The primordial non-Gaussianity of local type (f_NL) in the WMAP 5-year
  data: the length distribution of CMB skeleton
  We present skeleton studies of non-Gaussianity in the CMB temperature
anisotropy observed in the WMAP5 data. The local skeleton is traced on the 2D
sphere by cubic spline interpolation which leads to more accurate estimation of
the intersection positions between the skeleton and the secondary pixels than
conventional linear interpolation. We demonstrate that the skeleton-based
estimator of non-Gaussianity of the local type (f_NL) - the departure of the
length distribution from the corresponding Gaussian expectation - yields an
unbiased and sufficiently converged f_NL-likelihood.
  We analyse the skeleton statistics in the WMAP5 combined V- and W-band data
outside the Galactic base-mask determined from the KQ75 sky-coverage. The
results are consistent with Gaussian simulations of the the best-fitting
cosmological model, but deviate from the previous results determined using the
WMAP1 data. We show that it is unlikely that the improved skeleton tracing
method, the omission of Q-band data, the modification of the
foreground-template fitting method or the absence of 6 extended regions in the
new mask contribute to such a deviation. However, the application of the Kp0
base-mask in data processing does improve the consistency with the WMAP1
results.
  The f_NL-likelihoods of the data are estimated at 9 different smoothing
levels. It is unexpected that the best-fit values show positive correlation
with the smoothing scales. Further investigation argues against a point-source
or goodness-of-fit explanation but finds that about 30% of either Gaussian or
f_NL samples having better goodness-of-fit than the WMAP5 show a similar
correlation. We present the estimate f_NL=47.3+/-34.9 (1sigma error) determined
from the first four smoothing angles and f_NL=76.8+/-43.1 for the combination
of all nine. The former result may be overestimated at the 0.21sigma-level
because of point sources.
