Over-density of SMGs in fields containing z~0.3 galaxies: magnification bias and the implications for studies of galaxy evolution

TL;DR

This study finds a significant over-density of high-redshift SMGs around z~0.3 galaxies, mainly due to gravitational lensing, affecting interpretations of galaxy evolution and Herschel survey data.

Contribution

It demonstrates that lensing by galaxy halos causes over-density of SMGs and biases in Herschel flux measurements, impacting galaxy evolution studies.

Findings

SMG over-density is 4-7 times the background.

Lensing by halos of ~7x10^{13} M_sun explains the over-density.

SMGs cause significant contamination in Herschel fluxes.

Abstract

We report a remarkable over-density of high-redshift submillimetre galaxies (SMG), 4-7 times the background, around a statistically complete sample of twelve 250-micron selected galaxies at z=0.35, which were targeted by ALMA in a study of gas tracers. This over-density is consistent with the effect of lensing by the halos hosting the target z=0.35 galaxies. The angular cross-correlation in this sample is consistent with statistical measures of this effect made using larger sub-mm samples. The magnitude of the over-density as a function of radial separation is consistent with intermediate scale lensing by halos of order 7x 10^{13} M_o, which should host one or possibly two bright galaxies and several smaller satellites. This is supported by observational evidence of interaction with satellites in four out of the six fields with SMG, and membership of a spectroscopically defined group for a fifth. We also investigate the impact of these SMG on the reported Herschel fluxes of the z=0.35 galaxies, as they produce significant contamination in the 350 and 500-micron Herschel bands. The higher than random incidence of these boosting events implies a significantly larger bias in the sub-mm colours of Herschel sources associated with z<0.7 galaxies than has previously been assumed, with f_boost = 1.13, 1.26, 1.44 at 250, 350 and 500-microns. This could have implications for studies of spectral energy distributions, source counts and luminosity functions based on Herschel samples at z=0.2-0.7.