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Imaging CMB Anisotropies with CAT
Joanne C. Baker 1
Mullard Radio Astronomy Observatory 2 , Cavendish Laboratory, Madingley Road, Cambridge CB3 0HE, U.K.
The Cambridge Cosmic Anisotropy Telescope (CAT) has recently made the first detailed
image of primordial anisotropies in the cosmic microwave background (CMB) radiation at
15 GHz. Preliminary results from CAT observations of a second 2 ffi \Theta 2 ffi field are presented
here. These new data show excess power on angular scales of spherical harmonic multipoles
l = 330 \Gamma 680 (i.e. around half a degree), at levels consistent with the first published
detection. These two results mark the downturn in the CMB power spectrum establishing
the presence of the first Doppler peak, and by constraining its position also limit values of
H 0
and\Omega\Gamma 1 Introduction
Observations of structure in the cosmic microwave background (CMB) radiation over many scales,
from degrees to arcminutes, yield information on the power spectrum of initial density fluctuations,
and are crucial for discriminating between competing theories of structure formation. On scales
of 0:2 \Gamma 2 ffi , inflationary models predict that increased power should be seen in the CMB sky due to
Doppler scattering of photons during acoustic oscillations of the photon­baryon fluid at recombination,
and detection of these `Doppler peaks' is one of the primary goals of CMB astronomy. Furthermore,
the size and position of these peaks can be used to determine basic cosmological parameters.
Until last year, CMB experiments had found increasing power in CMB spatial fluctuations going
from 10 ffi to 1 ffi , but none had measured the expected turnover. In a paper by Scott et al. (1996)
(Paper I), a new detection by the Cambridge Cosmic Anisotropy Telescope (CAT) on scales of about
0:5 ffi provided the first evidence to support the existence of a ¸ 1 ffi peak in the power spectrum. This
was the first of a series of fields being observed by the CAT; preliminary results for a second field are
presented here.
2 The CAT
The CAT is a 3­element radio interferometer, built as a prototype for a larger instrument (the Very
Small Array; see e.g. Jones (1996)). Its three 70­cm horns are mounted on a turntable, with baselines
adjustable in the range 0.7­5 m. Operating at frequencies between 13 and 17 GHz, the CAT is sensitive
to structures on scales of 10 \Gamma 30 0 over a primary beam of 2 ffi FWHM (Robson et al. (1993)). Data are
taken in two orthogonal polarisations, but the polarisation axis rotates on the sky as the telescope
tracks. In 300 hours of good data CAT reaches a typical rms sensitivity of 10 mJy/beam (equivalent
to 18¯K rms) (O'Sullivan et al. (1995)).
As an interferometer, CAT is not sensitive to large­scale signals or ones which do not rotate with
the sky, and thus efficiently filters out unwanted atmospheric emission and groundspill. This is the
main reason why CAT can operate from an ordinary sea­level site (while other experiments seek high,
dry sites or go into space). Groundspill is also minimised by surrounding CAT with earth banks
covered with aluminium shielding.
To combat foregrounds including discrete extragalactic sources (mostly active galaxies) and emis­
sion from our Galaxy (thermal, free­free and synchrotron components) CAT uses two strategies:
observations are made at three frequencies to remove Galactic emission (whose spectrum differs from
the CMB), and we use the Ryle Telescope (RT; see Grainge in these proceedings) which operates at
15.2 GHz to measure and remove discrete radio sources. Using 5 of the east­west RT aerials in a com­
pact configuration (baselines up to 108m), the RT achieves 30 00 resolution. At this higher resolution,
1 mailto:jcb@mrao.cam.ac.uk
2 http://www.mrao.cam.ac.uk/
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the RT is used to identify sources at 15 GHz by making raster scans of the central 2 ffi \Theta 2 ffi of CAT
fields, down to a sensitivity of 2 mJy/beam rms, i.e. well below the sensitivity of CAT.
3 CAT2: The Second Field
Because CAT observes relatively small patches of sky with modest resolution, sample variance will be
important. With that in mind, CAT has been observing several 2 ffi \Theta2 ffi fields, all chosen to be relatively
free from strong radio sources at frequencies up to 5 GHz (Condon et al. (1989)), and lie at high
Galactic latitude (b ? 30 ffi ). The second of these fields, called `CAT2', is centred at 17 h 00 m +64 ffi 30 0
(B1950). To date, CAT2 has been observed at 13.5 GHz, 15.5 GHz and 16.5 GHz for 200­400 hours per
frequency (the greatest amount of data has been collected at the two extreme frequencies) in arrays
with baselines scaled with frequency. The data were analysed using standard procedures, as described
by O'Sullivan et al. (1995). After excluding excessively noisy regions of visibility data by eye, images
were made for each frequency with a final rms sensitivity of about 9 mJy/beam at 13.5 and 16.5 GHz.
