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A deep WSRT 21 CM survey down to 0.1 mJy in the Lynx area
Abstract
Observations of the field Lynx 3A, obtained at 21 cm with 5sigma peak
flux 85 microJy using the 3-km array of the Westerbork Synthesis Radio
Telescope for 16 12-h periods during 1984, are reported. Details of the
observations and the data reduction are given, and the results are
presented in extensive tables and maps. A complete sample of 321 objects
within the 0.5-deg attenuation radius is selected from the 349 objects
detected above 85 microJy, and the 1.412-GHz source counts down to 100
microJy are found to be in good agreement with VLA surveys of similar
depth. The median angular size of sources with total flux density
between 350 microJy and 10 mJy is shown to be less than 10 arcsec.
The local luminosity function was determined at v = 1.4 GHz from radio
observations of two low-redshift galaxy samples: (1) spiral and
irregular galaxies with apparent blue magnitudes and declinations and
(2) galaxies of all morphologies with blue angular diameters of 1.0
arcmin or greater in the declination range between -2.5 deg and +82 deg.
Separate luminosity functions for the radio source populations powered
by 'starbursts' and 'monsters' were obtained from the latter sample. The
amount of evolution required for the local luminosity function to
account for the faint sources is discussed. The cosmological evolution
of extragalactic radio sources appears to be so strong at all observed
luminosities that the local luminosity function and counts of all
sources between S of roughly 10 micro-Jy and S of roughly 10 Jy at v =
1.4 GHz can be matched with a model in which most sources are confined
to a hollow shell with z of roughly 0.8.
The temperature fluctuations of the microwave background due to
unresolved, randomly distributed sources have been computed for a wide
interval of angular scales (from a few arcseconds to several tens of
degrees) and for wavelengths ranging from several centimeters to below 1
mm. For wavelenghts greater than 6 cm, the present estimates follow
almost directly from the observed radio source counts. By properly
taking into account all of the available data, the uncertainties in the
extrapolations have been reduced to smaller scales and higher
frequencies. Only within a relatively narrow frequency range (from about
20 to 200 GHz) the contribution of discrete sources to the temperature
fluctuations on scales of less that 1 deg is expected to be Delta T/T
less than 0.00001. The effect of clustering that might strongly enhance
fluctuations at least on some angular scales is noted. The most
sensitive data on Delta T/T at 6 cm provide significant constraints on
the correlation function of radio sources.
A deep survey has been carried out with the VLA at 21 cm to a noise
level of 45 μJy. The region chosen was the Leiden-Berkeley Deep
Survey (LBDS) area Lynx.2, known to be devoid of radio sources stronger
than 10 mJy. A catalog of 124 sources is presented, of which 93 form a
sample complete to the 5 σ level. The normalized differential
source counts show a flattening below 5 mJy. Optical data are used to
probe the nature of the sub-millijansky radio source population. Deep
identifications and calibrated photometry are made on prime-focus plates
obtained in four bands with the Kitt Peak 4 m Mayall telescope. In the
complete sample 41 radio sources are reliably identified. It is shown
that for radio sources with 1 < S1.4 < 10 mJy a blue
radio galaxy population becomes increasingly important; these often have
a peculiar optical morphology indicative of interacting or merging
galaxies. Below 1 mJy the VLA sources consist of some red galaxies, but
the majority are faint blue galaxies mostly fainter than V ≈ 20.
A single primary-beam area centered on α =
13h00m37s, δ = 30°34arcmin
(1950.0) near the north galactic pole was mapped with the Very Large
Array at 1.49 GHz. There are 145 sources within 22.5 arcmin of the map
center having map peak flux densities sp ≥ 70 μJy, of
which 49 were optically identified with objects brighter than
BJ ≡ 22.5 on a Palomar Schmidt IIIa-J plate. The sky
density of sources stronger than 84 μJy was determined by direct
counts of individual sources within 17 arcmin of the map center. The
authors used the confusion P(D) distribution in the central 13×13
arcmin2 of the map to estimate the density of sources between
10 and 84 μJy. The mean radio-source sky densities probed are about
103per deg2 above 87 μJy and 104per
deg2 above 10.5 μJy, comparable with the surface densities
of all galaxies brighter than J ≡ +21 and J ≡ +23.5,
respectively. An evolutionary model which fits the 1.4-GHz counts of
radio sources associated with spiral galaxies was used to predict the
sky densities and median redshifts of far-infrared (λ = 60 μ)
sources as faint as 1 mJy.
A single primary beam area centered on alpha = 08h52m15s, delta = +17
deg 16 arcmin was mapped with the VLA at 5.8-arcsec resolution to
investigate the angular sizes of very faint radio sources at 1465 MHz.
Previously, this field (approximately one square degree) was mapped with
19-arcsec resolution to a 6.3 sigma flux density limit of 84 micro Jy.
The rms noise in the high resolution map is sigma equal to about 21
micro Jy; hence all previously detected point sources should be present
at a confidence level of four sigma. The low resolution map was used to
verify that sources were not resolved away by the high resolution
observations. The median angular size (theta med) found for the faintest
sources (greater than or equal to 84 and less than 400 micro Jy) is less
than or equal to 3 arcsec. It is significantly lower than the
extrapolation of earlier data and models in which theta med remains
constant in the range 7-10 arcsec for sources brighter than about 1 mJy
and fainter than 100 mJy. The observed dropoff in angular size is
consistent with most stronger sources being produced by elliptical
galaxies, while the fainter sources reside in spiral or Seyfert
galaxies. Thus a simple model can fit the new data without requiring
size evolution of radio sources.