Professor Roger Davies

Abstract:

We present a morphological analysis of 17 X-ray-selected clusters at z ∼ 0.25, imaged uniformly with the Hubble Space Telescope Wide Field Planetary Camera 2 (WFPC2). Eight of these clusters comprise a subsample selected for their low X-ray luminosities (≲1044 ergs s -1), called the low-Lx sample. The remaining nine clusters comprise a high-Lx subsample with Lx > 10 45 ergs s-1. The two subsamples differ in their mean X-ray luminosity by a factor of 30 and span a range of more than 300. The clusters cover a relatively small range in redshift (z = 0.17-0.3, σ z/Z ∼ 0.15), and the data are homogeneous in terms of depth, resolution (0″.17 = 1 h50-1 kpc at z = 0.25), and rest wavelength observed, minimizing differential corrections from cluster to cluster. We fit the two-dimensional surface brightness profiles of galaxies down to very faint absolute magnitudes, M702 ≤, -18.2 + 5 log h50 (roughly 0.01L*R) with parametric models, and quantify their morphologies using the fractional bulge luminosity (B/T). Within a single WFPC2 image, covering a field of ∼ 3′ (1h50-1 Mpc at z = 0.25) in the cluster center, we find that the low-Lx clusters are dominated by galaxies with low B/T (∼ 0), while the high-Lx clusters are dominated by galaxies with intermediate B/T (∼ 0.4). We test whether this difference could arise from a universal morphology-density relation due to differences in the typical galaxy densities in the two samples. We find that small differences in the B/T distributions of the two samples persist with marginal statistical significance (98% confidence based on a binned Χ2 test) even when we restrict the comparison to galaxies in environments with similar projected local galaxy densities. A related difference (also of low statistical significance) is seen between the bulge-luminosity functions of the two cluster samples, while no difference is seen between the disk luminosity functions. From the correlations between these quantities, we argue that the global environment affects the population of bulges, over and above trends seen with local density. On the basis of this result, we conclude that the destruction of disks through ram pressure stripping or harassment is not solely responsible for the morphology-density relation and that bulge formation is less efficient in low-mass clusters, perhaps reflecting a less rich merger history.

Abstract:

The Gemini-North Multiobject Spectrograph (CMOS) includes a powerful capability for integral field spectroscopy - the first to be installed and used on an 8-10m telescope. CMOS is switched to this mode by the remote insertion of an integral field unit (IFU) into the focal plane in place of the masks used for multiobject spectroscopy. With 1500 lenslet-coupled fibres, it provides a total field of view exceeding 50 square arcseconds, including a separate field dedicated to background subtraction. We describe the design, construction and testing of the IFU and present performance results obtained during commissioning.

Abstract:

The Gemini Multiobject Spectrograph (GMOS) installed on the Gemini-North telescope has a facility for integral field spectroscopy over the wavelength range 0.4-1.0 μm. GMOS is converted to this mode by the remote insertion of an integral field unit (IFU) into the beam in place of the masks used for the multiobject mode. With the IFU deployed, integral field spectroscopy is available over a fully filled contiguous field of 5″ × 7″ with a sampling of 0″.2. A separate field of half the area, but otherwise identical, is also provided to improve background subtraction. The IFU contains 1500 lenslet-coupled fibers and is the first facility of any type for integral field spectroscopy employed on an 8-10 m telescope. We describe the design, construction, and testing of the GMOS IFU and present measurements of the throughput both in the laboratory and at the telescope. We compare these with a theoretical prediction made before construction started. All are in good agreement with each other, with the on-telescope throughput exceeding 60% (averaged over wavelength). A second paper will verify the scientific performance by comparison with existing one- and two-dimensional data sets.

Abstract:

We describe MUSE (Multi Unit Spectroscopic Explorer), a second-generation integral-field spectrograph for the VLT, operating in the visible and near IR wavelength range. It combines a 1' × 1' Field of View with the improved spatial resolution (0.2″) provided by adaptive optics and covers a large simultaneous spectral range (0.48-1 μm). With this unique combination of capabilities, MUSE has a wide domain of application, and a large discovery potential. It will provide ultra deep fields with a limiting magnitude for spectroscopy of R = 28. After a brief presentation of the scientific case and the derived instrument requirements, we will focus on the MUSE optical design, including the overall architecture, the major trade-off that were conducted in order to optimize the cost and performance, and a provisional implementation scheme of the instrument on the VLT Nasmyth platform. Then the most important optical subsystems (as the 3 × 8 Field-splitter, the Image Slicers and the Spectrometers) are described. One of MUSE special feature is the impressive number of Image Slicer and Spectrometer modules which must be manufactured, that is 24. The realization of such series has been studied in collaboration with an industrial company. Finally, a preliminary estimation of the expected performance and a technological development program in order to secure the realization of the critical optical subsystems will be presented.