A Quantitative Evaluation of the Galaxy Component of COSMOS and APM Catalogs

We have carried out an independent quantitative evaluation of the galaxy component of the “COSMOS/UKST Southern Sky Object Catalogue” (SSC) and the “APM/UKST J Catalogue” (APM). Using CCD observations our results corroborate the accuracy of the photometry of both catalogs, which have an overall dispersion of about 0.2 mag in the range 17 ≤ bJ ≤ 21.5. The SSC presents externally calibrated galaxy magnitudes that follow a linear relation, while the APM instrumental magnitudes of galaxies, only internally calibrated by the use of stellar profiles, require second-order corrections. The completeness of both catalogs in a general field falls rapidly fainter than bJ = 20.0, being slightly better for APM. The 90% completeness level of the SSC is reached between bJ = 19.5 and 20.0, while for APM this happens between bJ = 20.5 and 21.0. Both SSC and APM are found to be less complete in a galaxy cluster field, where completeness reachs 90%, respectively, in the ranges bJ = 19.0-19.5 and bJ = 19.5-20.0. Galaxies misclassified as stars in the SSC receive an incorrect magnitude because the stellar ones take saturation into account besides using a different calibration curve. In both cases, the misclassified galaxies show a large diversity of colors that range from typical colors of early-types to those of blue star-forming galaxies. A possible explanation for this effect is that it results from the combination of low sampling resolutions with properties of the image classifier for objects with characteristic sizes close to the instrumental resolution. We find that the overall contamination by stars misclassified as galaxies is < 5% to bJ = 20.5, as originally estimated for both catalogs. Although our results come from small areas of the sky, they are extracted from two different plates and are based on the comparison with two independent datasets. We conclude that both the SSC and APM can be a particularly valuable resource for extragalactic studies in the Southern Hemisphere once their limitations are taken into account. Subject headings: catalogs galaxies: fundamental parameters (classification, photographic magnitudes) techniques: image processing Partly based on observations at Complejo Astronomico El Leoncito (CASLEO), operated under agreement between the Consejo Nacional de Investigaciones Cient́ıficas de la República Argentina and the National Universities of La Plata, Córdoba and San Juan; European Southern Observatory (ESO), under the ESO-ON agreement to operate the 1.52m telescope; and Observatório do Pico dos Dias, operated by the Laboratório Nacional de Astrof́ısica (LNA). Present address: UCO/Lick Observatory, University of California, Santa Cruz, 95064