Massive Young Clusters in the Disk of M31

We have studied the properties of a sample of 67 very blue and likely young massive clusters in M31 extracted from the Bologna Revised Catalog of globular clusters, selected according to their color [(B - V)0 ≤ 0.45] and/or the strength of their Hβ spectral index (Hβ ≥ 3.5 A). Their existence in M31 has been noted by several authors in the past; we show here that these blue luminous compact clusters (BLCCs) are a significant fraction (15%) of the whole globular cluster system of M31. Compared to the global properties of the M31 globular cluster system, they appear to be intrinsically fainter and morphologically less concentrated, with a shallower Balmer jump and enhanced Hβ absorption in their spectra. Empirical comparison with integrated properties of clusters with known ages, as well as with theoretical simple stellar population models, consistently indicates that their typical age is less than ~2 Gyr, so they are probably not as metal-poor as would be deduced if they were older. When selecting BLCCs by either their (B - V)0 colors or the strength of their Hβ index, the cluster sample turns out to be distributed on the outskirts of the M31 disk, sharing the kinematic properties of the thin, rapidly rotating disk component. If confirmed to be young and not metal-poor, these clusters indicate the occurrence of significant recent star formation in the thin disk of M31, although they do not set constraints on the epoch of its early formation.