Search

Aims.We present a radio morphological study and spectral analysis of a sample of 13 cD galaxies in rich and poor clusters of galaxies.

Aims.We present the low frequency (610 MHz) radio source counts of the VVDS-VLA field and investigate the radio spectral index properties of the sub-mJy population.

Context.Wide-field narrowband surveys are an efficient way of searching large volumes of high-redshift space for distant galaxies.

We report the spectroscopic confirmation of a compact Lyαemitting galaxy at $z=5.721$. A FORS2 spectrum of the source shows a strong asymmetric line with a flux of $5\times 10^{-17}$erg s-1cm-2, making it one of the brightest Lyαemitting galaxies at this redshift, and a line-of-sight velocity dispersion of 400 km s-1. We also have a tentative detection of a second, narrower component that is redshifted by 400 km s-1with respect to the main peak. A FORS2 image shows that the source is compact, with a FWHMof 0$\farcs$5, which corresponds to 3.2 kpc at this redshift. This source is a brighter example of J1236.8+6215([CITE], ApJ, 570, 92), another Lyαemit ting galaxy at $z\sim5.2$.

We present a gravitational lensing analysis of the cluster of galaxies MS 1008–1224 ($z=0.31$), based on very deep observations obtained using the VLT with FORS1 and ISAAC during the science verification phase. Two different mass reconstruction algorithms were applied to the B-, V-, R- and I-band data to obtain similar projected mass distributions in all the bands. The FORS1 (BVRI) and ISAAC (JK) data were combined to determine the photometric redshift distribution of galaxies within the ISAAC field and to estimate the mass. We inferred from weak shear a minimum mass of $2.3 \times 10^{14}$$h^{-1}~M_{\odot}$on large scales (within ~$ 700$h-1kpc, diameter) which agrees well with the X-ray mass mass estimate. The Mass-to-light ratios are also in excellent agreement. The observed mass profile is consistent with Pseudo-Isothermal Sphere models as well as a Navarro, Frenk and White model. In the inner regions the lensing mass is about twice as high as the X-ray mass which supports the long-held view that complex physical processes occuring in the innermost parts of lensing-clusters are mainly responsible for the X-ray-lensing mass discrepancy. We found that the central part of the cluster comprises two mass peaks whose center of mass is located 10–20 arcsec north of the cD galaxy. A similar offset between the cD and the peak of the X-ray distribution has been reported before. The optical, X-ray and the mass distributions show that MS 1008–1224 is composed of several subsystems which are probably undergoing a merger. It is likely that the gas is not in equilibrium in the innermost regions which vitiates the X-ray mass estimate there. We discovered that MS 1008–1224 shows a remarkable case of cluster-cluster lensing. The photometric redshifts show an excess of galaxies located 30 arcsec south-west of the cD galaxy at a redshift of ~0.9. This distant cluster is therefore also lensed by MS 1008–1224, which, if confirmed with spectroscopic data, would make this the first known case of magnification of a distant cluster by another one.

We present Giant Metrewave Radio Telescope (GMRT) observations of redshifted 21-cm absorption from the $z=0.437$metal line absorption system towards PKS 1243-072. HI absorption is clearly detected; the absorption profile has a velocity spread of ∼20 km s-1. Detection of 21-cm absorption indicates that the absorber has an HI column density large enough to be classified as a damped Lyman-αsystem. Follow up ground based optical imaging and spectroscopy allow us to identify the absorber with an $L \sim L^\star$galaxy at an impact parameter of ∼9.8 kpc from the line of sight to the QSO. The absorbing galaxy is unusual in that it has bright emission lines. On the basis of the optical spectrum we are unable to uniquely classify the galaxy since its emission line ratios lie in the transition region between starburst and Seyfert II type spectra.

Aims. We present new high sensitivity observations of the radio relic in A?521 carried out using the Giant Metrewave Radio Telescope at 327?MHz and with the Very Large Array at 4.9 and 8.5?GHz.Methods. We imaged the relic at these frequencies and carried out a detailed spectral analysis, based on the integrated radio spectrum between 235?MHz and 4.9?GHz, and on the spectral index image in the frequency range 327-610?MHz. In our present analysis we use our new GMRT observations in addition to proprietary and archival data. We search for a possible shock front co-located with the relic on a short archival Chandra X-ray observation of the cluster.Results. The integrated spectrum of the relic is consistent with a single power law; the spectral index image shows a clear trend of steepening going from the outer portion of the relic toward the cluster centre. We discuss the origin of the source in the light of theoretical models for the formation of cluster radio relics. Our results on the spectral properties of the relic are consistent with acceleration of relativistic electrons by a shock in the intracluster medium. This scenario is supported by our detection of an X-ray surface brightness edge coincident with the outer border of the radio relic. This edge is probably a shock front.

We report the discovery of a giant radio halo in the galaxy cluster RXC J1514.9-1523 at z= 0.22 with a relatively low X-ray luminosity, LX[0.1−2.4 kev]~ 7 × 1044erg s-1. This faint, diffuse radio source is detected with the Giant Metrewave Radio Telescope at 327 MHz. The source is barely detected at 1.4 GHz in a NVSS pointing that we have reanalyzed. The integrated radio spectrum of the halo is quite steep, with a slope α= 1.6 between 327 MHz and 1.4 GHz. While giant radio halos are common in more X-ray luminous cluster mergers, there is a less than 10% probability to detect a halo in systems with LX≲ 8 × 1044erg s-1. The detection of a new giant halo in this borderline luminosity regime can be particularly useful for discriminating between the competing theories for the origin of ultrarelativistic electrons in clusters. Furthermore, if our steep radio spectral index is confirmed by future deeper radio observations, this cluster would provide another example of the recently discovered population of ultra-steep spectrum radio halos, predicted by the model in which the cluster cosmic ray electrons are produced by turbulent reacceleration.