Where do galaxies end? Comparing measurement techniques of
  hydrodynamic-simulation galaxies' integrated properties
  Using the suite of high-resolution zoom re-simulations of individual haloes
by Martig et al., and the large-scale simulation \emph{MassiveBlack-II}, we
examine the differences in measured galaxy properties from techniques with
various aperture definitions of where galaxies end. We perform techniques
popular in the literature and present a new technique of our own, where the
aperture radius is based on the baryonic mass profiles of simulated
(sub)haloes. For the average Milky-Way-mass system, we find the two most
popular techniques in the literature return differences of order 30 per cent
for stellar mass, a factor of 3 for gas mass, 40 per cent for star formation
rate, and factors of several for gas accretion and ejection rates. Individual
cases can show variations greater than this, with the severity dependent on the
concentration of a given system. The average difference in integrated
properties for a more general galaxy population are not as striking, but are
still significant for stellar and gas mass, especially for optical-limit
apertures. The large differences that can occur are problematic for comparing
results from various publications. We stress the importance of both defining
and justifying a technique choice and discourage using popular apertures that
use an exact fraction of the virial radius, due to the unignorable variation in
galaxy-to-(sub)halo size. Finally, we note that technique choice does not
greatly affect simulated galaxies from lying within the scatter of observed
scaling relations, but it can alter the derived best-fit slope for the
Kennicutt-Schmidt relation.
