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Luminosity profiles and sizes of massive star clusters in NGC 7252

We present Hubble Space Telescope (HST) Wide-Field Camera 3 (WFC3) images of the merger remnant NGC 7252. In particular, we focus on the surface brightness profiles and effective radii, Reff, of 36 young massive clusters (YMCs) within the galaxy. All the clusters have masses exceeding 10^5 Msun and are, despite the 64 Mpc distance to the galaxy, (partly) resolved on the HST images. Effective radii can be measured down to ~2.5 pc, and the largest clusters have Reff approaching 20 pc. The median Reff of our sample clusters is ~6-7 pc, which is larger than typical radii of YMCs (~2.5 pc). This could be due to our sample selection (only selecting resolved sources) or to an intrinsic mass--radius relation within the cluster population. We find at least three clusters that have power-law profiles of the Elson, Fall, & Freeman (1987, "EFF") type extending out to >150 pc. Among them are the two most massive clusters, W3 and W30, which have profiles that extend to at least 500 and 250 pc, respectively. Despite their extended profiles, the effective radii of the three clusters are 17.2, 12.6 and 9.1 pc for W3, W26 and W30, respectively. We compare these extended profiles with those of YMCs in the LMC (R136 in 30 Dor), the Antennae galaxies (Knot S) and in the nearby spiral galaxy NGC 6946. Extended profiles seem to be a somewhat common feature, even though many nearby YMCs show distinct truncations. A continuous distribution between these two extremes, i.e. truncated or extremely extended, is the most likely interpretation. We suggest that the presence or absence of an extended envelope in {\em very young clusters} may be due to the gas distribution of the proto-cluster giant molecular cloud, in particular if the proto-cluster core becomes distinct from the surrounding gas before star formation begins.