The Shape of FIREbox Galaxies and a Potential Tension with Low-mass Disks

Understand

We study the intrinsic and observable shapes of approximately 700 star-forming galaxies with stellar masses of $10^8 - 10^{11}$ M$_\odot$ from the FIREbox simulation at $z=0$.

• We calculate intrinsic axis ratios using inertia tensors weighted by three morphology types: "All Stars," "Young Stars," and "Luminosity-weighted Stars." Young Stars shows mass-dependent 3D configurations, with spheroidal, elongated, and disky shapes dominant at stellar masses of $10^{8.5}$ M$_\odot$, $10^{9.5}$ M$_\odot$, and $10^{10.5}$ M$_\odot$, respectively.
• Using the radiative transfer code SKIRT, we construct mock images for each galaxy and show that projected short-to-long axis ratios, $q$, inferred from 2D S\'ersic fits are most closely related to Luminosity-weighted Stars tensor shapes and least resemble the All Stars' shapes.
• This suggests observed 2D shape distributions should not be compared to predictions based on 3D stellar mass shapes.