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 multiband 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 broadband filters. We reach a level of completeness of 50% down to $F555W=24.8$ mag ($0.7M_{\odot }$) and $F814W=23.3$ mag ($0.2M_{\odot }$) in the optical and $F125W=20.2$ mag and $F160W=19.4$ mag (both $0.12M_{\odot }$) 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 and the northern clump. From the spatial distribution of the completeness-corrected population, we find that their stellar surface densities are $1114stars{pc}^{-2}$ and $555stars{pc}^{-2}$, respectively, down to $F814W=21.8$ mag. We find that the 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 and most likely primordial. In addition, we report the detection of a stellar population of spatially uniformly distributed low-mass ($<0.15M_{\odot }$) 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.