The Millennium Galaxy Catalogue: The photometric accuracy, completeness and contamination of the 2dFGRS and SDSS-EDR/DR1 data sets

The Millennium Galaxy Catalogue (MGC) is a deep (μ _B,lim = 26 mag arcsec ^-2), wide-field, charge-coupled device imaging survey, covering 37.5 deg ^-2. The MGC survey region is completely contained within the Two-Degree Field Galaxy Redshift Survey (2dFGRS) and the Sloan Digital Sky Survey Early Data Release (SDSS-EDR). We compare the photometry and completeness of the 2dFGRS and the SDSS-EDR with the MGC over the range 16 < B < 20 mag. We have also undertaken a photometric comparison to SuperCosmos (SCOS) and the Sloan Digital Sky Survey First Data Release (SDSS-DR1). We find that B _MGC - B _2dF = (0.035 ± 0.005) mag with an uncertainty of 0.142 mag per galaxy; B _MGC - B _SCOS = (0-032 ± 0.005) mag with an uncertainty of 0.108 mag; B _MGC - B _SDSS-EDR = (0.032 ± 0.005) mag with an uncertainty of 0.094 mag; and B _MGC - B _SDSS-DRI = (0.039 ± 0.005) mag with an uncertainty of 0.086 mag. We find that high surface brightness 2dFGRS galaxies are systematically too faint, which leads to a significant scale error in magnitude. This effect is significantly reduced with the SCOS photometry. In the SDSS there is a weak non-linear scale error, which is negligible for faint galaxies. Low surface brightness galaxies in the SDSS are systematically fainter, consistent with the relative shallowness of this survey. We find that the 2dFGRS catalogue has (5.2 ± 0.3) per cent stellar contamination, (7.0 ± 0.4) per cent of objects resolved into two or more by the MGC, and is (8.7 ± 0.6) per cent incomplete compared to the MGC. From our all-object spectroscopic survey we find that the MGC is itself misclassifying (5.6 ± 1.3) per cent of galaxies as stars, and hence the 2dFGRS misses (14.3 ± 1.4) per cent of the galaxy population. The SDSS-EDR galaxy catalogue has (1.3 ± 0.1) per cent stellar contamination and (5.3 ± 1.0) per cent of galaxies misclassified as stars, with (0.18 ± 0.04) per cent of objects resolved into two or more by the MGC, and is (1.8 ± 0.1) per cent incomplete compared to the MGC. The total fraction of galaxies missing from the SDSS-EDR galaxy catalogue to B _MGC = 20 mag, from incompleteness and misclassification, is (7.1 ± 1.0) per cent.