Young massive star clusters in M51

Abstract

A search for young massive star clusters (YMCs) in the nearby face-on spiral galaxy M51 (NGC 5194) has been carried out using UBV CCD images from the prime focus camera on the Lick 3-m Shane telescope. The YMC population is found to be quite rich with a specific U-band luminosity T_L(U)~1.4, consistent with the high current star formation rate (SFR) of this galaxy. The brightest clusters have T_L(U) = 100[(L_Clusters(U))/((L_Galaxy(U))], M_V~-12.5, far brighter than any young clusters currently known in the Milky Way and even surpassing the luminosity of the R136 cluster in the 30 Dor complex in the Large Magellanic Cloud. A few of the YMCs are examined on archive images from the Wide Field Planetary Camera (WF/PC2) on board the Hubble Space Telescope (HST), confirming their cluster nature and providing estimates of their effective radii of 2-3pc. The number of YMCs in M51 is compatible with extrapolation of a power-law luminosity function with exponent ~-2 from a Milky Way-like population of open clusters. Both the SFR and T_L(U) of M51 are similar to those of other cluster-rich spiral galaxies like NGC 1313 and M83.