Young Massive Star Clusters in Spiral Galaxies. III. Correlations   between cluster populations and host galaxy properties
  We present an analysis of correlations between integrated properties of galaxies and their populations of young massive star clusters. Data for 21 nearby galaxies presented by Larsen & Richtler (1999) are used together with literature data for 10 additional galaxies, spanning a range in specific U-band luminosity T_L(U) from 0 to 15. We find that T_L(U) correlates with several observable host galaxy parameters, in particular the ratio of Far-Infrared (FIR) to B-band flux and the optical surface brightness. Taking the FIR luminosity as an indicator of the star formation rate (SFR), it is found that T_L(U) correlates very well with the SFR per unit area. A similar correlation is seen between T_L(U) and the atomic hydrogen surface density. The cluster formation efficiency seems to depend on the SFR in a continuous way, rather than being related to any particularly violent mode of star formation. We discuss fundamental features of possible scenarios for cluster formation. One possibility is that the correlation between T_L(U) and SFR is due to a common controlling parameter, most probably the density of the ISM. Another scenario conceives a high T_L(U) as resulting from the energy input from many massive stars in case of a high SFR. 
S. S. Larsen, T. Richtler