Astronomical instrumentation

Abstract:

We present a comparison of how well the large-scale structure of the universe is traced by clusters from the Abell catalog and from the Automated Plate Measuring Facility (APM). We investigate selection functions for both cluster catalogs, using samples of all clusters (including clusters with estimated redshifts) and samples of clusters with measured redshifts. We present a catalog of superclusters of galaxies, based on APM clusters up to a redshift Zlim = 0.13. We find that the distribution of rich superclusters, defined by all Abell and APM clusters, is similar in the volume covered by both cluster samples. We calculate the correlation function for Abell and APM cluster samples. We show that the supercluster-void network can be traced with both cluster samples; the network has a period of ∼ 120 h-1 Mpc. However, the APM cluster sample with measured redshifts covers a small volume, which contains only a few very rich superclusters. These superclusters surround one void and have exceptionally large mutual separations. Because of this property, the secondary maximum of the correlation function of APM clusters with measured velocities is located at larger scales than the corresponding feature in the correlation function of Abell clusters. We conclude that the APM sample is not representative of the large-scale structure as a whole because of the small volume covered. The Abell cluster catalog is presently the best sample to investigate the large-scale distribution of high-density regions in the universe.

Abstract:

We discuss a cryogenic, multi-object near-infrared spectrometer as a second generation instrument for the VLT. The spectrometer combines 20 to 40 independent integral field units (IFUs), which can be positioned by a cryogenic robot over the entire unvignetted field of the VLT (similar to 7'). Each IFU consists of a contiguous cluster of 20 to 30 pixels (0.15 to 0.25" per pixel). The individual IFUs have cold fore-optics and couple into the spectrograph with integrated fibers-microlenses. The spectrometer has resolving power of lambda/Deltalambdasimilar to4000 and simultaneously covers the J-, H-, and K-bands with three HAWAII 2 detectors. The system is designed for operation both in seeing limited and MCAO modes. Its speed is approximately 3500 times greater than that of ISAAC and 60 times greater than NIRMOS (in H-band). The proposed instrument aims at a wide range of science, ranging from studies of galaxies/clusters in the high-z Universe (dynamics and star formation in z>1 galaxies, evolution of ellipticals, properties of distant, obscured far-IR and X-ray sources), to investigations of nearby starbursts, star clusters and properties of young low mass stars and brown dwarfs.