Galaxies in the Illustris simulation as seen by the Sloan Digital Sky Survey - I: Bulge+disc decompositions, methods, and biases.

We present an image-based method for comparing the structural properties of galaxies produced in hydrodynamical simulations to real galaxies in the Sloan Digital Sky Survey. The key feature of our work is the introduction of extensive observational realism, such as object crowding, noise and viewing angle, to the synthetic images of simulated galaxies, so that they can be fairly compared to real galaxy catalogues. We apply our methodology to the dust-free synthetic image catalogue of galaxies from the Illustris simulation at z = 0, which are then fit with bulge+disc models to obtain morphological parameters. In this first paper in a series, we detail our methods, quantify observational biases and present publicly available bulge+disc decomposition catalogues. We find that our bulge+disc decompositions are largely robust to the observational biases that affect decompositions of real galaxies. However, we identify a significant population of galaxies (roughly 30 per cent of the full sample) in Illustris that are prone to internal segmentation, leading to systematically reduced flux estimates by up to a factor of 6, smaller half-light radii by up to a factor of ∼2 and generally erroneous bulge-to-total fractions of (B/T) = 0.