High-z Radio Sources Research Articles

High-resolution international LOFAR observations of 4C 43.15

It has long been known that radio sources with the steepest spectra are preferentially associated with the most distant galaxies, the α − z relation, but the reason for this relation is an open question. The spatial distribution of spectra in high-z radio sources can be used to study this relation, and low-frequency observations are particularly important in understanding the particle acceleration and injection mechanisms. However, the small angular sizes of high-z sources together with the inherently low resolution of low-frequency radio telescopes until now has prevented high angular resolution low-frequency observations of distant objects. Here we present subarcsecond observations of a z = 2.4 radio galaxy at frequencies between 121 and 166 MHz. We measure the spatial distribution of spectra, and discuss the implications for models of the α−z relation. We targeted 4C 43.15 with the High Band Antennas of the International LOFAR Telescope with a range of baselines up to 1300 km. At the central frequency of 143 MHz we achieve an angular resolution of ~0.3″. By complementing our data with archival Very Large Array data we study the spectral index distribution across 4C 43.15 between 55 MHz and 8.4 GHz at resolutions of 0.4″ and 0.9″. With a magnetic field strength of B = 5.2 nT and fitted injection indices of αinjnorth = −0.8 and αinjsouth = −0.6, fitting a Tribble spectral ageing model results in a spectral age of τspec = 1.1 ± 0.1 Myr. We conclude that our data on 4C 43.15 indicates that inverse Compton losses could become comparable to or exceed synchrotron losses at higher redshifts and that inverse Compton losses could be a viable explanation for the α−z relation. Statistical studies of these objects will become possible in the future when wide-area subarcsecond surveys start.

Spatially resolved studies of extragalactic jets in high redshift radio galaxies

AbstractThe correlation between radio spectral steepness and redshift has been successfully used to find high redshift (z ⩾ 2) radio galaxies, but the origin of this relation is unknown. The ultra-steep spectra of high-z radio sources make them ideally suited for studies with the Low Band Antenna of the new Low Frequency Array, which covers 10–80 MHz and has baselines up to about 1300 km. As part of an ongoing survey, we use the longest baselines to map the low-frequency (< 70 MHz) spatial distributions along the jets of 5 bright extended steep spectrum high-z radio sources. From this, we will determine whether the spectra change over these spatially resolved sources, thereby constraining particle acceleration processes. We present early results from our low-frequency survey of ultra-steep spectrum radio galaxies. The first low frequency long baseline images of these objects are presented.

The Australia Telescope Large Area Survey: 2.3 GHz observations of ELAIS-S1 and CDF-S

Context. The Australia Telescope Large Area Survey (ATLAS) aims to image a 7 deg2 region centred on the European Large Area ISO Survey - South 1 (ELAIS-S1) field and the Chandra Deep Field South (CDF-S) at 1.4 GHz with high sensitivity (up to ~10 \muJy) to study the evolution of star-forming galaxies (SFGs) and Active Galactic Nuclei (AGN) over a wide range of cosmic time. Aims. We present here ancillary radio observations at a frequency of 2.3 GHz obtained with the Australia Telescope Compact Array (ATCA). The main goal of this is to study the radio spectra of an unprecedented large sample of sources (~2000 observed, ~600 detected in both frequencies). Methods. With this paper, we provide 2.3 GHz source catalogues for both ATLAS fields, with a detection limit of 300 \muJy. We compute spectral indices between 1.4 GHz and 2.3 GHz using matched resolution images and investigate various properties of our source sample in dependence of their spectral indices. Results. We find the entire source sample to have a median spectral index of 0.74, in good agreement with both the canonical value of 0.7 for optically thin synchrotron radiation and other spectral index studies conducted by various groups. Regarding the radio spectral index as indicator for source type, we find only marginal correlations so that flat or inverted spectrum sources are usually powered by AGN and hence conclude that at least for the faint population the spectral index is not a strong discriminator. We investigate relation between spectral index and redshift for our source sample and find no such correlation at all. We do find a significant correlation between redshift and radio to near-infrared flux ratio, making this a much stronger tracer of high-z radio sources. We also find no evidence for a dependence of the radio-IR correlation on spectral index.

