The phrase "parent galaxies" refers to large galaxies that give birth to or produce smaller galaxies through a process called galaxy formation. Full definition
Because this scenario depends on the presence of nearby stars, we expect DCBHs to typically form in satellite galaxies that orbit around larger parent galaxies where Population III stars have already formed. (scientificamerican.com)
As the two parent galaxies smashed together, they gobbled up huge amounts of hydrogen, emptying that corner of the universe of the star - making gas. (keckobservatory.org)
Few galaxies have even more exotic nuclei, which are extremely compact and extremely bright, outshining their whole parent galaxy; these are called quasars (an acronym for QUAsi-StellAR objects). (maa.clell.de)