Experiments were performed with prototype antenna tiles for the Mileura Widefield Array Low Frequency Demonstrator (MWA LFD) to better understand the wide-field, wide-band properties of their design and to characterize the radio-frequency interference (RFI) between 80 and 300 MHz at the site in Western Australia. Observations acquired during the 6 month deployment confirmed the predicted sensitivity of the antennas, sky-noise-dominated system temperatures, and phase-coherent interferometric measurements. The radio spectrum is remarkably free of strong terrestrial signals, with the exception of two narrow frequency bands allocated to satellite downlinks, and rare bursts due to ground-based transmissions being scattered from aircraft and meteor trails. Results indicate the potential of the MWA LFD to make significant achievements in its three key science objectives: epoch of reionization science, heliospheric science, and radio transient detection.
Abstract Analyses of QSO absorption lines show that the H I content has evolved over the redshift range z = 5 to z = 0. The 21-cm line measurements of the z = 0 H I content avoid several biases inherent in the absorption-line technique, such as the influence of evolving dust content in the absorbers, and will produce a reliable measure to anchor theories of galaxy evolution. Examples of important questions to be addressed by local H I surveys are: (1) Is there a significant population of gas-rich galaxies or intergalactic clouds that is missing from the census of optically selected galaxies? (2) Is there an adequate reservoir of neutral gas to substantially prolong star formation at its present rate? (3) Are there massive objects of such low H I column density that they can have escaped detection in the ‘unbiased’ H I surveys that have been conducted so far?
Spatial filtering has traditionally been the province of antenna arrays, where both the Radio Frequency Interference (RFI) and celestial signals appear at all receivers. In the implementation described here, spatial filtering techniques are applied to data taken with the 20 cm multibeam receiver on the Parkes 64 m dish. Here, each of the feeds has independent celestial signal and system noise, but common RFI. The signal processing requirements of the technique are outside the capacity of the standard spectrometer for the multibeam receiver at the observatory. As a result, a customised baseband recoding system was used to record the raw voltage data to disk, which were then processed offline in software. The results demonstrate that spatial filtering methods provide powerful tools for interference mitigation with an array feed receiver.
We present results of two 21-cm surveys performed with the Australia Telescope Compact Array in the nearby Centaurus A and Sculptor galaxy groups. These surveys are sensitive to compact clouds and galaxies with masses as low as ~, and are therefore among the most sensitive extragalactic surveys to date. The surveys consist of sparsely spaced pointings that sample approximately 2% of the groups' area on the sky. We detected previously known group members, but we found no new clouds or galaxies down to the sensitivity limit of the surveys. If the mass function had a faint end slope of below in these groups, we would have expected ~ new objects. Cold dark matter theories of galaxy formation predict the existence of a large number low mass dark matter sub-halos that might appear as tiny satellites in galaxy groups. Our results support and extend similar conclusions derived from previous surveys that a rich population of these satellites does not exist.
Abstract We present a measurement of the H I column density distribution function f ( N hi ) at the present epoch for column densities > 10 20 cm −2 . These high column densities compare to those measured in damped Lyα lines seen in absorption against background quasars. Although observationally rare, it appears that the bulk of the neutral gas in the Universe is associated with these damped Lyα systems. In order to obtain a good anchor point at z = 0 we determine f ( N hi ) in the local Universe by using 21 cm synthesis observations of a complete sample of spiral galaxies. We show that f ( N hi ) for damped Lyα systems has changed significantly from high z to the present and that change is greatest for the highest column densities. The measurements indicate that low surface brightness galaxies make a minor contribution to the cross section for H I , especially for N HI > 10 21 cm −2 .
We have used the GMOS instrument on the Gemini-South telescope to obtain spatially-resolved two-colour imaging and IFU spectroscopy of a sample of ten nearby E+A galaxies. Surface brightness profiles measured using our imaging data show the isophotal profiles of our sample are generally r^{1/4}-like, consistent with a sample dominated by early-type galaxies. This is further underscored by all galaxies having early-type morphological classifications, and showing a behaviour in the central velocity dispersion-absolute magnitude plane that is consistent with the Faber-Jackson relation, once the transitory brightening that occurs in the E+A phase is corrected for. In addition, two-thirds of our sample shows clear evidence of either ongoing or recent tidal interactions/mergers. While all the galaxies in our sample have total integrated colours that are relatively blue, they show a diversity of colour gradients, possessing central core regions that are either redder, bluer, or indistinct in colour relative to their outer regions. Kinematically, the most striking property is the significant and unambiguous rotation that is seen in all our E+A galaxies, with it being generally aligned close to the photometric major axis. This is contrary to the findings of Norton et al. 2001, who found little or no evidence for rotation in a very similar sample of nearby E+A galaxies. We also clearly demonstrate that our E+A galaxies are, in all but one case, consistent with being "fast rotators" (Emsellem et al. 2007). We argue that the combination of disturbed morphologies and significant rotation in these galaxies supports their production via gas-rich galaxy mergers and interactions. The large fraction of fast rotators argues against equal mass mergers being the dominant progenitor to the E+A population.
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