The radio observations were carried out with two main goals in mind.
The first aim was to detect the ESP galaxies in order to derive the
"local'' ()
bivariate luminosity function. We therefore tried to keep the
sensitivity as uniform as possible over the whole ESP area, while
at the same time reaching flux densities well below
mJy (see
Sect. 2.2).
The second aim was to have a complete catalogue of faint radio sources in order to study the sub-mJy population through a programme of optical identification of complete radio source samples extracted from the ATESP survey, exploiting the available data, i.e. deep CCD images.
As the survey is intended to achieve uniform sensitivity over a large area it is necessary to make use of the mosaicing technique.
In planning a mosaicing experiment, the main issue to be decided is the pointing grid pattern, i.e. geometry and pointing spacings. For a detection experiment on a large area of sky (like the ATESP survey) the main requirement is uniform sensitivity over the entire region together with high observing efficiency. Such requirement can be satisfied by choosing opportunely the pointing grid pattern.
The mosaic noise standard deviation,
,
can be obtained from
the error propagation of Eq. (1) and the uniform sensitivity
constraint is expressed by
The ATCA primary beam pattern can be approximated by a circular Gaussian
function (Wieringa & Kesteven 1992)
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(3) |
To make the final choice for the ATESP survey grid pattern, we have
performed a series of simulations of the bidimensional quantity
In general a very good compromise between
uniform sensitivity and observing efficiency is represented by a grid
pattern with pointing spacings of the order of
.
For our particular case, the best choice turned out to be a
spacing rectangular grid. The mosaic noise variations over a
reference area of 1 sq. degr. for such a grid configuration are shown
in Fig. 1.
As expected, in the region of interest
(central box) the noise is rather constant: variations are
,
except at the region borders (
). We point out that
hexagonal grids should be preferred when imaging wide
areas of sky (like for the NVSS and FIRST radio surveys), but are not
very efficient in the case of a narrow 1-degree wide strip of sky.
At 20 cm (the longest
observing wavelength available at the ATCA) the field of view
is largest (Gaussian primary beam
)
and the system
noise is lowest.
Thus, observing at 20 cm allows minimization of both the number of
fields (i.e. pointings) required to complete the survey and the observing
time spent on each field.
The ATCA can observe at two frequencies simultaneously
(for instance 20 and 13 cm).
However, we decided to optimize sensitivity at the expense of spectral
information, by setting both receivers in the 20 cm band and observing in
continuum mode
(
MHz bandwidth, each divided into
MHz channels in
order to reduce the bandwidth smearing effect).
This choice was also influenced by another
consideration: since the field of view depends on the observing frequency,
the grid pattern could not be optimized to get uniform
sensitivity for both 13 and 20 cm bands simultaneously.
The observations were carried out at full resolution (ATCA 6 km
configuration), since the identification follow up benefits from high
spatial resolution, and the expected fraction of
very extended sources (that could be resolved out and lost) is low at the
ATESP resolution. Using the angular size
distribution given by Windhorst et al. (1990) for radio sources, we estimate
that
of the mJy and sub-mJy sources would appear point-like
at the ATESP resolution, and
would have
angular sizes twice the beam size or larger.
Since the ESP sample distance distribution peaks at
and
"normal'' galaxies are typically low-power radio sources, deep radio
observations were needed to ensure detections of a statistically significant
number of ESP galaxies. We considered satisfactory a point source radio limit
of the order of
mJy (
), which corresponds to a
detection threshold of
WHz-1at
(H0 =100 km s-1 Mpc-1).
Furthermore a large sample of sub-mJy radio sources can be constructed
at a
detection limit, corresponding to a flux limit of
mJy.
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