Evolution in the X‐Ray Cluster Luminosity Function Revisited

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Abstract
We present new X-ray data taken from the ROSAT Position Sensitive Proportional Counter pointing archive for 21 clusters in the Einstein Extended Medium-Sensitivity Survey (EMSS). We have supplemented these data with new optical follow-up observations found in the literature, and, overall, 32 of the original 67 z > 0.14 EMSS clusters now have new information. Using this revised sample, we find no systematic difference, as a function of X-ray flux, between our measured X-ray cluster fluxes and those in the original EMSS (30% scatter). However, we do detect a marginal correlation between this observed difference in the flux and the redshift of the clusters, with the lower redshift systems having a greater scatter by nearly a factor of 2. We have also determined the X-ray extent of these reobserved EMSS clusters and find that 14 of them have significant extents compared with the ROSAT point-spread function. Combining these data with extended clusters seen in the original EMSS sample, at least 40% of z > 0.14 clusters now have an observed X-ray extent, thus justifying their classification as X-ray clusters. Using our improved EMSS sample, we have redetermined the EMSS X-ray cluster luminosity function as a function of redshift. We have removed potential misclassifications and included our new measurements of the clusters' X-ray luminosities and redshifts. We find similar luminosity functions to those originally presented by Henry et al., albeit with two important differences. First, we show that the original low-redshift EMSS luminosity function is insufficiently constrained. Second, the power-law shape of our new determination of the high-redshift EMSS luminosity function (z = 0.3-0.6) has a shallower slope than that seen by Henry et al. We have compared our new EMSS luminosity functions with those recently derived from a nearby sample of X-ray clusters and find that the overall degree of observed luminosity function evolution is mild at best. This is a result of the shallower slope seen in our EMSS high-redshift luminosity function and a more robust low-redshift determination of the X-ray cluster luminosity function from the literature. We have quantified the degree of evolution seen in the X-ray cluster luminosity by using several statistical tests. The most restrictive analysis indicates that our low- and high-redshift EMSS luminosity functions are statistically different at the 95% level. However, other tests indicate that these low- and high-redshift luminosity functions only differ by as little as 1 σ. These data are, therefore, consistent with no evolution in the X-ray cluster luminosity function out to z 0.5.
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