Mock galaxy redshift catalogues from simulations: implications for Pan-STARRS1

We describe a method for constructing mock galaxy catalogues which are well suited for use in conjunction with large photometric surveys. We use the semi-analytic galaxy formation model of Bower et al. implemented in the Millennium N-body simulation of the evolution of dark matter clustering in a Lambda cold dark matter cosmology. We apply our method to the specific case of the surveys soon to commence with PS1, the first of four telescopes planned for the Pan-STARRS. PS1 has five photometric bands, g, r, i, z and y and will carry out an all-sky '3 pi' survey and a medium deep survey (MDS) over 84 deg(2). We calculate the expected magnitude limits for extended sources in the two surveys. We find that, after 3 year, the 3 pi survey will have detected over 10(8) galaxies in all five bands, 10 million of which will lie at redshift z > 0.9, while the MDS will have detected over 10(7) galaxies with 0.5 million lying at z > 2. These numbers at least double if the detection in the shallowest band, y, is not required. We then evaluate the accuracy of photometric redshifts estimated using an off-the-shelf photo-z code. With the grizy bands alone, it is possible to achieve an accuracy in the 3 pi survey of Delta z/(1 + z) similar to 0.06 in the range 0.25 < z < 0.8, which could be reduced by about 15 per cent using near-infrared photometry from the Infrared Deep Sky Survey (UKIDDS) survey, but would increase by about 25 per cent for the deeper sample without the y-band photometry. For the MDS, an accuracy of Delta z/(1 + z) similar to 0.05 is achievable for 0.02 < z < 1.5 using grizy. A dramatic improvement in accuracy is possible by selecting only red galaxies. In this case, Delta z/(1 + z) similar to 0.02-0.04 is achievable for similar to 100 million galaxies at 0.4 < z < 1.1 in the 3p survey and for 30 million galaxies in the MDS at 0.4 < z < 2. We investigate the effect of using photometric redshifts in the estimate of the baryonic acoustic oscillation scale. We find that PS1 will achieve a similar accuracy in this estimate as a spectroscopic survey of 20 million galaxies.