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Observations using the 7-mm receiver system on the Australia Telescope Compact Array have revealed large reservoirs of molecular gas in two high-redshift radio galaxies: HATLAS J090426.9+015448 (z = 2.37) and HATLAS J140930.4+003803 (z = 2.04). Optically, the targets are very faint, and spectroscopy classifies them as narrow-line radio galaxies. In addition to harbouring an active galactic nucleus the targets share many characteristics of sub-mm galaxies. Far-infrared data from Herschel-Astrophysical Terahertz Large Area Survey suggest high levels of dust (>109 M⊙) and a correspondingly large amount of obscured star formation (~1000 M⊙ yr-1). The molecular gas is traced via the J = 1 → 0 transition of 12CO, its luminosity implying total H2 masses of (1.7 ± 0.3) × 1011 and (9.5 ± 2.4) × 1010 (aCO/0.8) M⊙ in HATLAS J090426.9+015448 and HATLAS J140930.4+003803, respectively. Both galaxies exhibit molecular line emission over a broad (~1000 km s-1) velocity range and feature double-peaked profiles. We interpret this as evidence of either a large rotating disc or an on-going merger. Gas depletion time-scales are ~100 Myr. The 1.4-GHz radio luminosities of our targets place them close to the break in the luminosity function. As such they represent 'typical' z > 2 radio sources, responsible for the bulk of the energy emitted at radio wavelengths from accretion-powered sources at high redshift, and yet they rank amongst the most massive systems in terms of molecular gas and dust content. We also detect 115-GHz rest-frame continuum emission, indicating a very steep high-radio-frequency spectrum, possibly classifying the targets as compact steep spectrum objects. [ABSTRACT FROM AUTHOR]

We analyse optical and radio properties of radio galaxies detected in the Sloan Digital Sky Survey. The sample of radio sources is selected from the catalogue of Kimball & Ivezić with flux densities at 325, 1400 and 4850 MHz, using Westerbork Northern Sky Survey, NRAO VLA Sky Survey and Green Bank 6 cm radio surveys and from flux measurements at 74 MHz taken from Very Large Array Low-Frequency Sky Survey ( Cohen et al. 2006 ). We study radio galaxy spectral properties using radio colour–colour diagrams and find that our sample follows a single power law from 74 to 4850 MHz. The spectral index versus spectroscopic redshift relation is not significant for our sample of radio sources. We analyse a subsample of radio sources associated with clusters of galaxies identified from the maxBCG catalogue and find that about 40 per cent of radio sources with ultrasteep spectra (USS; , where ) are associated with galaxy clusters or groups of galaxies. We construct a Hubble diagram of USS radio sources in the optical r band up to and compare our results with those for normal galaxies selected from different optical surveys and find that USS radio sources are around as luminous as the central galaxies in the maxBCG cluster sample and typically more than 4 mag brighter than normal galaxies at . We study correlations between spectral index, richness and luminosity of clusters associated with radio sources. We find that USS at low redshift are rare, most of them reside in regions of unusually high ambient density, such of those found in rich cluster of galaxies. Our results also suggest that clusters of galaxies associated with steeper than the average spectra have higher richness counts and are populated by luminous galaxies in comparison with those environments associated to radio sources with flatter than the average spectra. A plausible explanation for our results is that radio emission is more pressure confined in higher gas density environments such as those found in rich clusters of galaxies and as a consequence radio lobes in rich galaxy clusters will expand adiabatically and lose energy via synchrotron and inverse Compton losses, resulting in a steeper radio spectra. [ABSTRACT FROM AUTHOR]

It has been known for nearly three decades that high-redshift radio galaxies exhibit steep radio spectra, and hence ultrasteep spectrum radio sources provide candidates for high-redshift radio galaxies. Nearly all radio galaxies with have been found using this redshift–spectral index correlation. We have started a programme with the Giant Metrewave Radio Telescope (GMRT) to exploit this correlation at flux density levels about 10 to 100 times deeper than the known high-redshift radio galaxies which were identified primarily using the already available radio catalogues. In our programme, we have obtained deep, high-resolution radio observations at 150 MHz with GMRT for several ‘ deep’ fields which are well studied at higher radio frequencies and in other bands of the electromagnetic spectrum, with an aim to detect candidate high-redshift radio galaxies. In this paper we present results from the deep 150-MHz observations of the LBDS-Lynx field, which has been already imaged at 327, 610 and 1412 MHz with the Westerbork Synthesis Radio Telescope and at 1400 and 4860 MHz with the Very Large Array. The 150-MHz image made with GMRT has an rms noise of 0.7 mJy beam and a resolution of arcsec . It is the deepest low-frequency image of the LBDS-Lynx field. The source catalogue of this field at 150 MHz has about 765 sources down to 20 per cent of the primary beam response, covering an area of about 15 deg . Spectral index was estimated by cross-correlating each source detected at 150 MHz with the available observations at 327, 610, 1400 and 4860 MHz and also using available radio surveys such as the Westerbork Northern Sky Survey at 327 MHz and the NRAO VLA Sky Survey and Faint Images of the Radio Sky at Twenty-cm at 1400 MHz. We find about 150 radio sources with spectra steeper than 1. About two-third of these are not detected in the Sloan Digital Sky Survey, hence are strong candidate high-redshift radio galaxies, which need to be further explored with deep infrared imaging and spectroscopy to estimate the redshift. [ABSTRACT FROM AUTHOR]

We present basic observational data on the 6C** sample. This is a new sample of radio sources drawn from the 151-MHz 6C survey, which was filtered with radio criteria chosen to optimize the chances of finding radio galaxies at . The filtering criteria are a steep-spectral index and a small angular size. The final sample consists of 68 sources from a region of sky covering 0.421 sr. We present Very Large Array radio maps, and the results of K-band imaging and optical spectroscopy. Near-infrared counterparts are identified for 66 of the 68 sources, down to a 3σ limiting magnitude of K∼ 22 mag in a 3-arcsec aperture. Eight of these identifications are spatially compact, implying an unresolved nuclear source. The K-magnitude distribution peaks at a median K≈ 18.7 mag, and is found to be statistically indistinguishable from that of the similarly selected 6C* sample, implying that the redshift distribution could extend to z≳ 4. Redshifts determined from spectroscopy are available for 22 (32 per cent) of the sources, over the range of 0.2 ≲ z≲ 3.3. We measure 15 of these, whereas the other seven were previously known. Six sources are at . Four sources show broad emission lines in their spectra and are classified as quasars. Three of these show also an unresolved K-band identification. 11 sources fail to show any distinctive emission and/or absorption features in their spectra. We suggest that these could be (i) in the so-called ‘redshift desert’ region of or (ii) at a greater redshift, but feature weak emission-line spectra. [ABSTRACT FROM AUTHOR]