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 R _ { eff } of 36 young massive clusters ( YMCs ) within the galaxy .
All the clusters have masses exceeding 10 ^ { 5 } M _ { \odot } and are , despite the 64 Mpc distance to the galaxy , ( partly ) resolved on the HST images .
Effective radii can be measured down to \sim 2.5 pc , and the largest clusters have R _ { eff } approaching 20 pc .
The median R _ { eff } of our sample clusters is \sim 6 - 7 pc , which is larger than typical radii of YMCs ( \sim 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 \ga 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 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 .