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Register a new userA search for distant radio galaxies from SUMSS and NVSS: I. Sample definition, radio and K-band imaging
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Authors
Carlos De Breuck
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We present the first results from a pilot study to search for distant radio galaxies in the southern hemisphere (delta < -32). Within a 360 deg^2 region of sky, we define a sample of 76 ultra-steep spectrum (USS) radio sources from the 843 MHz Sydney University Molonglo Sky Survey (SUMSS) and 1.4 GHz NRAO VLA Sky Survey (NVSS) radio surveys with alpha_843^1400 < -1.3 and S_1400 > 15 mJy. We observed 71 sources without bright optical or near-infrared counterparts at 1.385 GHz with the ATCA, providing ~5 resolution images and sub-arcsec positional accuracy. To identify their host galaxies, we obtained near-IR K-band images with IRIS2 at the AAT and SofI at the NTT. We identify 92% of the USS sources down to K~20.5. The SUMSS-NVSS USS sources have a surface density more than 4 times higher than USS sources selected at lower frequencies. This is due to the higher effective selection frequency, and the well-matched resolutions of both surveys constructed using the same source fitting algorithm. The scattering of alpha >-1.3 sources into the USS sample due to spectral index uncertainties can account for only 35% of the observed USS sources. Since our sample appears to contain a similar fraction of very distant (z>3) galaxies, selecting USS sources from SUMSS-NVSS should allow us to identify large numbers of massive galaxies at high redshift.
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This is the second in a series of papers presenting observations and results for a sample of 76 ultra-steep-spectrum (USS) radio sources in the southern hemisphere designed to find galaxies at high redshift. Here we focus on the optical spectroscopy program for 53 galaxies in the sample. We report 35 spectroscopic redshifts, based on observations with the Very Large Telescope (VLT), the New Technology Telescope (NTT) and the Australian National Universitys 2.3m telescope; they include five radio galaxies with z>3. Spectroscopic redshifts for the remaining 18 galaxies could not be confirmed: three are occulted by Galactic stars, eight show continuum emission but no discernible spectral lines, whilst the remaining seven galaxies are undetected in medium-deep VLT integrations. The latter are either at very high redshift (z >~7) or heavily obscured by dust. A discussion of the efficiency of the USS technique is presented. Based on the similar space density of z>3 radio galaxies in our sample compared with other USS-selected samples, we argue that USS selection at 843-1400 MHz is an efficient and reliable technique for finding distant radio galaxies.
This paper introduces a new program to find high-redshift radio galaxies in the southern hemisphere through ultra-steep spectrum (USS) selection. We define a sample of 234 USS radio sources with spectral indices alpha_408^843 < -1.0 and flux densities S_408 > 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 deg < delta < -30 deg. 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 anti-correlated 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.
We cross-identified the ROSAT Bright Source Catalog (RBSC) and the NRAO VLA Sky Survey (NVSS) to construct the RBSC-NVSS sample of the brightest X-ray sources (>= 0.1 counts/s or ~1E-12 ergs/cm/cm/s in the 0.1-2.4 keV band) that are also radio sources (S >= 2.5 mJy at 1.4 GHz) in the 7.8 sr of extragalactic sky with |b| > 15 degrees. and delta > -40 degrees. The sky density of NVSS sources is low enough that they can be reliably identified with RBSC sources having average rms positional uncertainties = 10 arcsec. We used the more accurate radio positions to make reliable X-ray/radio/optical identifications down to the POSS plate limits. We obtained optical spectra for many of the bright identifications lacking published redshifts. The resulting X-ray/radio sample is unique in its size (N ~ 1500 objects), composition (a mixture of nearly normal galaxies, Seyfert galaxies, quasars, and clusters), and low average redshift (<z> ~ 0.1).
We review the properties and nature of luminous high-redshift radio galaxies (HzRGs, z > 2) and the environments in which they are located. HzRGs have several distinct constituents which interact with each other - relativistic plasma, gas in various forms, dust, stars and an active galactic nucleus (AGN). These building blocks provide unique diagnostics about conditions in the early Universe. We discuss the properties of each constituent. Evidence is presented that HzRGs are massive forming galaxies and the progenitors of brightest cluster galaxies in the local Universe. HzRGs are located in overdense regions in the early Universe and are frequently surrounded by protoclusters. We review the properties and nature of these radio-selected protoclusters. Finally we consider the potential for future progress in the field during the next few decades. A compendium of known HzRGs is given in an appendix.
We report the results of the spectroscopic observations carried out at the SAO RAS 6-m telescope for the optical components of nine new extended radio sources found in the NVSS catalog. The measured redshifts of the host galaxies are in the range of z=0.1-0.4. The physical sizes of radio sources were calculated within the standard cosmological model. The two most extended objects, 0003+1512 and 0422+0351 reach the sizes of 2.1 Mpc and 4.0 Mpc, respectively. This is close to the maximum size of known radio sources.
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