The K–selected Butcher-Oemler Effect

We investigate the Butcher-Oemler effect using samples of galaxies brighter than observed frame K * + 1.5 in 33 clusters at 0.1 z 0.9. We attempt to duplicate as closely as possible the methodology of Butcher & Oemler. Apart from selecting in the K-band, the most important difference is that we use a brightness limit fixed at 1.5 magnitudes below an observed frame K – 2 – the nominal limit of rest frame M(V) = −20 used by Butcher & Oemler. For an early type galaxy at z = 0.1 our sample cutoff is 0.2 magnitudes brighter than rest frame M(V) = −20, while at z = 0.9 our cutoff is 0.9 magnitudes brighter. If the blue galaxies tend to be faint, then the difference in magnitude limits should result in our measuring lower blue fractions. A more minor difference from the Butcher & Oemler methodology is that the area covered by our galaxy samples has a radius of 0.5 or 0.7 Mpc at all redshifts rather than R 30 , the radius containing 30% of the cluster population. In practice our field sizes are generally similar to those used by Butcher & Oemler. We find the fraction of blue galaxies in our K-selected samples to be lower on average than that derived from several optically selected samples, and that it shows little trend with redshift. However, at the redshifts z < 0.6 where our sample overlaps with that of Butcher & Oemler, the difference in f B as determined from our K-selected samples and those of Butcher & Oemler is much reduced. The large scatter in the measured f B , even in small redshift ranges, in our study indicates that determining the f B for a much larger sample of clusters from K-selected galaxy samples is important. As a test of our methods, our data allow us to construct optically-selected samples down to rest frame M(V) = −20, as used by Butcher & Oemler, for four clusters that are common between our sample and that of Butcher & Oemler. For these rest V selected samples, we find similar fractions of blue galaxies to Butcher & Oemler, while the K selected samples for the same 4 clusters yield blue fractions which are typically half as large. This comparison indicates that selecting in the K-band is the primary difference between our study and previous optically-based studies of the Butcher & …