Imaging and spectroscopy of ultrasteep spectrum radio sources

Bornancini, C. G.; Breuck, C. De; Vries, Wim de; Croft, Steve; Breugel, Wil van; Röttgering, H. J. A.; Minniti, D.

doi:10.1111/j.1365-2966.2007.11769.x

Cited by 29 publications

(32 citation statements)

References 44 publications

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“…Most of the currently known radio galaxies at z > 4 display a compact steep radio spectrum, with curvature occurring at low observed frequencies (∼100 MHz), data which Klamer et al did not have for their sample. Bornancini et al (2007) also confirm the presence of curvature at low MHz frequencies for their USS sample.…”

Section: The Origin Of the α– Correlation In Flux‐limited Samplessupporting

confidence: 63%

New insights on the -α correlation from complete radio samples

Ker

Best

Rigby

et al. 2012

Monthly Notices of the Royal Astronomical Society

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The existence of a correlation between observed radio spectral index and redshift has long been used as a method for selecting high‐redshift radio galaxy candidates. We use nine highly spectroscopically complete radio samples, selected at different frequencies and flux limits, to determine the efficiency of this method and compare consistently observed correlations between spectral index (α), luminosity (P), linear size (D) and redshift (z) in our samples. We observe a weak correlation between z and α which remains even when Malmquist bias is removed. The strength of the z–α correlation is dependent on both the k‐correction and sample selection frequency, in addition to the frequency at which α is measured, and consistent results for both high‐ and low‐frequency‐selected samples are only seen if analysis is restricted to just extended radio galaxies. This fits with the popular interpretation that the spectra steepen with z because the radio lobes work against a denser intergalactic medium environment as z increases, out to z∼ 2–3. However, we also note that the majority of sources known at z > 4 are very compact and often display a negatively curved or peaked spectrum, indicative of youth or merger activity, and therefore the low‐frequency radio spectrum as a whole should be determined; this is something for which the new LOw Frequency ARray will be crucial. We quantify both the efficiency and the completeness of various techniques used to select high‐z radio candidates. A steep‐spectrum cut applied to low‐frequency‐selected samples can more than double the fraction of high‐z sources, but at a cost of excluding over half of the high‐z sources present in the original sample. An angular size cut is an almost as equally effective radio‐based method as a steep‐spectrum cut for maximizing the high‐z content of large radio samples, and works for both high‐ and low‐frequency‐selected samples. In multiwavelength data, selection first of infrared‐faint radio sources remains by far the most efficient method of selecting high‐z sources. We present a simple method for selecting high‐z radio sources, based purely on combining their observed radio properties of α and angular size, with the addition of the K‐band magnitude if available.

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Section: The Origin Of the α– Correlation In Flux‐limited Samplessupporting

confidence: 63%

New insights on the -α correlation from complete radio samples

Ker

Best

Rigby

et al. 2012

Monthly Notices of the Royal Astronomical Society

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“…(a) Despite the fact that most previous searches are limited to sources with spectral index steeper than 1.3, the median spectral index of known HzRGs is 1.31. (b) Though Klamer et al (2006) have shown that the spectra for the majority of HzRGs are straight over a large frequency range, evidence for spectral curvature cannot be ruled out completely (Bornancini et al 2007). This means that at frequencies as low as 150 MHz, a higher cut‐off in the spectral index will translate into even higher cut‐off at the rest frame of high‐redshift objects.…”

Section: Resultsmentioning

confidence: 99%

Deep GMRT 150-MHz observations of the LBDS-Lynx region: ultrasteep spectrum radio sources

Ishwara‐Chandra

Sirothia

Wadadekar

et al. 2010

Monthly Notices of the Royal Astronomical Society

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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 z > 3 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,

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“…Using data from the VLSS, the 352‐MHz Westerbork in the Southern Hemisphere Survey (WISH; De Breuck et al 2002b) and NVSS, Bornancini et al (2007) found significant curvature below 352 MHz in a subset of 12 USS sources with α 1400 352 ≤−1.3 (WISH–NVSS selection, see Table 9): the median values of the two‐point spectral indices are and . We can test whether we see the same amount of curvature in the MRCR–SUMSS sample over a similar frequency range by using our 74, 408 and 1400 MHz data.…”

Section: Discussionmentioning

confidence: 99%

A new search for distant radio galaxies in the Southern hemisphere – I. Sample definition and radio properties

Broderick¹,

Bryant²,

Hunstead³

et al. 2007

Monthly Notices of the Royal Astronomical Society

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This paper introduces a new program to find high‐redshift radio galaxies in the Southern hemisphere through ultrasteep spectrum (USS) selection. We define a sample of 234 USS radio sources with spectral indices α843408≤−1.0 (Sν∝να) and flux densities S408≥ 200 mJy in a region of 0.35 sr, chosen by cross‐correlating the revised 408 MHz Molonglo Reference Catalogue, the 843 MHz Sydney University Molonglo Sky Survey and the 1400 MHz NRAO VLA Sky Survey in the overlap region −40° < δ < −30°. We present Australia Telescope Compact Array (ATCA) high‐resolution 1384 and 2368 MHz radio data for each source, which we use to analyse the morphological, spectral index and polarization properties of our sample. We find that 85 per cent of the sources have observed‐frame spectral energy distributions that are straight over the frequency range 408–2368 MHz, and that, on average, sources with smaller angular sizes have slightly steeper spectral indices and lower fractional linear polarization. Fractional polarization is anticorrelated with flux density at both 1400 and 2368 MHz. We also use the ATCA data to determine observed‐frame Faraday rotation measures for half of the sample.

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Imaging and spectroscopy of ultrasteep spectrum radio sources

Cited by 29 publications

References 44 publications

New insights on the -α correlation from complete radio samples

New insights on the -α correlation from complete radio samples

Deep GMRT 150-MHz observations of the LBDS-Lynx region: ultrasteep spectrum radio sources

A new search for distant radio galaxies in the Southern hemisphere – I. Sample definition and radio properties

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