The K-band luminosity functions of super star clusters in luminous infrared galaxies, their slopes and the effects of blending

Abstract

Super star clusters (SSCs) are typically found in interacting galaxies and trace an extreme form of star formation. We present a K-band study of SSC candidates in a sample of local luminous infrared galaxies (LIRGs) using two adaptive optics instruments: the Very Large Telescope's Nasmyth Adaptive Optics System-COude Near Infrared CAmera (NACO) and Gemini's ALTtitude conjugate Adaptive optics for the InfraRed (ALTAIR)/Near InfraRed Imager and Spectrometer (NIRI). In addition to facilitating SSC detections in obscured environments, this work introduces SSC studies in hosts with higher star-formation rates (SFRs) than most previous studies. We find that the luminosity functions (LFs) of the clusters are reasonably well-fitted by a single power law with values of the index α ranging between 1.5 and 2.4 with an average value of α ≈ 1.9. This value appears to be less steep than the average α ≈ 2.2 in normal spiral galaxies. Due to the host galaxy distances involved (median D_L ∼ 70 Mpc), blending effects have to be taken into account and are investigated using Monte Carlo simulations of blending effects for LFs and a photometric SSC analysis of the well-studied Antenna system, which is artificially redshifted to the distance of our sample. While blending tends to flatten LFs, our analyses show that Δα is less than ∼0.1 in our sample. The simulations also show that in the luminosity range M_K < -13 considered in this work, the extracted SSC luminosities are generally dominated by a single dominant star cluster rather than several knots of star formation. We present resolution- and distance-dependent SSC surface-density confusion limits and show how blending rates and aperture sizes affect the LFs. The smallest possible apertures should be used in crowded regions.