosmological model, and these are the first observations which cover a broad enough l range to show this decrease in a single experiment. The C_l we have measured enable us to place limits on the density parameter, Omega(tot) = 0.7 (90% confidence)., 5 pages including 2 figures. Corrected an error in the comparison with Boomerang and Maxima

Design and performance details are given for a 10-GHz bandwidth analog filterbank correlator with 1-GHz spectral resolution. The correlator is based on a 1-GHz bandwidth hybrid module which contains all the multipliers and signal distribution for a 13-antenna array.

We present predicted full-sky maps of submillimeter and microwave emission from the diffuse interstellar dust in the Galaxy. These maps are extrapolated from the 100 micron emission and 100/240 micron flux ratio maps that Schlegel, Finkbeiner, & Davis (1998; SFD98) generated from IRAS and COBE/DIRBE data. Results are presented for a number of physically plausible emissivity models. We find that no power law emissivity function fits the FIRAS data from 200 - 2100 GHz. In this paper we provide a formalism for a multi-component model for the dust emission. A two-component model with a mixture of silicate and carbon-dominated grains (motivated by Pollack et al., 1994}) provides a fit to an accuracy of about 15% to all the FIRAS data over the entire high-latitude sky. Small systematic differences are found between the atomic and molecular phases of the ISM. Our predictions for the thermal (vibrational) emission from Galactic dust at \nu < 3000 GHz are available for general use. These full-sky predictions can be made at the DIRBE resolution of 40' or at the higher resolution of 6.1 arcmin from the SFD98 DIRBE-corrected IRAS maps.

We report on a measurement of the angular spectrum of the CMB between $l\approx 100$ and $l\approx 400$ made at 144 GHz from Cerro Toco in the Chilean altiplano. When the new data are combined with previous data at 30 and 40 GHz, taken with the same instrument observing the same section of sky, we find: 1) a rise in the angular spectrum to a maximum with $\delta T_l \approx 85~\mu$K at $l\approx 200$ and a fall at $l>300$, thereby localizing the peak near $l\approx 200$; and 2) that the anisotropy at $l\approx 200$ has the spectrum of the CMB.

We consider corrections to the underlying cosmology due to secondary contributions from weak gravitational lensing, the integrated Sachs-Wolfe effect, and the Sunyaev-Zel'dovich effect contained in the trispectrum. We incorporate these additional contributions to the covariance of a binned angular power spectrum of temperature anisotropies in the analysis of current and prospective data sets. Although recent experiments such as ACBAR and CBI are not particularly sensitive to these additional non-Gaussian effects, the interpretation of Planck and CMBPol anisotropy spectra will require an accounting of non-Gaussian covariance leading to a degradation in cosmological parameter estimates by up to 20% and 30%, respectively., 9 pages. Fixed some typos

We present evidence for anomalous microwave emission in the RCW175 \hii region. Motivated by 33 GHz $13\arcmin$ resolution data from the Very Small Array (VSA), we observed RCW175 at 31 GHz with the Cosmic Background Imager (CBI) at a resolution of $4\arcmin$. The region consists of two distinct components, G29.0-0.6 and G29.1-0.7, which are detected at high signal-to-noise ratio. The integrated flux density is $5.97\pm0.30$ Jy at 31 GHz, in good agreement with the VSA. The 31 GHz flux density is $3.28\pm0.38$ Jy ($8.6\sigma$) above the expected value from optically thin free-free emission based on lower frequency radio data and thermal dust constrained by IRAS and WMAP data. Conventional emission mechanisms such as optically thick emission from ultracompact \hii regions cannot easily account for this excess. We interpret the excess as evidence for electric dipole emission from small spinning dust grains, which does provide an adequate fit to the data., Comment: 5 pages, 2 figures, submmited to ApJ Letters

