An orientation bias in observations of submillimetre galaxies

Abstract

Recent high-resolution interferometric images of submillimetre galaxies (SMGs) reveal fascinatingly complex morphologies. This raises a number of questions: how does the relative orientation of a galaxy affect its observed submillimetre emission, and does this result in an `orientation bias' in the selection and analysis of such galaxies in flux-limited cosmological surveys? We investigate these questions using the Simba cosmological simulation paired with the dust radiative transfer code Powderday. We select eight simulated SMGs ($S_{850}\gtrsim2$ mJy) at $z = 2$, and measure the variance of their `observed' emission over 50 random orientations. Each galaxy exhibits significant scatter in its emission close to the peak of the thermal dust emission, with variation in flux density of up to $\sim$50 mJy at the peak. This results in an appreciable dispersion in the inferred dust temperatures and infrared luminosities ($16^{\mathrm{th}}-84^{\mathrm{th}}$ percentile ranges of 5 K and 0.1 dex, respectively) and therefore a fundamental uncertainty in derived parameters such as dust mass and star formation rate ($\sim$30% for the latter using simple calibrations). Using a Monte Carlo simulation we also assess the impact of orientation on flux-limited surveys, finding a bias in the selection of SMGs towards those with face-on orientations, as well as those at lower redshifts. We predict that the orientation bias will affect flux-limited single-dish surveys, most significantly at THz frequencies, and this bias should be taken into account when placing the results of targeted follow-up studies in a statistical context.