ALMACAL XI: Over-densities as signposts to proto-clusters?

Authors:
  • RIR. J. Ivison
    R. J. Ivison
  • CPCeline Peroux
    Celine Peroux
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Abstract
Perhaps unsurprisingly, the most common approach to finding proto-clusters is to search for over-densities of galaxies. Upgrades to submillimetre interferometers and the advent of the James Webb Space Telescope mean that we will soon find more distant candidate proto-clusters in deep sky surveys without spectroscopic confirmation. In this letter, we report the serendipitous discovery of an extremely dense region behind the blazar J0217-0820 at z=0.6 in the ALMACAL sky survey. Its overdensity is eight times higher than the predicted by the blind sky surveys. Among the seven submillimetre-bright galaxies, three are conventional, single-dish submm galaxies with S 870{\mu}m > 3 mJy. The over-density is thus comparable to the densest known, confirmed proto-cluster cores. However, their spectral properties suggest a wide range of redshifts. We investigate the likelihood of line-of-sight projection effects using the light cones from cosmological simulations, and find that the deeper we search, the higher the chance that we will suffer from such projection effects. Meanwhile, this extreme overdensity is very likely produced by the projection effects of the large-scale structures. We must therefore question the fidelity of galaxy proto-cluster candidates selected via galaxy photometric over-densities, and the cosmic variance in deep submm surveys, where the negative K correction eases the detection of dusty galaxies along an extraordinarily long line of sight.