The 100-m Robert C. Byrd Green Bank Telescope (GBT) and the Owens Valley Radio Observatory (OVRO) 40-m radio telescope have been used to conduct a survey of 3165 known extragalactic radio sources over 143 square degrees of the sky. Target sources were selected from the NRAO VLA Sky Survey in fields observed by the Cosmic Background Imager (CBI); most are extragalactic active galactic nuclei (AGN) with 1.4 GHz flux densities of 3 to 10 mJy. The resulting 31 GHz catalogs are presented in full online. Using a Maximum-Likelihood analysis to obtain an unbiased estimate of the distribution of the 1.4 to 31 GHz spectral indices of these sources, we find a mean 31 to 1.4 GHz flux ratio of 0.110 +/- 0.003 corresponding to a spectral index of alpha=-0.71 +/- 0.01 (S ~ nu^alpha); 9.0 +/- 0.8 % of sources have alpha > -0.5 and 1.2 +/- 0.2 % have alpha > 0. By combining this spectral index distribution with 1.4 GHz source counts we predict 31 GHz source counts in the range 1 mJy < S_31 < 4 mJy, N(>S_31) = (16.7 +/- 1.7) deg^2 (S_31/1 mJy)^(-0.80 +/- 0.07). We also assess the contribution of mJy-level (S_1.4 < 3.4 mJy) radio sources to the 31 GHz CMB power spectrum, finding a mean power of ell (ell+1) C^src_ell/(2 pi) = 44 +/- 14 micro-Kelvin^2 and a 95% upper limit of 80 micro-Kelvin^2 at ell = 2500. Including an estimated contribution of 12 micro-Kelvin^2 from the population of sources responsible for the turn-up in counts below S_1.4 = 1 mJy this amounts to 21 +/- 7 % of what is needed to explain the CBI high-ell excess signal, 275 +/- 63 micro-Kelvin^2. These results are consistent with other measurements of the 31 GHz point source foreground., Replace with accepted version, incorporating referee comments. Significant revisions for clarity, added comparison to recent results. Quoted error bars on source counts are larger and the range of validity more limited; results on CBI excess unchanged

Using large numbers of simulations of the microwave sky, incorporating the Cosmic Microwave Background (CMB) and the Sunyaev-Zel'dovich (SZ) effect due to clusters, we investigate the statistics of the power spectrum at microwave frequencies between spherical multipoles of 1000 and 10000. From these virtual sky maps, we find that the spectrum of the SZ effect has a larger standard deviation by a factor of 3 than would be expected from purely Gaussian realizations, and has a distribution that is significantly skewed towards higher values, especially when small map sizes are used. The standard deviation is also increased by around 10 percent compared to the trispectrum calculation due to the clustering of galaxy clusters. We also consider the effects of including residual point sources and uncertainties in the gas physics. This has implications for the excess power measured in the CMB power spectrum by the Cosmic Background Imager and BIMA experiments. Our results indicate that the observed excess could be explained using a lower value of $\sigma_8$ than previously suggested, however the effect is not enough to match $\sigma_8=0.825$. The uncertainties in the gas physics could also play a substantial role. We have made our maps of the SZ effect available online., Comment: 21 pages, 23 figures, 3 tables. Accepted by MNRAS. We have made our maps of the SZ effect available online

We present the results of the flux density measurements at 4.85 GHz and 10.45 GHz of a sample of 5998 NVSS radio sources with the Effelsberg 100 m telescope. The initial motivation was the need to identify the NVSS radio sources that could potentially contribute significant contaminating flux in the frequency range at which the Cosmic Background Imager experiment operated. An efficient way to achieve this challenging goal has been to compute the high frequency flux density of those sources by extrapolating their radio spectrum. This is determined by the three-point spectral index measured on the basis of the NVSS entry at 1.4 GHz and the measurements at 4.85 GHz and 10.45 GHz carried out with the 100 m Effelsberg telescope. These measurements are important since the targeted sample probes the weak part of the flux density distribution, hence the decision to make the data available. We present the table with flux density measurements of 3434 sources that showed no confusion allowing reliable measurements, their detection rates, their spectral index distribution and an interpretation which explains satisfactorily the observed uncertainties., Comment: 13 pages, 9 figures, in press

