The Millennium Run Observatory: first light

Simulations of galaxy evolution aim to capture our current understanding as well as to make predictions for testing by future experiments. Simulation s and observations are often compared in an indirect fashion: physical quantities are estim ated from the observational data and compared to models. However, many applications can benefit f rom a more direct approach, where the observing process is also simulated, so that the models are seen fully from the observer’s perspective. To facilitate this, we have develo ped the Millennium Run Observatory (MRObs), a theoretical virtual observatory which uses virtual telescopes to ‘observe’ semi-analytic galaxy formation simulations based on the suite of Millennium Run (MR) dark matter simulations. The MRObs produces data that can be processed and analyzed using the standard observational software packages developed for re al observations. At present, we produce images in forty filters covering the rest-frame UV to inf rared for two stellar population synthesis models, for three different models of absorption by the intergalactic medium, and in two cosmologies (WMAP1 and 7). Galaxy distributions for a large number of mock lightcones can be ‘observed’ using models of major ground- and spa ce-based telescopes. The data include lightcone catalogues linked to structural pro perties of galaxies, pre-observation model images, mock telescope images, and Source Extractor products that can all be traced back to the higher-level dark matter, semi-analytic galaxy, and lightcone catalogues available in the MR database. Here, we describe our methods and announce a first public release of simulated observations that emulate a large number of extra-galactic surveys (e.g. SDSS, CFHT-LS, GOODS, GOODS/ERS, CANDELS, and HUDF). The MRObs br owser, an online tool, further facilitates exploration of the simulated dat a. We demonstrate the benefits of a direct approach through a number of example applications: (1) deep galaxy number counts in the CANDELS survey; (2) observed properties of galaxy clusters; (3) structural parameters of galaxies; and (4) identification of drop-out galaxies. Th e MRObs enhances the range of questions that can be asked of semi-analytic models, allowing observers and theorists to work toward each other with virtually complete freedom of where to meet.