Point sources were clearly visible in these images, although most were weaker on average than those
in the first `CAT1' field.
Nevertheless, a total of 27 discrete sources with S 15 ? 10 mJy were removed from the CAT2 field
after RT raster scanning (simultaneous with the CAT observations). Long term monitoring with the
RT showed that the brightest one was very highly variable, its flux density fluctuating by as much
as a factor of two in a few days. This illustrates the importance of simultaneous monitoring for
source subtraction. Comparison of the final source­subtracted maps with reconstructed maps of the
foreground radio sources only showed no significant correlation.
After source subtraction, excess power was seen in all three maps, mostly at 16.5 GHz, consistent
with the detection of CMB signal. Figure 1 shows the 16.5 GHz image of CAT2 after the sources have
been subtracted--there is excess power in the centre, although a 5­oe negative feature dominates the
actual map (note the map has not been CLEANed, i.e. instrumental response has not been taken out).
We have checked that this feature is real by splitting the data in time, frequency and polarisation, and
seeing it in all subsets. Also, the feature disappears in a polarisation­difference map. Furthermore,
the strength of the feature in the maps at different frequencies is well described by the spectrum of the
CMB. In any case, simulations of Gaussian CMB skies show clearly that it is not necessarily unusual
to see a 5­oe feature on its own in a map with low resolution and a relatively small field of view. Again,
this convinces us of that observations of several fields are essential to obtain representative average
values.
4 Analysis
To confirm the detection of CMB anisotropy in the CAT2 field statistically, a Bayesian maximum­
likelihood analysis (Hobson et al. (1995)) was performed on the raw complex visibilities, which were
split first into two bins of low and high l. CMB and Galactic signals were modelled as independent
Gaussian distributions, the CMB with a fixed spectral index of 2 in flux density and the Galactic
spectral index variable between 0 and 1. After marginalising over the Galactic parameters, this
analysis confirmed that the bulk of the power in the 16.5 GHz map is due to CMB signal, with
relative amplitudes in the two bins of \DeltaT =T = 1:7 +0:3
\Gamma0:2 \Theta 10 \Gamma5 and \DeltaT =T = 1:6 +0:5
\Gamma0:4 \Theta 10 \Gamma5 centred on
(spherical harmonic) multipoles l = 422 and l = 615 respectively.
These values of \DeltaT =T in the CAT2 field are slightly lower than found in CAT1 but fall within
the error bars, consistent with sample variance. The good agreement between the two independent
measurements in two different patches of sky reinforces the impact of the CAT points on the power
spectrum, and together with other data on larger scales, clearly marks the presence of the first Doppler
peak.
5 Cosmological implications from CAT detections
The mean broad­band power of the CMB fluctuations measured in the CAT1 and CAT2 fields, about
\DeltaT =T = 2:0 +0:4
\Gamma0:4 \Theta10 \Gamma5 , is broadly consistent with COBE­normalised CDM models (Scott et al. (1996);
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Figure 1: 16.5 GHz CAT image of 6 ffi \Theta 6 ffi area centred on the CAT2 field, after discrete sources
have been subtracted. Excess power can be seen in the central 2 ffi \Theta 2 ffi primary beam (because the
sensitivity drops sharply outside this area, the outer regions are a good indicator of the noise level on
the map). The flux density range scale spans \Sigma40 mJy per beam.
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Hancock et al. (1997)). This \DeltaT =T value is lower than that measured by other experiments at ¸ 1 ffi
scales, pointing to the existence of the first Doppler peak and constraining its position in l­space (i.e.
below l = 400 \Gamma 500). This in turn places restrictions on cosmological parameters,
showing\Omega total ? 0:3
and implying a low value of H 0 (Hancock & Rocha (1996), Hancock et al. (1997)). The existence of
a peak at all also raises problems for defect models (see Turok, these proceedings).
6 Conclusions
We have imaged a 2 ffi \Theta 2 ffi patch of CMB sky at 13--17 GHz with the CAT; this is the second field
observed by CAT. Significant anisotropies on scales of 0:5 ffi are detected in the new field, CAT2. This
new result agrees remarkably well with the first detection made by CAT in a completely different area
of sky.
Acknowledgements
The CAT team includes Mike Hobson, Mike Jones, Anthony Lasenby, Guy Pooley, Richard Saunders,
Paul Scott, Liz Waldram and others at MRAO.
References
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Jones M.E., 1996, in Microwave background anisotropies, Procs 31st Rencontre de Moriond, (Editions
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O'Sullivan C. et al., 1995, MNRAS, 274, 861
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