In the context of Brans-Dicke scalar tensor theory of gravitation, the cosmological Friedmann equation which relates the expansion rate $H$ of the universe to the various fractions of energy density is analyzed rigorously. It is shown that Brans-Dicke scalar tensor theory of gravitation brings a negligible correction to the matter density component of Friedmann equation. Besides, in addition to $\Omega_{\Lambda}$ and $\Omega_{M}$ in standard Einstein cosmology, another density parameter, $\Omega_{_{\Delta}}$, is expected by the theory. This implies that if $\Omega_{_{\Delta}}$ is found to be nonzero, data will favor this model instead of the standard Einstein cosmological model with cosmological constant and will enable more accurate predictions for the rate of change of Newtonian gravitational constant in the future., Comment: minor reference changes

The primordial magnetic field (PMF) can strongly affect the cosmic microwave background (CMB) power spectrum and the formation of large scale structure. In this presentation, we calculate the CMB temperature anisotropies generated by including a power-law magnetic field at the photon last scattering surface (PLSS). We then deduce an upper limit on the primordial magnetic field based upon our theoretical analysis of the power excess on small angular scales. We have taken into account several important effects such as the modified matter sound speed in the presence of a magnetic field. An upper limit to the field strength of $|B_\lambda|\lesssim$ 4.7 nG at the present scale of 1 Mpc is deduced. This is obtained by comparing the calculated theoretical result including the Sunyaev-Zeldovich (SZ) effect with recent observed data on the small scale CMB anisotropies from the Wilkinson Microwave Anisotropy Probe (WMAP), the Cosmic Background Imager (CBI) and the Arcminute Cosmology Bolometer Array Receiver (ACBAR). We discuss several possible mechanisms for the generation and evolution of the PMF before, during and after the BBN., Comment: 5 pages, 1 figures Proceedings of the International Symposium on Nuclear Astrophysics "Nuclei in the Cosmos - IX", CERN, Geneva, June 25-30, 2006

Recent observations of the cosmic microwave background (CMB) have extended the measured power spectrum to higher multipoles $l\gtrsim$1000, and there appears to be possible evidence for excess power on small angular scales. The primordial magnetic field (PMF) can strongly affect the CMB power spectrum and the formation of large scale structure. In this paper, we calculate the CMB temperature anisotropies generated by including a power-law magnetic field at the photon last-scattering surface (PLSS). We then deduce an upper limit on the PMF based on our theoretical analysis of the power excess on small angular scales. We have taken into account several important effects such as the modified matter sound speed in the presence of a magnetic field. An upper limit to the field strength of $|B_\lambda|\lesssim$ 4.7 nG at the present scale of 1 Mpc is deduced. This is obtained by comparing the calculated theoretical result including the Sunyaev-Zeldovich (SZ) effect with recent observed data on the small-scale CMB anisotropies from the $Wilkinson Microwave Anisotropy Probe$ (WMAP), the Cosmic Background Imager (CBI), and the Arcminute Cosmology Bolometer Array Receiver (ACBAR). We discuss several possible mechanisms for the generation and evolution of the PMF., Comment: 27 pages, 4 figures, accepted to ApJ April 10, 2006

We compute the power spectrum of the Cosmic Microwave Background temperature anisotropies generated by the Intergalactic Medium. To estimate the electron pressure along the line of sight and its contribution to the Sunyaev-Zeldovich component of the CMB anisotropies, we assume the non-linear baryonic density contrast is well described by a log-normal distribution. For model parameters in agreement with observations and for an experiment operating in the Rayleigh-Jeans regime, the largest IGM contribution is expected at scales corresponding to multipole numbers of about 2000. The amplitude is rather uncertain and could be as large as 100-200 [mikroK]^2, comparable to the contribution of galaxy clusters. The actual value is strongly dependent on the gas polytropic index and the amplitude of the matter power spectrum sigma_8. At all redshifts, the largest contribution comes from scales very close to the comoving baryon Jeans length. This scale is not yet resolved in numerical simulations that follow the evolution of gas on cosmological scales. The anisotropy generated by the Intergalactic Medium could make compatible the excess of power measured by Cosmic Background Imager (CBI) on scales of l > 2000 with sigma_8=0.9. Taking the CBI result as an upper limit, the polytropic index can be constrained to < 1.5 at 2-sigma level at redshifts z ~ 0.1-0.4. With its large frequency coverage, the PLANCK satellite will be able to measure the secondary anisotropies coming from hot gas. Cluster and Intergalactic Medium contributions could be separated by cross correlating galaxy/cluster catalogs with CMB maps. This measurement will determine the state of the gas at low and intermediate redshifts., 22 pages, 7 figures, accepted for publication in the Astrophysical Journal

