The projected 2-point correlation function of the galaxy density field Original Research Article

We present formulas relating the angular 2-point correlation function of the galaxy density field to the spatial one. Our solution was obtained for arbitrary angles. We argue that it is particularly well applicable for middle angular scales (up to 30 – 40°), where the small angle approximation may be not valid any more but the large angle result using series of spherical harmonics (quite good for numerical purposes) is too sophisticated. We check the applicability of our result for the power law power spectrum and for a realistic model, based on the APM galaxy survey. We discuss the accuracy of our results compared to the cosmic error and find that for the middle angular scales the corrections introduced by our solution are worth to take into account.