Young Stars at the Edge: Stellar Clustering in the Outer Regions of the M33 Disk

Abstract: We investigate the distribution of bright main sequence stars near the northern edge of the M33 disk. Clustering on sub-kpc scales is seen among stars with ages near 10 Myr, and two large star-forming complexes are identified. Similar large-scale grouping is not evident among stars with ages 100 Myr. These stars are also distributed over a much larger area than those with younger ages, and it is argued that random stellar motions alone, as opposed to orderly motions of the type spurred by large scale secular effects, can re-distribute stars out to distances of at least 2 kpc (i.e. one disk scale length) from their birth places on 100 Myr timescales. Such random motions may thus play a significant role in populating the outer regions of the M33 disk. Finally, it is suggested that - to the extent that the ambient properties of the outer disk mirror those in the main body of the disk - stars in this part of M33 may have formed in star clusters with masses 50 - 250 solar, which is substantially lower than the peak of the solar neighborhood initial cluster mass function.