  We present near-infrared K-band spectroscopy of 21 elliptical or cD Brightest
Cluster Galaxies (BCGs), for which we have measured the strength of the 2.293
micron CO stellar absorption feature. We find that the strength of this feature
is remarkably uniform among these galaxies, with a smaller scatter in
equivalent width than for the normal elliptical population in the field or
clusters. The scatter for BCGs is 0.156 nm, compared with 0.240 nm for Coma
cluster ellipticals, 0.337 nm for ellipticals from a variety of other clusters,
and 0.422 nm for field ellipticals. We interpret this homogeneity as being due
to a greater age, or more uniform history, of star formation in BCGs than in
other ellipticals; only a small fraction of the scatter can be due to
metallicity variations, even in the BCGs. Notwithstanding the small scatter,
correlations are found between CO strength and various galaxy properties,
including R-band absolute magnitude, which could improve the precision of these
galaxies as distance indicators in measurements of cosmological parameters and
velocity flows.
