25,983 research outputs found
Abundance of moderate-redshift clusters in the Cold + Hot dark matter model
Using a set of \pppm simulation which accurately treats the density
evolution of two components of dark matter, we study the evolution of clusters
in the Cold + Hot dark matter (CHDM) model. The mass function, the velocity
dispersion function and the temperature function of clusters are calculated for
four different epochs of . We also use the simulation data to test
the Press-Schechter expression of the halo abundance as a function of the
velocity dispersion . The model predictions are in good agreement
with the observational data of local cluster abundances (). We also
tentatively compare the model with the Gunn and his collaborators' observation
of rich clusters at and with the x-ray luminous clusters at
of the {\it Einstein} Extended Medium Sensitivity Survey. The
important feature of the model is the rapid formation of clusters in the near
past: the abundances of clusters of \sigma_v\ge 700\kms and of \sigma_v\ge
1200 \kms at are only 1/4 and 1/10 respectively of the present values
(). Ongoing ROSAT and AXAF surveys of distant clusters will provide
sensitive tests to the model. The abundance of clusters at would
also be a good discriminator between the CHDM model and a low-density flat CDM
model both of which show very similar clustering properties at .Comment: 21 pages + 6 figures (uuencoded version of the PS files), Steward
Preprints No. 118
Measurement of the c-axis optical reflectance of AFeAs (A=Ba, Sr) single crystals: Evidence of different mechanisms for the formation of two energy gaps
We present the c-axis optical reflectance measurement on single crystals of
BaFeAs and SrFeAs, the parent compounds of FeAs based
superconductors. Different from the ab-plane optical response where two
distinct energy gaps were observed in the SDW state, only the smaller energy
gap could be seen clearly for \textbf{E}c-axis. The very pronounced
energy gap structure seen at a higher energy scale for
\textbf{E}ab-plane is almost invisible. We propose a novel picture
for the band structure evolution across the SDW transition and suggest
different driving mechanisms for the formation of the two energy gaps.Comment: 4 page
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