Testing the Recovery of Intrinsic Galaxy Sizes and Masses of z ∼ 2 Massive Galaxies Using Cosmological Simulations

Accurate measurements of galaxy masses and sizes are key to tracing galaxy evolution over time. Cosmological zoom-in simulations provide an ideal test bed for assessing the recovery of galaxy properties from observations. Here, we utilize galaxies with M-* similar to 10(10)-10(11.5) M-circle dot at z similar to 1.7-2 from the MassiveFIRE cosmological simulation suite, part of the Feedback in Realistic Environments (FIRE) project. Using mock multi-band images, we compare intrinsic galaxy masses and sizes to observational estimates. We find that observations accurately recover stellar masses, with a slight average underestimate of similar to 0.06 dex and a similar to 0.15 dex scatter. Recovered half-light radii agree well with intrinsic half-mass radii when averaged over all viewing angles, with a systematic offset of similar to 0.1 dex (with the half-light radii being larger) and a scatter of similar to 0.2 dex. When using color gradients to account for mass-to-light variations, recovered half-mass radii also exceed the intrinsic half-mass radii by similar to 0.1 dex. However, if not properly accounted for, aperture effects can bias size estimates by similar to 0.1 dex. No differences are found between the mass and size offsets for star-forming and quiescent galaxies. Variations in viewing angle are responsible for similar to 25% of the scatter in the recovered masses and sizes. Our results thus suggest that the intrinsic scatter in the mass-size relation may have previously been overestimated by similar to 25%. Moreover, orientation-driven scatter causes the number density of very massive galaxies to be overestimated by similar to 0.5 dex at M-* similar to 10(11.5) M-circle dot.