  In recent years the number of known galaxy clusters beyond z > 0.2 has
increased drastically, with the release of multiple catalogs containing >30,000
optically-detected galaxy clusters over the range 0 < z < 0.6. Combining these
catalogs with the availability of optical spectroscopy of the brightest cluster
galaxy from the Sloan Digital Sky Survey allows for the evolution of optical
emission-line nebulae in cluster cores to be quantified. For the first time,
the continuous evolution of optical line emission in brightest cluster galaxies
over the range 0 < z < 0.6 is determined. A minimum in the fraction of BCGs
with optical line emission is found at z \sim 0.3, suggesting that complex,
filamentary emission in systems such as Perseus A are a recent phenomenon.
Evidence for an upturn in the number of strongly-emitting systems is reported
beyond z > 0.3, hinting at an earlier epoch of strong cooling. We compare the
evolution of emission line nebulae to the X-ray-derived cool core (CC) fraction
from the literature over the same redshift range and find overall agreement,
with the exception that an upturn in the strong CC fraction is not observed at
z > 0.3. The overall agreement between the evolution of cool cores and optical
line emission at low redshift suggests that emission-line surveys of galaxy
clusters may provide an efficient method of indirectly probing the evolution of
cool cores and, thus, provide insights into the balance of heating and cooling
processes at early cosmic times.
