How Robust Are the Size Measurements of High-redshift Compact Galaxies?
  Massive quiescent galaxies at $z \approx 2$ are apparently much more compact
than galaxies of comparable mass today. How robust are these size measurements?
We perform comprehensive simulations to determine possible biases and
uncertainties in fitting single-component light distributions to real galaxies.
In particular, we examine the robustness of the measurements of the luminosity,
size, and other structural parameters. We devise simulations with increasing
realism to systematically disentangle effects due to the technique
(specifically using GALFIT) and the intrinsic structures of the galaxies. By
accurately capturing the detailed substructures of nearby elliptical galaxies
and then rescaling their sizes and signal-to-noise to mimic galaxies at
different redshifts, we confirm that the massive quiescent galaxies at $z
\approx 2$ are significantly more compact intrinsically than their local
counterparts. Their observed compactness is not a result of missing faint outer
light due to systematic errors in modeling. In fact, we find that fitting
multi-component galaxies with a single S\'ersic profile, the procedure most
commonly adopted in the literature, biases the inferred sizes higher by up to
10% - 20%, which accentuates the amount of size evolution required. If the sky
estimation has been done robustly and the model for the point-spread function
is fairly accurate, GALFIT can retrieve the properties of single-component
galaxies over a wide range of signal-to-noise ratios without introducing any
systematic errors.
