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The Very Small Array (VSA) is a synthesis telescope designed to image faint structures in the cosmic microwave background on degree and sub-degree angular scales. The VSA has key differences from other CMB interferometers with the result that different systematic errors are expected. We have tested the operation of the VSA with a variety of blank-field and calibrator observations and cross-checked its calibration scale against independent measurements. We find that systematic effects can be suppressed below the thermal noise level in long observations; the overall calibration accuracy of the flux density scale is 3.5 percent and is limited by the external absolute calibration scale.
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We investigate the constraints on basic cosmological parameters set by the first compact-configuration observations of the Very Small Array (VSA), and other cosmological data sets, in the standard inflationary LambdaCDM model. Using a weak prior 40 < H_0 < 90 km/s/Mpc and 0 < tau < 0.5 we find that the VSA and COBE_DMR data alone produce the constraints Omega_tot = 1.03^{+0.12}_{-0.12}, Omega_bh^2 = 0.029^{+0.009}_{-0.009}, Omega_cdm h^2 = 0.13^{+0.08}_{-0.05} and n_s = 1.04^{+0.11}_{-0.08} at the 68 per cent confidence level. Adding in the type Ia supernovae constraints, we additionally find Omega_m = 0.32^{+0.09}_{-0.06} and Omega_Lambda = 0.71^{+0.07}_{-0.07}. These constraints are consistent with those found by the BOOMERanG, DASI and MAXIMA experiments. We also find that, by combining all the recent CMB experiments and assuming the HST key project limits for H_0 (for which the X-ray plus Sunyaev--Zeldovich route gives a similar result), we obtain the tight constraints Omega_m=0.28^{+0.14}_{-0.07} and Omega_Lambda= 0.72^{+0.07}_{-0.13}, which are consistent with, but independent of, those obtained using the supernovae data.
We present the power spectrum of the fluctuations in the cosmic microwave background detected by the Very Small Array (VSA) in its first season of observations in its compact configuration. We find clear detections of first and second acoustic peaks at l~200 and l~550, plus detection of power on scales up to l=800. The VSA power spectrum is in very good agreement with the results of the Boomerang, Dasi and Maxima telescopes despite the differing potential systematic errors.
We have observed the cosmic microwave background temperature fluctuations in eight fields covering three separated areas of sky with the Very Small Array at 34 GHz. A total area of 101 square degrees has been imaged, with sensitivity on angular scales 3.6 - 0.4 degrees (equivalent to angular multipoles l=150-900). We describe the field selection and observing strategy for these observations. In the full-resolution images (with synthesised beam of FWHM ~ 17 arcmin) the thermal noise is typically 45 microK and the CMB signal typically 55 microK. The noise levels in each field agree well with the expected thermal noise level of the telescope, and there is no evidence of any residual systematic features. The same CMB features are detected in separate, overlapping observations. Discrete radio sources have been detected using a separate 15 GHz survey and their effects removed using pointed follow-up observations at 34 GHz. We estimate that the residual confusion noise due to unsubtracted radio sources is less than 14 mJy/beam (15 microK in the full-resolution images), which added in quadrature to the thermal noise increases the noise level by 6 %. We estimate that the rms contribution to the images from diffuse Galactic emission is less than 6 microK. We also present images which are convolved to maximise the signal-to-noise of the CMB features and are co-added in overlapping areas, in which the signal-to-noise of some individual CMB features exceeds 8.
The Very Small Array (VSA) is a fourteen-element interferometer designed to study the cosmic microwave background on angular scales of 2.4 to 0.2 degrees (angular multipoles l = 150 to 1800). It operates at frequencies between 26 and 36 GHz, with a bandwidth of 1.5 GHz, and is situated at the Teide Observatory, Tenerife. The instrument also incorporates a single-baseline interferometer, with larger collecting area, operating simultaneously with and at the same frequency as the VSA main array. This provides accurate flux measurements of contaminating radio sources in the VSA observations. Since September 2000, the VSA has been making observations of primordial CMB fluctuations. We describe the instrument, observing strategy and current status of the first year of observations.
We review the first science results from the Arcminute Cosmology Bolometer Array Receiver (ACBAR); a multifrequency millimeter-wave receiver optimized for observations of the Cosmic Microwave Background (CMB) and the Sunyaev-Zeldovich (SZ) effect in clusters of galaxies. ACBAR was installed on the 2 m Viper telescope at the South Pole in January 2001 and the results presented here incorporate data through July 2002. We present the power spectrum of the CMB at 150 GHz over the range ell = 150 - 3000 measured by ACBAR as well as estimates for the values of the cosmological parameters within the context of Lambda-CDM models. We find that the inclusion of Omega_Lambda greatly improves the fit to the power spectrum. We also observe a slight excess of small-scale anisotropy at 150 GHz; if interpreted as power from the SZ effect of unresolved clusters, the measured signal is consistent with CBI and BIMA within the context of the SZ power spectrum models tested.
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