[LPH96]201.663+1.643, a diffuse H{\sc ii} region, has been reported to be a candidate for emission from rapidly spinning dust grains. Here we present Cosmic Background Imager (CBI) observations at 26-36 GHz that show no evidence for significant anomalous emission. The spectral index within the CBI band, and between CBI and Effelsberg data at 1.4/2.7 GHz, is consistent with optically thin free-free emission. The best-fitting temperature spectral index from 2.7 to 31 GHz, $\beta=-2.06 \pm 0.03$, is close to the theoretical value, $\beta=-2.12$ for $T_{e}=9100$ K. We place an upper limit of 24% ~ (2\sigma) for excess emission at 31 GHz as seen in a $6\arcmin$ FWHM beam. Current spinning dust models are not a good fit to the spectrum of LPH96. No polarized emission is detected in the CBI data with an upper limit of 2% on the polarization fraction., Comment: 5 pages, 3 figures, submitted to ApJL

The spectral energy distribution of the dark cloud LDN1622, as measured by Finkbeiner using WMAP data, drops above 30GHz and is suggestive of a Boltzmann cutoff in grain rotation frequencies, characteristic of spinning dust emission. LDN1622 is conspicuous in the 31 GHz image we obtained with the Cosmic Background Imager, which is the first cm-wave resolved image of a dark cloud. The 31GHz emission follows the emission traced by the four IRAS bands. The normalised cross-correlation of the 31 GHz image with the IRAS images is higher by 6.6sigma for the 12um and 25um bands than for the 60um and 100um bands: C(12+25) = 0.76+/-0.02 and C(60+100) = 0.64+/-0.01. The mid-IR -- cm-wave correlation in LDN 1622 is evidence for very small grain (VSG) or continuum emission at 26-36GHz from a hot molecular phase. In dark clouds and their photon-dominated regions (PDRs) the 12um and 25um emission is attributed to stochastic heating of the VSGs. The mid-IR and cm-wave dust emissions arise in a limb-brightened shell coincident with the PDR of LDN1622, where the incident UV radiation from the Ori OB1b association heats and charges the grains, as required for spinning dust., accepted for publication in ApJ - the complete article with uncompressed figures may be downloaded from http://www.das.uchile.cl/~simon/ftp/l1622.pdf

There is accumulating observational evidence (based on SnIa data) that the dark energy equation of state parameter $w$ may be evolving with time and crossing the phantom divide barrier $w=-1$ at recent times. The confirmation of these indications by future data would indicate that minimally coupled quintessence can not reproduce the observed expansion rate $H(z)$ for any scalar field potential. Here we explicitly demonstrate that scalar tensor theories of gravity (extended quintessence) can predict crossing of the phantom divide barrier. We reconstruct phenomenologically viable scalar-tensor potentials $F(\Phi)$ and $U(\Phi)$ that can reproduce a polynomial best fit expansion rate $H(z)$ and the corresponding dark energy equation of state parameter $w(z)$ crossing the $w=-1$ line. The form of the reconstructed scalar tensor potentials is severely constrained but is not uniquely determined. This is due to observational uncertainties in the form of $H(z)$ and also because a single observed function $H(z)$ does not suffice for the reconstruction of the two potential functions $F(\Phi)$ and $U(\Phi)$., Comment: 9 pages, 5 figures. Presentation modified, references added

