Size-Mass Relations for Simulated Low-Mass Galaxies: Mock Imaging versus Intrinsic Properties
arxiv(2024)
摘要
The observationally-inferred size versus stellar-mass relationship for
low-mass galaxies provides an important test for galaxy formation models.
However, the relationship relies on assumptions that relate observed luminosity
profiles to underlying stellar mass profiles. We use the Feedback in Realistic
Environments (FIRE-2) simulations of low-mass galaxies to explore how the
predicted size-mass relation (SMR) changes depending on whether one uses
star-particle counts directly or mock observations. We reproduce the SMR found
in the ELVES survey remarkably well only when we infer stellar masses and sizes
using mock surface brightness images and the same color-inferred mass-to-light
ratio (CMLR) used in deriving the observed relation. However, when we use star
particles to directly infer stellar masses and half-mass radii, we find that
our galaxies are too large and obey a SMR with too little scatter compared to
observations. The reason for this discrepancy between the "true" galaxy size
and mass and those derived in the mock observation approach is twofold. First,
our simulated galaxies have higher and more varied MLRs at a fixed color than
those commonly-adopted because their star-formation-histories are more
temporally extended and not well represented by exponential star formation
models. Using a standard CMLR therefore tends to underestimate their stellar
masses compared to their true, simulated values. Second, our galaxies have
radially increasing MLR gradients. Using a single MLR tends to under-predict
the mass in the outer regions. Similarly, the true half-mass radius is larger
than the half-light radius because the light is more concentrated than the
mass. If our simulations are accurate representations of the real universe,
then the relationship between galaxy size and stellar mass is even tighter for
low-mass galaxies than is commonly inferred from observed relations.
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