Abstract
It is often claimed that overdensities of (or even individual bright)
submillimetre-selected galaxies (SMGs) trace the assembly of the most-massive
dark matter structures in the Universe. We test this claim by performing a
counts-in-cells analysis of mock SMG catalogues derived from the Bolshoi
cosmological simulation to investigate how well SMG associations trace the
underlying dark matter structure. We find that SMGs exhibit a relatively
complex bias: some regions of high SMG overdensity are underdense in terms of
dark matter mass, and some regions of high dark matter overdensity contain no
SMGs. Because of their rarity, Poisson noise causes scatter in the SMG
overdensity at fixed dark matter overdensity. Consequently, rich associations
of less-luminous, more-abundant galaxies (i.e. Lyman-break galaxy analogues)
trace the highest dark matter overdensities much better than SMGs. Even on
average, SMG associations are relatively poor tracers of the most significant
dark matter overdensities because of `downsizing': at z < ~2.5, the
most-massive galaxies that reside in the highest dark matter overdensities have
already had their star formation quenched and are thus no longer SMGs.
Furthermore, because of Poisson noise and downsizing, some of the highest
overdensities are not associated with any SMGs. Conversely, some bright SMGs
are in underdense regions.
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