  We describe the construction of MegaZ-LRG, a photometric redshift catalogue
of over one million luminous red galaxies (LRGs) in the redshift range 0.4 < z
< 0.7 with limiting magnitude i < 20. The catalogue is selected from the
imaging data of the Sloan Digital Sky Survey Data Release 4. The 2dF-SDSS LRG
and Quasar (2SLAQ) spectroscopic redshift catalogue of 13,000
intermediate-redshift LRGs provides a photometric redshift training set,
allowing use of ANNz, a neural network-based photometric-redshift estimator.
The rms photometric redshift accuracy obtained for an evaluation set selected
from the 2SLAQ sample is sigma_z = 0.049 averaged over all galaxies, and
sigma_z = 0.040 for a brighter subsample (i < 19.0). The catalogue is expected
to contain ~5 per cent stellar contamination. The ANNz code is used to compute
a refined star/galaxy probability based on a range of photometric parameters;
this allows the contamination fraction to be reduced to 2 per cent with
negligible loss of genuine galaxies. The MegaZ-LRG catalogue is publicly
available on the World Wide Web from http://www.2slaq.info .
