Are ultra-diffuse galaxies Milky Way-sized?
  Now almost 70 years since its introduction, the effective or half-light
radius has become a very popular choice for characterising galaxy size.
However, the effective radius measures the concentration of light within
galaxies and thus does not capture the intuitive definition of size which is
related to the edge or boundary of objects. For this reason, we aim to
demonstrate the undesirable consequence of using the effective radius to draw
conclusions about the nature of faint 'ultra-diffuse galaxies' (UDGs) when
compared to dwarfs and Milky Way-like galaxies. Instead of the effective
radius, we use a measure of galaxy size based on the location of the gas
density threshold required for star formation. Compared to the effective
radius, this physically motivated definition places the sizes much closer to
the boundary of a galaxy. Therefore, considering the sizes and stellar mass
density profiles of UDGs and regular dwarfs, we find that the UDGs have sizes
that are within the size range of dwarfs. We also show that currently known
UDGs do not have sizes comparable to Milky Way-like objects. We find that, on
average, UDGs are ten times smaller in extension than Milky Way-like galaxies.
These results show that the use of size estimators sensitive to the
concentration of light can lead to misleading results.
