An assessment of the evidence from ATLAS3D for a variable initial mass
  function
  The ATLAS3D Survey has reported evidence for a non-universal stellar initial
mass function (IMF) for early type galaxies (ETGs) (Cappellari et al. 2012,
2013b,a). The IMF was constrained by comparing stellar mass measurements from
kinematic data with those from spectral energy distribution (SED) fitting. Here
we investigate possible effects of scatter in the reported stellar mass
measurements and their potential impact on the IMF determination. We find that
a trend of the IMF mismatch parameter with the kinematic mass to light ratio,
comparable to the trend observed by Cappellari et al. (2012), could arise if
the Gaussian errors of the kinematic mass determination are typically 30%.
Without additional data, it is hard to separate between the option that the IMF
has a true large intrinsic variation or the option that the errors in the
determination are larger than anticipated. A correlation of the IMF with other
properties would help to make this distinction, but no strong correlation has
been found yet. The strongest correlation is with velocity dispersion. However,
it has a large scatter and the correlation depends on sample selection and
distance measurements. The correlation with velocity dispersion could be partly
caused by the colour-dependent calibration of the surface brightness
fluctuation distances of Tonry et al. (2001). We find that the K-band
luminosity limited ATLAS3D Survey is incomplete for the highest M/L galaxies
below 10^10.3 M_sun. There is a significant IMF - velocity dispersion trend for
galaxies with SED masses above this limit, but no trend for galaxies with
kinematic masses above this limit. We also find an IMF trend with distance, but
no correlation between nearest neighbour ETGs, which excludes a large
environmental dependence. Our findings do not rule out the reported IMF
variations, but they suggest that further study is needed.
