Deep GMRT 150 MHz observations of the LBDS-Lynx region: Ultra-Steep
  Spectrum Radio Sources
  It has been known for nearly three decades that high redshift radio galaxies
exhibit steep radio spectra, and hence ultra-steep spectrum radio sources
provide candidates for high-redshift radio galaxies. Nearly all radio galaxies
with z > 3 have been found using this redshift-spectral index correlation. We
have started a programme with GMRT to exploit this correlation at flux density
levels about 10 to 100 times deeper than the known high-redshift radio galaxies
which were identified primarily using the already available radio catalogues.
In our programme, we have obtained deep, high resolution radio observations at
150 MHz with GMRT for several 'deep' fields which are well studied at higher
radio frequencies and in other bands of the electromagnetic spectrum, with an
aim to detect candidate high redshift radio galaxies. In this paper we present
results from the deep 150 MHz observations of LBDS-Lynx field, which has been
already imaged at 327, 610 and 1412 MHz with the WSRT and at 1400 and 4860 MHz
with the VLA. The 150 MHz image made with GMRT has a rms noise of ~0.7 mJy/beam
and a resolution of ~19" X 15". It is the deepest low frequency image of the
LBDS-Lynx field. The source catalog of this field at 150 MHz has about 765
sources down to ~20% of the primary beam response, covering an area of about 15
degree$^2$. Spectral index was estimated by cross correlating each source
detected at 150 MHz with the available observations at 327, 610, 1400 and 4860
MHz and also using available radio surveys such as WENSS at 327 MHz and NVSS
and FIRST at 1400 MHz. We find about 150 radio sources with spectra steeper
than 1. About two-third of these are not detected in SDSS, hence are strong
candidate high-redshift radio galaxies, which need to be further explored with
deep infra-red imaging and spectroscopy to estimate the redshift.
