A Very Hot, High Redshift Cluster of Galaxies: More Trouble for Omega_0 = 1

We have observed the most distant (z=0.829) cluster of galaxies in the Einstein Extended Medium Sensitivity Survey, with the ASCA and ROSAT satellites. We find an X-ray temperature of 12.3 +3.1/-2.2 keV for this cluster, and the ROSAT map reveals significant substructure. The high temperature of MS1054-0321 is consistent with both its approximate velocity dispersion, based on the redshifts of 12 cluster members we have obtained at the Keck and the Canada-France-Hawaii telescopes, and with its weak lensing signature. The X-ray temperature of this cluster implies a virial mass ~ 7.4 x 10^14 h^-1 solar masses, if the mean matter density in the universe equals the critical value, or larger if Omega_0 < 1. Finding such a hot, massive cluster in the EMSS is extremely improbable if clusters grew from Gaussian perturbations in an Omega_0 = 1 universe. Combining the assumptions that Omega_0 = 1 and that the intial perturbations were Gaussian with the observed X-ray temperature function at low redshift, we show that the probability of this cluster occurring in the volume sampled by the EMSS is less than a few times 10^{-5}. Nor is MS1054-0321 the only hot cluster at high redshift; the only two other $z > 0.5$ EMSS clusters already observed with ASCA also have temperatures exceeding 8 keV. Assuming again that the initial perturbations were Gaussian and Omega_0 = 1, we find that each one is improbable at the < 10^{-2} level. These observations, along with the fact that these luminosities and temperatures of the high-$z$ clusters all agree with the low-z L_X-T_X relation, argue strongly that Omega_0 < 1. Otherwise, the initial perturbations must be non-Gaussian, if these clusters' temperatures do indeed reflect their gravitational potentials.Comment: 20 pages, 4 figures, To appear in 1 Aug 1998 ApJ (heavily revised version of original preprint

FlyEyes: integrating CCID-35 into PUEO AO system at CFHT

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FlyEyes: A CCD-based Wavefront Sensor for PUEO, the CFHT Curvature AO System

International audienceAdaptive optics wavefront sensing imposes stringent requirements on detectors, due to the simultaneous need for extremely low read noise and high frame rates. Curvature wavefront sensing measurements are based on the normalized intensity of the signal in a given subaperture, and avalance photodiodes (APDs) have traditionally been used as detectors in curvature systems such as the Canada-France-Hawaii Telescope (CFHT) adaptive optics (AO) bonnette, called PUEO after the endemic Hawaiian owl. Passively quenched APDs are robust but have low QE (~40%), while actively quenched APDs can have much higher QE, but have been known to fail. Furthermore, curvature systems with large numbers of subapertures are now in operation, and the cost of individual APDs may become prohibitive for such systems. Thus, a CCD-based alternative appears very attractive, and development of a specific chip was initiated at ESO 10 years ago. In this article, we report on the performance of the FlyEyes camera, a project that was conceived to compare the performance of the backside-illuminated custom-designed CCD detector with an array of APDs, used in an operational and well-characterized curvature wavefront AO system. The on-sky performance is demonstrated to be unaffected on bright guide stars (i.e., negligible latency), and although the faint end suffers from the 2.5 e- read noise, the performance can be regained by lowering the frame rate on the wavefront sensor. In this article, we report on results that show that the CCD can be used to replace an array of expensive APDs. This would enable a cost-effective upgrade of PUEO to a higher-order system, as has been proposed at various occasions