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Using the suite of high-resolution zoom re-simulations of individual haloesby Martig et al., and the large-scale simulation \emph{MassiveBlack-II}, weexamine the differences in measured galaxy properties from techniques withvarious aperture definitions of where galaxies end. We perform techniquespopular in the literature and present a new technique of our own, where theaperture radius is based on the baryonic mass profiles of simulated(sub)haloes. For the average Milky-Way-mass system, we find the two mostpopular techniques in the literature return differences of order 30 per centfor stellar mass, a factor of 3 for gas mass, 40 per cent for star formationrate, and factors of several for gas accretion and ejection rates. Individualcases can show variations greater than this, with the severity dependent on theconcentration of a given system. The average difference in integratedproperties for a more general galaxy population are not as striking, but arestill significant for stellar and gas mass, especially for optical-limitapertures. The large differences that can occur are problematic for comparingresults from various publications. We stress the importance of both definingand justifying a technique choice and discourage using popular apertures thatuse an exact fraction of the virial radius, due to the unignorable variation ingalaxy-to-(sub)halo size. Finally, we note that technique choice does notgreatly affect simulated galaxies from lying within the scatter of observedscaling relations, but it can alter the derived best-fit slope for theKennicutt-Schmidt relation.

Where do galaxies end? Comparing measurement techniques of hydrodynamic-simulation galaxies' integrated properties