Search

We present the results of a survey for HI 21 cm and OH 18 cm absorption in seven strong CO emitters at z ≳ 3. Despite reaching limits comparable to those required to detect 21 cm absorption at lower redshifts, we do not detect either transition in any of the objects searched. We believe that this is due to the high-redshift selection causing all of our targets to have ultraviolet (UV) luminosities above the critical value, where all of the atomic gas in the host galaxy disc is suspected to be ionized. However, not only are all of our targets bright in CO emission, but detection of CO above the critical UV luminosity is generally not uncommon. This suggests that the molecular gas is shielded from the radiation or is physically remote from the source of the continuum emission, as it appears to be from CO observations of high-redshift radio galaxies. [ABSTRACT FROM AUTHOR]

Deep radio observations of the galaxy cluster Abell 781 have been carried out using the Giant Metrewave Radio Telescope at 325 MHz and have been compared to previous 610-MHz observations and to archival Very Large Array (VLA) 1.4-GHz data. The radio emission from the cluster is dominated by a diffuse source located at the outskirts of the X-ray emission, which we tentatively classify as a radio relic. We detected residual diffuse emission at the cluster centre at the level of S∼ 15-20 mJy. Our analysis disagrees with Govoni et al., and on the basis of simple spectral considerations, we do not support their claim of a radio halo with the flux density of 20-30 mJy at 1.4 GHz. Abell 781, a massive and merging cluster, is an intriguing case. Assuming that the residual emission is indicative of the presence of a radio halo barely detectable at our sensitivity level, it could be a very steep spectrum source. [ABSTRACT FROM AUTHOR]

We present the results of a mini-survey for associated H 21-cm absorption at z≤ 0.42 with the Giant Metrewave Radio Telescope. Our targets are radio galaxies, selected on the basis that the λ≈ 1216 Å luminosities are below L∼ 10 W Hz, above which there has never been a detection of 21-cm absorption. Of the three sources for which we obtained good data, two are unclassified active galactic nuclei (AGNs) and one is type-2. Being a non-detection, the type-2 object is consistent with our previous result that 21-cm absorption in radio sources is not dictated by unified schemes of AGN. In the case of the detection, the absorption only occurs towards one of the two resolved radio lobes in PKS 1649-062. If the absorption is due to another intervening galaxy, or cool H gas in the intergalactic medium, covering only the south-west (SW) lobe, then, being at the same redshift, this is likely to be gravitationally bound to the optical object identified as PKS 1649-062. If the absorption is due to an inclined disc centred between the lobes, intervening the SW lobe while being located behind the north-east (NE) lobe, by assuming that it covers the emission peak at ≈150 kpc from the nucleus, we estimate a dynamical mass of ≈3 × 10 M for the disc. [ABSTRACT FROM AUTHOR]

We are currently undertaking a large survey for redshifted atomic and molecular absorption lines at radio frequencies. In this paper, we present the results from the first phase of this survey: the search for H i 21-cm and OH 18-cm absorption lines in the hosts of reddened quasars and radio galaxies. Although we observed each source for up to several hours with two of the world's most sensitive radio telescopes, the Giant Metrewave Radio Telescope (GMRT) and Westerbork Synthesis Radio Telescope (WSRT), only one clear and one tentative detection were obtained: H i absorption at in PKS 1555−140 and OH absorption at in PKS 2300−189, respectively, with the Australia Telescope Compact Array (ATCA). For the latter, no H i absorption was detected at the same redshift as the borderline OH detection. In order to determine why no clear molecular absorption was detected in any of the 13 sources searched, we investigate the properties of the five redshifted systems currently known to exhibit OH absorption. In four of these, molecules were first detected via millimetre-wave transitions, and the flat radio spectra indicate compact background continuum sources, which may suggest a high degree of coverage of the background source by the molecular clouds in the absorber. Furthermore, for these systems we find a relationship between the molecular line strength and red optical–near-infrared ( V− K) colours, thus supporting the notion that the reddening of these sources is due to dust, which provides an environment conducive to the formation of molecules. Upon comparison with the V− K colours of our sample, this relationship suggests that, presuming the reddening occurs at the host galaxy redshift at least in some of the targets, many of our observations still fall short of the sensitivity required to detect OH absorption, although a confirmation of the ‘detection’ of OH in 2300−189 could contravene this. [ABSTRACT FROM AUTHOR]