  The COSMOS field has been the subject of a wide range of observations, with a
number of studies focusing on reconstructing the 3D dark matter density field.
Typically, these studies have focused on one given method or tracer. In this
paper, we reconstruct the distribution of mass in the COSMOS field out to a
redshift z=1 by combining Hubble Space Telescope weak lensing measurements with
zCOSMOS spectroscopic measurements of galaxy clustering. The distribution of
galaxies traces the distribution of mass with high resolution (particularly in
redshift, which is not possible with lensing), and the lensing data empirically
calibrates the mass normalisation (bypassing the need for theoretical models).
Two steps are needed to convert a galaxy survey into a density field. The first
step is to create a smooth field from the galaxy positions, which is a point
field. We investigate four possible methods for this: (i) Gaussian smoothing,
(ii) convolution with truncated isothermal sphere, (iii) fifth nearest
neighbour smoothing and (iv) a muliti-scale entropy method. The second step is
to rescale this density field using a bias prescription. We calculate the
optimal bias scaling for each method by comparing predictions from the smoothed
density field with the measured weak lensing data, on a galaxy-by-galaxy basis.
In general, we find scale-independent bias for all the smoothing schemes, to a
precision of 10%. For the nearest neighbour smoothing case, we find the bias to
be 2.51\pm 0.25. We also find evidence for a strongly evolving bias, increasing
by a factor of ~3.5 between redshifts 0<z<0.8. We believe this strong evolution
can be explained by the fact that we use a flux limited sample to build the
density field.