The Phoenix Deep Survey: The Clustering and Environment of Extremely Red Objects

In this paper we explore the clustering properties and environment of the extremely red objects (EROs; I - K > 4 mag) detected in a ≈180 arcmin2 deep (Ks ≈ 20 mag) Ks-band survey of a region within the Phoenix Deep Survey, an ongoing multiwavelength program aiming to investigate the nature and evolution of faint radio sources. Using our complete sample of 289 EROs brighter than Ks = 20 mag, we estimate a statistically significant (≈3.7 σ) angular correlation function signal with amplitude Aw = 8.7 × 10-3 (assuming w(θ) = Awθ-0.8, with θ in degrees), consistent with earlier work based on smaller samples. This amplitude suggests a clustering length in the range ro = 12-17 h-1 Mpc, implying that EROs trace regions of enhanced density. Using a novel method, we further explore the association of EROs with galaxy overdensities by smoothing the K-band galaxy distribution using the matched filter algorithm of Postman et al. (1996) and then cross-correlating the resulting density maps with the ERO positions. Our analysis provides direct evidence that EROs are associated with overdensities at redshifts z 1. We also exploit the available deep radio 1.4 GHz data (limiting flux 60 μJy) to explore the association of EROs and faint radio sources and whether the two populations trace similar large-scale structures. Cross-correlation of the two samples (after excluding 17 EROs with radio counterparts) gives a 2 σ signal only for the subsample of high-z radio sources (z > 0.6). Although the statistics are poor, this suggests that it is the high-z radio subsample that traces similar structures with EROs.

The 21 Centimeter Forest: Radio Absorption Spectra as Probes of Minihalos before Reionization

We study the absorption along lines of sight toward high-z radio sources caused by the 21 cm transition of neutral hydrogen in the intergalactic medium (IGM) before reionization. Using semianalytic methods, we compute the number density of observable features caused by both minihalos (bound objects that are unable to cool efficiently because of their small virial temperatures) and protogalactic disks. We show that both sets of features should be observable by the next generation of low-frequency radio telescopes, including the Low Frequency Array and the Square Kilometer Array, provided that sufficiently bright background sources exist. The statistics of minihalo absorption features seen along lines of sight to radio-loud quasars offer a way to measure the evolution of the radiation background and the IGM temperature with cosmic time. Intersections with disks are much less common but cause significantly deeper absorption features that would be visible in the spectra of both radio-loud quasars and gamma-ray bursts (GRBs). The absorption feature caused by H I in the host galaxy of a GRB should be observable, offering a route to determine spectroscopically the burst redshift.

Hi21 Centimeter Absorption beyond the Epoch of Reionization

We explore the possibility of detecting H I 21 cm absorption by the neutral intergalactic medium (IGM) toward very high redshift radio sources. The epoch considered is between the time when the first ionizing sources form and when the bulk of the neutral IGM becomes ionized. Because of the extreme Lyα opacities of the neutral IGM, objects within this "gray age" can be observed only at wavelengths longer than about 1 μm. We use the latest simulations of the evolution of the IGM in the context of Λ cold dark matter structure formation models constrained by observations of the highest redshift QSOs to predict the optical depth as a function of frequency of the neutral IGM due to the H I 21 cm line. We then simulate radio spectra assuming observational parameters for future large-area radio telescopes. These spectra show that H I 21 cm absorption studies could be a powerful probe of the rich structure of the neutral IGM prior to the epoch of reionization, including ~1% absorption by the mean neutral IGM, plus deeper, narrow lines (≥5% and a few kilometers per second). Most of the variations in transmissivity are due to mild density inhomogeneities with typical values of the cosmic overdensity δ ~ 10, precisely the structures that at later times give rise to the Lyα forest. We also consider sensitivity limits and the evolution of radio source populations and conclude that it is reasonable to hypothesize the existence of an adequate number of high-z radio sources against which such absorption studies could be performed.