If the voids that we see today in the distribution of galaxies existed at recombination, they will leave an imprint on the cosmic microwave background (CMB). On the other hand, if these voids formed much later, their effect on the CMB will be negligible and will not be observed with the current generation of experiments. In this paper presented at the 2004 Annual Scientific Meeting of the Astronomical Society of Australia, we discuss our ongoing investigations into voids of primordial origin. We show that if voids in the cold dark matter distribution existed at the epoch of decoupling, they could contribute significantly to the apparent rise in CMB power on small scales detected by the Cosmic Background Imager (CBI) Deep Field. Here we present our improved method for predicting the effects of primordial voids on the CMB in which we treat a void as an external source in the cold dark matter (CDM) distribution employing a Boltzmann solver. Our improved predictions include the effects of a cosmological constant (Lambda) and acoustic oscillations generated by voids at early times. We find that models with relatively large voids on the last scattering surface predict too much CMB power in an Einstein--de Sitter background cosmology but could be consistent with the current CMB observations in a Lambda-CDM universe., 9 pages, 8 figures, paper presented at the 2004 Annual Scientific Meeting of the Astronomical Society of Australia, accepted for publication in PASA

We report upper limits on the polarization of the CMBR as measured with the Cosmic Background Imager, a 13 element interferometer that operates in the 26-36 GHz band and is sited on Llano de Chajnantor in northern Chile. The array consists of 90-cm Cassegrain antennas mounted on a steerable platform that can be rotated about the optical axis to facilitate polarization observations. The CBI employs single-mode circularly polarized receivers and it samples multipoles from l~400 to l~3500. The polarization data were calibrated on 3C279 and Tau A. The polarization observations consist of 278 hours of data on two fields taken in 2000, during the first CBI observing season. A joint likelihood analysis of the two fields yields three upper limits (95% c.l.) for \mathcal{C}_l^{EE} = C^{EE}l(l+1)/(2pi) under the assumption that \mathcal{C}_l^{BB}=0: 49.0 microK^2 (l=603); 164 microK^2 (l=1144); and 630 microK^2 (l=2048)., 17 pages, 3 figures, scheduled for 10 April 2005 ApJ

This work presents a reduced chi^2_nu test to search for non-gaussian signals in the CMBR TT power spectrum of recent CMBR data, WMAP, ACBAR and CBI data sets, assuming a mixed density field including adiabatic and isocurvature fluctuations. We assume a skew positive mixed model with adiabatic inflation perturbations plus additional isocurvature perturbations possibly produced by topological defects. The joint probability distribution used in this context is a weighted combination of Gaussian and non-Gaussian random fields. Results from simulations of CMBR temperature for the mixed field show a distinct signature in CMB power spectrum for very small deviations (~ 0.1%) from a pure Gaussian field, and can be used as a direct test for the nature of primordial fluctuations. A reduced chi^2_nu test applied on the most recent CMBR observations reveals that an isocurvature fluctuations field is not ruled out and indeed permits a very good description for a flat geometry Lambda-CDM universe, chi^2_930 ~ 1.5, rather than the simple inflationary standard model with chi^2_930 ~ 2.3. This result may looks is particular discrepant with the reduced chi^2 of 1.07 obtained with the same model in Spergel et al. (2003) for temperature only, however, our work is restricted to a region of the parameter space that does not include the best fit model for TT only of Spergel et al. (2003)., Comment: Accepted for publication in Physical Review D

