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UBC Theses and Dissertations

The most luminous galaxies in the Universe Hill, Ryley


Submillimetre galaxies have become essential tools in studying the high redshift Universe. Reaching luminosities well over 10¹³ L⊙, they constitute the vast majority of star formation during this early epoch. Their combined infrared and submillimetre emission output is comparable in energy density to all of the optical and ultraviolet light emitted by all of the galaxies in the observable Universe. We have used the Submillimeter Array at 860 μm to observe the brightest submillimetre sources in 4 deg² of the Cosmology Legacy Survey. Previous interferometric studies have found a significant amount of multiplicity at the bright end of the single-dish number counts, suggesting a steepening in the drop-off brighter than 10 mJy, but these studies suffered from small-number statistics. We have targeted 75 of the brightest flux density-ordered single-dish SCUBA-2 sources down to approximately 10 mJy, achieving an average synthesized beam size of 2.4 arcsec and an average depth of 1.5 mJy in our primary beam-corrected maps, corresponding to 4σ detections of about 6 mJy. Our data is sufficient to distinguish between intrinsically bright galaxies and systems that break up into two ≳6 mJy galaxies with flux ratios less than 2 and separated by about 2 arcsec or more, corresponding to a physical distance of around 20 kpc at z=2. We include in our study 28 archival observations of similar nature, bringing our sample size to 103. We statistically deboost our flux density measurements and use these to compute the cumulative and differential number counts of our sample, finding them to be consistent with previous single-dish survey number counts within the uncertainties but with a systematic offset between 2 and 20 per cent. We compute the probability that a >10 mJy single-dish submm source resolves into two or more galaxies with a brightest to second-brightest flux density ratio less than 2 to be about 15 per cent. Assuming the remaining 85 per cent of the targets are ultra-luminous galaxies between redshifts 2 and 3, we find the surface density of >500 M⊙ yr-¹ sources to be 8^{+2}_{-1} deg-² and a likely volume density of 660^{+140}_{-120} Gpc-³.

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