Systematics of the 4000 Angstrom Break in the Spectra of Galaxies

The authors have measured the strength of the 4000 Å break from moderate-resolution spectra of 950 galaxies in the fields of 12 rich clusters. They find only a weak dependence of the 4000 Å break amplitude on either the total or bulge luminosity of the galaxy. Since the U-V colors of spheroidal systems appear to correlate with absolute magnitude, a correlation attributed to line blanketing, the authors' result indicates that the 4000 Å break amplitude is insensitive to changes in metal abundance, at least over the range characteristic of galactic bulges. On the other hand, the authors show that the break is a sensitive probe of star formation, by correlating the break amplitude with emission and absorption features indicative of recent star formation. This sensitivity to star formation but insensitivity to metal abundance makes the 4000 Å break a powerful diagnostic for studying evolution with observations of galaxies at large lookback times.