These are cluster members showing similarly blue colors as starburst galaxies only with moderate to strong H-delta absorption.
Third are cluster members showing broad and/or high excitation line spectra, often found in active galactic nuclei.
This study bolsters the conclusion put forward in their original 1978 paper, that there has been "strong, recent evolution of galaxies in clusters".
The Butcher–Oemler conclusion generated numerous investigations of the cores of rich clusters at intermediate redshift (0.3 ≤ z ≤ 0.9): Couch and Newell (1984),[4] like Butcher and Oemler, acquired broadband photometry of such environments.
These authors independently arrived at the conclusion that every observation of a merger between cluster members, both of which showed blue colors, had the spectroscopic signature of a starburst or post-starburst object.