Infrared constraints on the dark mass concentration observed in the cluster Abell 1942

We present a deep H-band image of the region in the vicinity of the cluster Abell 1942 containing the puzzling dark matter concentration detected in a n optical weak lensing study by Erben et al. We demonstrate that our limiti ng magnitude, H = 22, would be sufficient to detect clusters of appropriate mass out to redshifts comparable with the mean redshift of the background sources. Despite this, our infrared image reveals no obvious overdensity of sources at the location of the lensing mass peak, nor an excess of sources in the I - H versus H colour-magnitude diagram. We use this to constrain f urther the luminosity and mass-to-light ratio of the putative dark clump as a function of its redshift. We find that for spatially flat cosmologies, b ackground lensing clusters with reasonable mass-to-light ratios lying in th e redshift range 0 < z < 1 are strongly excluded, leaving open the possibil ity that the mass concentration is a new type of truly dark object.