# Testing the recovery of intrinsic galaxy sizes and masses of z ∼ 2 massive galaxies using cosmological simulations

Price, Sedona H; Kriek, Mariska; Feldmann, Robert; Quataert, Eliot; Hopkins, Philip F; Faucher-Giguère, Claude-André; Kereš, Dušan; Barro, Guillermo (2017). Testing the recovery of intrinsic galaxy sizes and masses of z ∼ 2 massive galaxies using cosmological simulations. The Astrophysical Journal. Letters, 844(1):6.

## Abstract

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 ${M}_{* }\sim {10}^{10}\mbox{--}{10}^{11.5}\,{M}_{\odot }$ at z ~ 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 $\sim 0.06\,\mathrm{dex}$ and ${\rm{a}}\sim 0.15\,\mathrm{dex}$ scatter. Recovered half-light radii agree well with intrinsic half-mass radii when averaged over all viewing angles, with a systematic offset of $\sim 0.1\,\mathrm{dex}$ (with the half-light radii being larger) and a scatter of $\sim 0.2\,\mathrm{dex}$. When using color gradients to account for mass-to-light variations, recovered half-mass radii also exceed the intrinsic half-mass radii by $\sim 0.1\,\mathrm{dex}$. However, if not properly accounted for, aperture effects can bias size estimates by $\sim 0.1\,\mathrm{dex}$. No differences are found between the mass and size offsets for star-forming and quiescent galaxies. Variations in viewing angle are responsible for ~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 ~25%. Moreover, orientation-driven scatter causes the number density of very massive galaxies to be overestimated by $\sim 0.5\,\mathrm{dex}$ at ${M}_{* }\sim {10}^{11.5}\,{M}_{\odot }$.

## Abstract

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 ${M}_{* }\sim {10}^{10}\mbox{--}{10}^{11.5}\,{M}_{\odot }$ at z ~ 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 $\sim 0.06\,\mathrm{dex}$ and ${\rm{a}}\sim 0.15\,\mathrm{dex}$ scatter. Recovered half-light radii agree well with intrinsic half-mass radii when averaged over all viewing angles, with a systematic offset of $\sim 0.1\,\mathrm{dex}$ (with the half-light radii being larger) and a scatter of $\sim 0.2\,\mathrm{dex}$. When using color gradients to account for mass-to-light variations, recovered half-mass radii also exceed the intrinsic half-mass radii by $\sim 0.1\,\mathrm{dex}$. However, if not properly accounted for, aperture effects can bias size estimates by $\sim 0.1\,\mathrm{dex}$. No differences are found between the mass and size offsets for star-forming and quiescent galaxies. Variations in viewing angle are responsible for ~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 ~25%. Moreover, orientation-driven scatter causes the number density of very massive galaxies to be overestimated by $\sim 0.5\,\mathrm{dex}$ at ${M}_{* }\sim {10}^{11.5}\,{M}_{\odot }$.