Simulated observations of sub-millimetre galaxies: the impact of single-dish resolution and field variance

Abstract

Recent observational evidence suggests that the coarse angular resolution ($\sim 20"$ FWHM) of single-dish telescopes at sub-mm wavelengths has biased the observed galaxy number counts by blending together the sub-mm emission from multiple sub-mm galaxies (SMGs). We use lightcones computed from an updated implementation of the \galform semi-analytic model to generate $50$ mock sub-mm surveys of $0.5$ deg${}^2$ at $850$ $\hat{I}¼$m, taking into account the effects of the finite single-dish beam in a more accurate way than has been done previously. We find that blending of SMGs does lead to an enhancement of source extracted number counts at bright fluxes ($S_{850\hat{I}¼\mathrm{m}}\gtrsim 1$ mJy). Typically, $\sim 3-6$ galaxies contribute $90\%$ of the flux of an $S_{850\hat{I}¼\mathrm{m}}=5$ mJy source and these blended galaxies are physically unassociated. We find that field-to-field variations are comparable to Poisson fluctuations for our $S_{850\hat{I}¼\mathrm{m}}>5$ mJy SMG population, which has a median redshift $z_{50}=2.0$, but are greater than Poisson for the $S_{850\hat{I}¼\mathrm{m}}>1$ mJy population ($z_{50}=2.8$). In a detailed comparison to a recent interferometric survey targeted at single-dish detected sources, we reproduce the difference between single-dish and interferometer number counts and find a median redshift ($z_{50}=2.5$) in excellent agreement with the observed value ($z_{50}=2.5\pm 0.2$). We also present predictions for single-dish survey number counts at $450$ and $1100$ $\hat{I}¼$m, which show good agreement with observational data.

15 pages, 18 figures, Accepted to MNRAS