Astronomy and Astrophysics – Astronomy
Scientific paper
Nov 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003apj...598...20d&link_type=abstract
The Astrophysical Journal, Volume 598, Issue 1, pp. 20-35.
Astronomy and Astrophysics
Astronomy
29
Galaxies: Clusters: General, Galaxies: Evolution, Galaxies: Formation, Galaxies: High-Redshift
Scientific paper
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 mag below an observed-frame K* rather than the nominal limit of rest-frame MV=-20 used by Butcher & Oemler. For an early-type galaxy at z=0.1, our sample cutoff is 0.2 mag brighter than rest-frame MV=-20, while at z=0.9, our cutoff is 0.9 mag 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 R30, the radius containing 30% of the cluster population. In practice our field sizes are generally similar to those used by Butcher & Oemler. We find that the fraction of blue galaxies in our K-selected samples is 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, at which our sample overlaps with that of Butcher & Oemler, the difference in fB as determined from our K-selected samples and those of Butcher & Oemler is much reduced. The large scatter in the measured fB, even in small redshift ranges, in our study indicates that determining the fB 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 MV=-20, as used by Butcher & Oemler, for four clusters that are common between our sample and theirs. For these rest-frame V-selected samples, we find similar fractions of blue galaxies to Butcher & Oemler, while the K-selected samples for the same four clusters yield blue fractions that 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 & Oemler effect. Selecting in the observed K band is more nearly a process of selecting galaxies by their mass than is the case for optically selected samples. Our results suggest that the Butcher-Oemler effect is at least partly due to low-mass galaxies whose optical luminosities are boosted. These lower mass galaxies could evolve into the rich dwarf population observed in nearby clusters.
de Propris Roberto
Dickinson Mark
Eisenhardt Peter R.
Stanford Spencer A.
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