High resolution imaging of the ATLBS regions: the radio source counts
  The Australia Telescope Low-brightness survey (ATLBS; \cite{SESS10}) regions
have been mosaic imaged at a radio frequency of 1.4 GHz with $6\arcsec$ angular
resolution and 72 $\mu$Jy beam$^{-1}$ rms noise. The images (centered at RA:
$00^{h}\ 35^{m}\ 00^{s}$, DEC: $-67^{\circ}\ 00^{'}\ 00^{"}$ and RA: $00^{h}\
59^{m}\ 17^{s}$, DEC: $-67^{\circ}\ 00^{'}\ 00^{"}$ (J2000 epoch)) cover 8.42
square degrees sky area and have no artifacts or imaging errors above the image
thermal noise. Multi-resolution radio and optical r-band images (made using the
4-m CTIO Blanco telescope) were used to recognize multi-component sources and
prepare a source list; the detection threshold was 0.38 mJy in a low resolution
radio image made with beam FWHM of $50\arcsec$. Radio source counts in the flux
density range 0.4-8.7 mJy are estimated, with corrections applied for noise
bias, effective area correction and resolution bias. The resolution bias is
mitigated using low resolution radio images, while effects of source confusion
are removed by using high resolution images for identifying blended sources.
Below 1 mJy the ATLBS counts are systematically lower than the previous
estimates. Showing no evidence for an upturn down to 0.4 mJy, they do not
require any changes in the radio source population down to the limit of the
survey. The work suggests that automated image analysis for counts may be
dependent on the ability of the imaging to reproduce connecting emission with
low surface brightness and on the ability of the algorithm to recognize
sources, which may require that source finding algorithms effectively work with
multi-resolution and multi-wavelength data. The work underscores the importance
of using source lists-as opposed to component lists-and correcting for the
noise bias in order to precisely estimate counts close to the image noise and
determine the upturn at sub-mJy flux density.