We study quintessence and phantom field theory models based on linear-negative potentials of the form $V(\phi)=s \phi$. We investigate the predicted redshift dependence of the equation of state parameter $w(z$ for a wide range of slopes $s$ in both quintessence and phantom models. We use the gold dataset of 157 SnIa and place constraints on the allowed range of slopes $s$. We find $s=0\pm 1.6$ for quintessence and $s=\pm 0.7\pm 1$ for phantom models (the range is at the $2\sigma$ level and the units of $s$ are in $\sqrt{3}M_p H_0^2\simeq 10^{-38}eV^3$ where $M_p$ is the Planck mass). In both cases the best fit is very close to $s\simeq 0$ corresponding to a cosmological constant. We also show that specific model independent parametrizations of $w(z)$ which allow crossing of the phantom divide line $w=-1$ (hereafter PDL) provide significantly better fits to the data. Unfortunately such crossings are not allowed in any phantom or quintessence single field model minimally coupled to gravity. Mixed models (coupled phantom-quintessence fields) can in principle lead to a $w(z)$ crossing the PDL but a preliminary investigation indicates that this does not happen for natural initial conditions., Comment: 8 pages, 7 figures. References added, discussion modified. The mathematica file with the numerical analysis of the paper can be downloaded from http://leandros.physics.uoi.gr/negpot.htm

Polarization observations of the cosmic microwave background with the Cosmic Background Imager from September 2002 to May 2004 provide a significant detection of the E-mode polarization and reveal an angular power spectrum of polarized emission showing peaks and valleys that are shifted in phase by half a cycle relative to those of the total intensity spectrum. This key agreement between the phase of the observed polarization spectrum and that predicted based on the total intensity spectrum provides support for the standard model of cosmology, in which dark matter and dark energy are the dominant constituents, the geometry is close to flat, and primordial density fluctuations are predominantly adiabatic with a matter power spectrum commensurate with inflationary cosmological models., 28 pages including 11 color figures (reduced in quality for astro-ph). Version 2 includes supplementary material (window functions etc.) and matches the published paper

Using cosmological simulations, we make predictions for the distribution of clusters in a plausible non-gaussian model where primordial voids nucleated during inflation act together with scale-invariant adiabatic gaussian fluctuations as seeds for the formation of large-scale structure. This model agrees with most recent observations of the anisotropies of the cosmic microwave background (CMB) and can account for the excess of power measured on cluster scales by the Cosmic Background Imager (CBI), the large empty regions apparent in nearby galaxy redshift surveys and the number of giant arcs measured in deep cluster lensing surveys. We show that the z=0 cluster mass function differs from predictions for a standard LCDM cosmology with the same sigma_8. Moreover, as massive clusters also form much earlier in the "void" scenario, we show that integrated number counts of SZ sources and simple statistics of strong lensing can easily falsify this model.

We have summarized previous results from the Cosmic Background Imager, as well as current results and prospects for current CBI observing campaigns focused on higher l resolution, small angular scales, and CMB polarization.

We present observations of the Vela X region at 31 GHz using the Cosmic Background Imager (CBI). We find a strong compact radio source (5.9'x4.1', FWHM) about the Vela pulsar, which we associate with the Vela pulsar wind nebula (PWN) recently discovered at lower radio-frequencies. The CBI's 4' resolution for a 45' field of view allows the PWN to be studied in the large-scale context of Vela X. Filamentary structure in Vela X, which stands out in lower frequency maps, is very low-level at 31 GHz. By combining the 10 CBI channels, which cover 26-36 GHz, and 8.4 GHz archive data, we study the spectral energy distribution (SED) of the PWN and the brightest filaments. Our results show that the spectral index alpha (F_{nu} propto nu^alpha) of the PWN is flat, or even marginally positive, with a value of alpha_{8.4}^{31}=0.10+-0.06, while the Vela X filamentary structure has a negative spectral index of alpha_{8.4}^{31}=-0.28+-0.09. The SED inhomogeneity observed in Vela X suggests different excitation processes between the PWN and the filaments. We investigate whether the PWN's flat spectrum is a consequence of variability or truly reflects the SED of the object. The peak of the PWN at 31 GHz is 80''+-20'' south-west of the peak at 8.4 GHz. This shift is confirmed by comparing the 31 GHz CBI image with higher resolution 5 GHz Australia Telescope Compact Array observations, and is likely to be due to SED variations within the PWN. We also investigate the nature of the Vela X filamentary structure., Comment: Accepted for publication in The Astrophysical Journal. 25 pages, 9 figures