GALAXIES WITH DENIS: Preliminary star/galaxy separation and first results

The numerous extragalactic and cosmological motivations of the DENIS and 2MASS near infrared surveys are outlined. The performance of the DENIS survey is estimated from 50deg of high galactic latitude data (20◦ < |b| < 60◦). Simple star/galaxy separation methods are presented and comparison with 300 visually classified objects as well as COSMOS and APM classifications. We find that the peak intensity over isophotal area is an excellent star/galaxy separation algorithm, fairly robust to variations of the PSF within the frames, achieving 98.5% completeness and 92.5% reliability for I < 16.5, in comparison with visual classification. A new estimate of the photometric accuracy for galaxies is presented. The limiting factors for homogeneous galaxy extraction at high galactic latitudes are completeness and photometric accuracy in K, photometric accuracy in J and star/galaxy separation in I (also used for classification in J and K). Galaxy counts are presented on 50 deg. The I counts are in excellent agreement with a Euclidean extrapolation of the published counts around I = 16−17 (more so than in all previous studies), and thus point to a high normalization at the bright end, in contrast with the counts published from the APM and COSMOS plate scans. The J-band differential galaxy counts follow the relation N(J) = 12± 1 dex(0.6 [J − 14]) degmag. Extrapolation of these high latitude counts suggest that DENIS will produce highly homogeneous catalogs of ≃ 6000 (K < 11), ≃ 700 000 (J < 14.8) and, ≃ 1 000 000 (I < 16.5) galaxies, respectively with photometric accuracy of 0.08 in I and 0.20 in J and K. Larger highly homogeneous samples are expected with improvements to the camera and the algorithms. 2 GARY A. MAMON ET AL.