ROSAT Observations of Butcher-Oemler Clusters

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Session 67 -- Galaxy Clusters and the Extragalactic Distance Scale
Oral presentation, Thursday, 10:30-12:00, Zellerbach Auditorium Room

[67.04] ROSAT Observations of Butcher-Oemler Clusters

D.S.Davis (U Md/GSFC), Richard F. Mushotzky (NASA/GSFC)

In our continuing study of medium redshift clusters (0.2 $<$ z $<$0.3) we have obtained ROSAT PSPC observations of Abell 222/223, Abell 777 and Abell 1758. These objects exhibit the "Butcher-Oemler" effect, that is they have an increased fraction of blue galaxies when compared to lower redshift clusters. Our ROSAT PSPC observations reveal that these clusters consist of multiple components. Abell 222/223 and Abell 777 are double clusters and Abell 1758 is at least a double system. These X-ray observations show another extended x-ray source $\sim$ 6$'$ to the south of the optical center of A1758. Fits to the ROSAT spectra for A1758 give a best fit temperature of T $\sim$ 3.4 kev, much lower than for nearby rich clusters indicating that this cluster may not be fully virialized. The southern source is well fit with a thermal plasma model with a temperature of 2.9 keV and very low metal abundance. If this source is at the same redshift as A1758 then this cluster may be in the process of a double merger.

One interesting result from the X-ray data is that most of the clusters which show strong structure have a low blue galaxy fraction and the clusters with a higher fraction of blue galaxies have no multiple components. Of the Butcher-Oemler clusters between 0.2 $<$ z $<$0.3 with X-ray data only 13\% of the clusters with a blue fraction above 0.10 have multiple X-ray components while 60\% of the clusters with a blue fraction below 0.10 have multiple components. While this result is not statistically compelling it may be a clue to the origin of the excess blue galaxies.

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