Astronomical instrumentation

Abstract:

Aims. We describe a 6-12 GHz analogue correlator that has been developed for use in radio interferometers. Methods. We use a lag-correlator technique to synthesis eight complex spectral channels. Two schemes were considered for sampling the cross-correlation function, using either real or complex correlations, and we developed prototypes for both of them. We opted for the "add and square" detection scheme using Schottky diodes over the more commonly used active multipliers because the stability of the device is less critical. Results. We encountered an unexpected problem, in that there were errors in the lag spacings of up to ten percent of the unit spacing. To overcome this, we developed a calibration method using astronomical sources which corrects the effects of the non-uniform sampling as well as gain error and dispersion in the correlator. © ESO 2007.

Abstract:

Using integral-field spectroscopy, the SAURON survey has shown that early-type galaxies, once thought to be essentially devoid of gas, commonly show ionised gas emission. This emission is found with a rich variety of distributions and kinematics, ranging from very uniform disks or rings, and large-scale twisted structures, to flocculent and irregular streams. Such variety is missed in conventional long-slit spectroscopy, and integral-field spectroscopic data allow accurate removal of the underlying stellar continuum compared with imaging surveys, giving very low detection limits. Moreover, spectral data can simultaneously provide the stellar kinematics and populations as well as the emission-line properties. We investigate the connection between the stellar and gas properties using integral-field spectroscopy from SAURON, OASIS and GMOS, and find that, although some global trends exist, the connection between the stellar population parameters and the gas properties is in some cases puzzlingly unclear. (c) 2006 Elsevier B.V. All rights reserved.

Abstract:

By detecting ionised-gas emission in 75% of the cases, the SAURON integral-field spectroscopic survey has further demonstrated that early-type galaxies often display nebular emission. Furthermore, the SAURON data have shown that such emission comes with an intriguing variety of morphologies, kinematic behaviours and line ratios. Perhaps most puzzling was the finding that round and slowly rotating objects generally display uncorrelated stellar and gaseous angular momenta, consistent with an external origin for the gas, whereas flatter and fast rotating galaxies host preferentially co-rotating gas and stars, suggesting internal production of gas. Alternatively, a bias against the internal production of ionised gas and against the acquisition of retrograde material may be present in these two kinds of objects, respectively. In light of the different content of hot gas in these systems, with slowly rotating objects being the only systems capable of hosting massive X-ray halos, we suggest that a varying importance of evaporation of warm gas in the hot interstellar medium can contribute to explain the difference in the relative behaviour of gas and stars in these two kinds of objects. Namely, whereas in X-ray bright and slowly rotating galaxies stellar-loss material would quickly evaporate in the hot medium, in X-ray faint and fast rotating objects such material would be allowed to lose angular momentum and settle in a disk, which could also obstruct the subsequent acquisition of retrograde gas. Evidence for a connection between warm and hot gas phases, presumably driven by heat conduction, is presented for four slowly rotating galaxies with Chandra observations. (c) 2006 Elsevier B.V. All rights reserved.