The Initial Mass Functions in the Super–Star Clusters NGC 1569A and NGC 1705‐1

Abstract

I use recent photometric and stellar velocity dispersion measurements of the super-star clusters (SSCs) NGC 1569A and NGC 1705-1 to determine their present-day luminosity/mass ratios (LV/M). I then use the inferred LV/M, together with population synthesis models of evolving star clusters, to constrain the initial mass functions (IMFs) in these objects. I find that (LV/M)? = 28.9 in 1569A and (LV/M)? = 126 in 1705-1. It follows that in 1569A the IMF is steep with ? ~ 2.5 for m-?dm IMFs that extend to 0.1 M?. This implies that most of the stellar mass in 1569A is contained in low-mass (<1 M?) stars; however, in 1705-1 the IMF is either flat, with ? 2, or it is truncated at a lower mass limit between 1 and 3 M?. I compare the inferred IMFs with the mass functions (MFs) of Galactic globular clusters. It appears that 1569A has a sufficient reservoir of low-mass stars for it plausibly to evolve into an object similar to Galactic globular clusters; however, the apparent deficiency of low-mass stars in 1705-1 may make it difficult for this SSC to become a globular cluster. If low-mass stars do dominate the cluster mass in 1705-1, the large LV/M in this SSC may be evidence that the most massive stars have formed close to the cluster cores.