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arXiv · 2106.11588

An Orientation Bias in Observations of Submillimetre Galaxies

C. C. Lovell·J. E. Geach·R. Davé·D. Narayanan·K. E. K. Coppin·Q. Li·M. Franco·G. C. Privon
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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 investigated these questions using the \textsc{Simba} cosmological simulation paired with the dust radiative transfer code \textsc{Powderday}. We selected eight simulated SMGs ($S_{850}\gtrsim2$ mJy) at $z = 2$, and measured 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 a factor of 2.7. 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 assessed 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.

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C. C. Lovell, J. E. Geach, R. Davé, D. Narayanan, K. E. K. Coppin, Q. Li, M. Franco, G. C. Privon. 2022-08-30. An Orientation Bias in Observations of Submillimetre Galaxies. https://doi.org/10.1093/mnras%2Fstac2008

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