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Looking at the SPT data, we see point sources. This is washed out in the lower-resolution space-based Plank data.

Most of these points are quasars and are in known catalogs. However, there were some bright, dusty sources found with SPT that were not in any catalogs.

Follow-up observations with ALMA show these to be distant galaxies lens by a foreground galaxy into ring shapes. Spectra of the galaxies gives you a redshift from the CO line.