A High-Resolution Multiband Survey of Westerlund 2 With the Hubble Space
  Telescope. III. The present-day stellar mass function
  We present a detailed analysis of the spatial distribution of the stellar
population and the present-day mass function (PDMF) of the Westerlund 2 (Wd2)
region using the data from our high resolution multi-band survey with the
Hubble Space Telescope. We used state-of-the-art artificial star tests to
determine spatially resolved completeness maps for each of the broad-band
filters. We reach a level of completeness of 50 % down to F555W=24.8 mag (0.7
$M_\odot$) and F814W=23.3 mag (0.2 $M_\odot$) in the optical and F125W=20.2 mag
and F160W=19.4 mag (both 0.12 $M_\odot$) in the infrared throughout the field
of view. We had previously reported that the core of Wd2 consists of two
clumps: namely the main cluster (MC) and the northern clump (NC). From the
spatial distribution of the completeness corrected population, we find that
their stellar surface densities are 1114 stars pc$^{-2}$ and 555 stars
pc$^{-2}$, respectively, down to F814W=21.8 mag. We find that the present-day
mass function (PDMF) of Wd2 has a slope of $\Gamma=-1.46 \pm 0.06$, which
translates to a total stellar cluster mass of $(3.6 \pm 0.3) \cdot 10^4
M_\odot$. The spatial analysis of the PDMF reveals that the cluster population
is mass-segregated, most likely primordial. In addition, we report the
detection of a stellar population of spatially uniformly distributed low-mass
(<0.15 $M_\odot$) stars, extending into the gas ridges of the surrounding gas
and dust cloud, as well as a confined region of reddened stars, likely caused
by a foreground CO cloud. We find hints that a cloud-cloud collision might be
the origin of the formation of Wd2.
