Artist’s impression of the protocluster observed by ALMA.

Artist’s impression of the protocluster observed by ALMA. It shows the central starburst galaxy AzTEC-3 along with its labeled cohorts of smaller, less active galaxies. New ALMA observations suggest that AzTEC-3 recently merged with another young galaxy and that the whole system represents the first steps toward forming a galaxy cluster. | Credit: B. Saxton (NRAO/AUI/NSF)