The Anisotropy of the Microwave Background to l = 3500: Mosaic Observations with the Cosmic Background Imager

TL;DR

The paper presents mosaic observations with the Cosmic Background Imager (CBI) to image the CMB and measure its angular power spectrum ${\cal C}_{\ell}$ up to ${\ell} \sim 3500$, using lead–trail differencing to remove ground contamination and a robust foreground treatment based on OVRO and NVSS surveys. A maximum-likelihood framework estimates band powers in contiguous ${\ell}$ bins, incorporating instrumental noise, residual sources, and constraint matrices that project out known point sources; uv-grid interpolation and the BJK98 relaxation yield precise estimates with well-characterized window functions. The joint mosaic results cover ${\ell} \lesssim 3000$, showing a spectrum consistent with inflationary models and prior measurements, including indications of second and third acoustic peaks and evidence for the damping tail, while demonstrating the method’s sensitivity and limitations given foregrounds. The work underscores the viability of ground-based interferometric mosaicing for high-${\ell}$ CMB studies and motivates future surveys and polarization measurements to further constrain cosmological parameters and the primordial perturbation spectrum, aided by improved low-noise foreground surveys at 31 GHz.

Abstract

Using the Cosmic Background Imager, a 13-element interferometer array operating in the 26-36 GHz frequency band, we have observed 40 sq deg of sky in three pairs of fields, each ~ 145 x 165 arcmin, using overlapping pointings (mosaicing). We present images and power spectra of the cosmic microwave background radiation in these mosaic fields. We remove ground radiation and other low-level contaminating signals by differencing matched observations of the fields in each pair. The primary foreground contamination is due to point sources (radio galaxies and quasars). We have subtracted the strongest sources from the data using higher-resolution measurements, and we have projected out the response to other sources of known position in the power-spectrum analysis. The images show features on scales ~ 6 - 15 arcmin, corresponding to masses ~ (5 - 80)*10^{14} Msun at the surface of last scattering, which are likely to be the seeds of clusters of galaxies. The power spectrum estimates have a resolution Delta-l = 200 and are consistent with earlier results in the multipole range l <~ 1000. The power spectrum is detected with high signal-to-noise ratio in the range 300 <~ l <~ 1700. For 1700 <~ l <~ 3000 the observations are consistent with the results from more sensitive CBI deep-field observations. The results agree with the extrapolation of cosmological models fitted to observations at lower l, and show the predicted drop at high l (the "damping tail").

Figures (15)

• Figure 1: Radial profile of the CBI primary beam in one of the ten frequency channels; data from all 78 baselines are superimposed. The observations ( error bars) were made on 2000 Nov 12; Tau A was observed for about 20 s at each point on a $13\times13$ grid in azimuth and elevation. The grid points were separated by 7$^\prime$, so the grid extended to $\pm42\hbox{$^\prime$}$. Measurements within 45$^\prime$ of the central position were used to fit for the amplitude scale and pointing offset (3 parameters) for each of the 780 datasets (78 baselines $\times$ 10 channels). The red curve shows the adopted profile, which was computed by taking the square of the Fourier transform of the aperture illumination pattern, assumed to be circularly symmetric. Taking the outer radius of the aperture (0.45 m) as $r=1$, the inner part $r < 0.172$ is blocked by the secondary and is assumed to have zero illumination. The illumination is tapered from center to edge: we approximate this as a Gaussian $\exp[-(r/r_0)^2]$. The parameter $r_0 = 0.683$ was chosen to give the best fit to all ten channels. It corresponds to an edge taper of $-18.6$ dB and a beam FWHM of $\theta_{\rm FWHM} = 45\hbox{$.\mkern-4mu^\prime$}2 \times (31\,{\rm GHz}/\nu)$. The 13 antennas have very similar beams, but they have relative pointing offsets of 1--2$^\prime$, up to 5$^\prime$ in the worst case.
• Figure 2: Typical $(u,v)$ sampling obtained for a single pointing (C1444$-$0230). For this observation only 12 antennas were used. Left: sampling for a single frequency channel. Right: sampling with 10 channels; a separate dot has been used for each channel. The different channels are sensitive to slightly different angular scales.
• Figure 3: Images of the 02$^{\mathrm h}$ mosaic. Left: raw data; right: after subtraction of sources measured at OVRO. The images show the difference of the emission in the lead and trail fields. The coordinates are J2000. The right ascension scale applies to the lead field; add 8$^{\mathrm m}$ to obtain the right ascension of objects in the trail field. The same gray-scale range has been used for both images, and it does not show the full brightness range of the discrete sources. Light (positive) spots are discrete sources in the lead field, while dark (negative) spots are discrete sources in the trail field. The brightest source in this image has a flux density of about 67 mJy at 31 GHz. These images were made from the entire dataset using natural weighting, and they have been corrected for the primary beam response as described in § \ref{['sec:images']}. They have a resolution (FWHM) of $5\hbox{$.\mkern-4mu^\prime$}2$--$5\hbox{$.\mkern-4mu^\prime$}5$ (the resolution varies slightly across the image, depending on the $(u,v)$ coverage obtained for each pointing).
• Figure 4: Images of the 14$^{\mathrm h}$ mosaic. Left: raw data; right: after subtraction of sources measured at OVRO. For details, see caption to Fig. \ref{['fig:mos02']}.
• Figure 5: Image of the 20$^{\mathrm h}$ mosaic. Left: raw data; right: after subtraction of sources measured at OVRO. For details, see caption to Fig. \ref{['fig:mos02']}.