A High-Resolution Multiband Survey of Westerlund 2 With the Hubble Space Telescope. III. The present-day stellar mass function

Abstract

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.

21 pages, 9 figures, 5 tables, accepted to AJ

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