The SUNBIRD survey: the K-band luminosity functions of young massive clusters in intensely star-forming galaxies

Strongly star-forming galaxies are prolific in producing the young and most massive star clusters still forming today. This work investigates the star cluster luminosity functions (CLFs, dN/dL ∝ L^-α) of 26 starburst and luminous infrared galaxies taken from the SUNBIRD survey. The targets were imaged using near-infrared K-band adaptive optics systems. Single power-law fits of the derived CLFs result in a slope α ranging between 1.53 and 2.41, with the median and average of 1.87 ± 0.23 and 1.93 ± 0.23, respectively. Possible biases such as blending effects and the choice of binning should only flatten the slope by no more than ~0.15, especially for cases where the luminosity distance of the host galaxy is below 100 Mpc. Results from this follow-up study strengthen the conclusion from our previous work: the CLF slopes are shallower for strongly star-forming galaxies in comparison to those with less intense star formation activity. There is also a (mild) correlation between α and both the host galaxy's star formation rate (SFR) and SFR density (Σ_SFR), i.e. the CLF flattens with an increasing SFR and Σ_SFR. Finally, we also find that CLFs on subgalactic scales associated with the nuclear regions of cluster-rich targets (N ≈ 300) have typically shallower slopes than the ones of the outer field by ~0.5. Our analyses suggest that the extreme environments of strongly star-forming galaxies are likely to influence the cluster formation mechanisms and ultimately their physical properties.