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

TL;DR

This paper develops an image-based method to compare simulated and real galaxy structures by incorporating observational realism, applying bulge+disc decompositions, and analyzing biases in the Illustris simulation at z=0.

Contribution

It introduces a novel approach to incorporate observational effects into synthetic galaxy images and provides a publicly available catalog of morphological parameters.

Findings

Bulge+disc decompositions are robust to observational biases.

Approximately 30% of simulated galaxies suffer from internal segmentation issues.

Segmentation leads to underestimation of flux, size, and incorrect bulge-to-total ratios.

Abstract

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 catalogs. We apply our methodology to the dust-free synthetic image catalog 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 catalogs. 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\% 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 $\sim$ 2, and generally erroneous bulge-to-total fractions of (B/T)=0.