The SAMI Galaxy Survey: Revising the Fraction of Slow Rotators in IFS
  Galaxy Surveys
  The fraction of galaxies supported by internal rotation compared to galaxies
stabilized by internal pressure provides a strong constraint on galaxy
formation models. In integral field spectroscopy surveys, this fraction is
biased because survey instruments typically only trace the inner parts of the
most massive galaxies. We present aperture corrections for the two most widely
used stellar kinematic quantities $V/\sigma$ and $\lambda_{R}$. Our
demonstration involves integral field data from the SAMI Galaxy Survey and the
ATLAS$^{\rm{3D}}$ Survey. We find a tight relation for both $V/\sigma$ and
$\lambda_{R}$ when measured in different apertures that can be used as a linear
transformation as a function of radius, i.e., a first-order aperture
correction. We find that $V/\sigma$ and $\lambda_{R}$ radial growth curves are
well approximated by second order polynomials. By only fitting the inner
profile (0.5$R_{\rm{e}}$), we successfully recover the profile out to one
$R_{\rm{e}}$ if a constraint between the linear and quadratic parameter in the
fit is applied. However, the aperture corrections for $V/\sigma$ and
$\lambda_{R}$ derived by extrapolating the profiles perform as well as applying
a first-order correction. With our aperture-corrected $\lambda_{R}$
measurements, we find that the fraction of slow rotating galaxies increases
with stellar mass. For galaxies with $\log M_{*}/M_{\odot}>$ 11, the fraction
of slow rotators is $35.9\pm4.3$ percent, but is underestimated if galaxies
without coverage beyond one $R_{\rm{e}}$ are not included in the sample
($24.2\pm5.3$ percent). With measurements out to the largest aperture radius
the slow rotator fraction is similar as compared to using aperture corrected
values ($38.3\pm4.4$ percent). Thus, aperture effects can significantly bias
stellar kinematic IFS studies, but this bias can now be removed with the method
outlined here.
