Clusters of galaxies are the largest gravitationally bound objects in the Universe, containing about 10^15 solar masses of hot (10^8 K) gas, galaxies and dark matter in a typical volume of about 10 Mpc^3. Magnetic fields and relativistic particles are mixed with the gas as revealed by giant radio haloes, which arise from diffuse, megaparsec-scale synchrotron radiation at cluster center. Radio haloes require that the emitting electrons are accelerated in situ (by turbulence), or are injected (as secondary particles) by proton collisions into the intergalactic medium. They are found only in a fraction of massive clusters that have complex dynamics, which suggests a connection between these mechanisms and cluster mergers. Here we report a radio halo at low frequencies associated with the merging cluster Abell 521. This halo has an extremely steep radio spectrum, which implies a high frequency cut-off; this makes the halo difficult to detect with observations at 1.4 GHz (the frequency at which all other known radio haloes have been best studied). The spectrum of the halo is inconsistent with a secondary origin of the relativistic electrons, but instead supports turbulent acceleration, which suggests that many radio haloes in the Universe should emit mainly at low frequencies.Comment: 18 pages, 4 figures, Nature 455, 94

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We present the results of a recent re-reduction of the data from the Very Large Array (VLA) Low-frequency Sky Survey (VLSS). We used the VLSS catalog as a sky model to correct the ionospheric distortions in the data and create a new set of sky maps and corresponding catalog at 73.8 MHz. The VLSS Redux (VLSSr) has a resolution of 75 arcsec, and an average map RMS noise level of σ ∼ 0.1 Jy beam −1 . The clean bias is 0.66 × σ and the theoretical largest angular size is 36 arcmin. Six previously unimaged fields are included in the VLSSr, which has an unbroken sky coverage over 9.3 sr above an irregular southern boundary. The final catalog includes 92,964 sources. The VLSSr improves upon the original VLSS in a number of areas including imaging of large sources, image sensitivity, and clean bias; however the most critical improvement is the replacement of an inaccurate primary beam correction which caused source flux errors which vary as a function of radius to nearest pointing center in the VLSS.

We present optical and near-infrared ground-based imaging results on four low-redshift damped Lyα (DLA) galaxies. The corresponding DLA systems were discovered in our Hubble Space Telescope spectroscopic surveys for DLA lines in known strong Mg II absorption-line systems toward the quasars B2 0827+243 (z DLA = 0.525), PKS 0952+179 (z DLA = 0.239), PKS 1127−145 (z DLA = 0.313), and PKS 1629+120 (z DLA = 0.532). Two of the four DLA galaxies have confirmed slit redshifts, one has a photometric redshift consistent with the absorption-line redshift, and the fourth identification is based on the galaxy's proximity to the quasar sight line. The DLA galaxies span a mixture of morphological types from patchy, irregular, and low surface brightness to spiral galaxies. The luminosities range from 0.02L * K to 1.2L * K . We also discovered several extremely red objects (EROs) in two of these fields and discuss the possibility that they are associated with the DLA galaxies. These observations add to the small but growing list of DLA galaxies at low redshift. At the present time, 14 DLA galaxies in the redshift range 0.05 z 1 have been studied. The distributions of DLA galaxy properties for these 14 cases are discussed and some important trends emerge. Low-luminosity dwarf galaxies with small impact parameters dominate this small sample. Also, four of the five highest column density systems, which dominate in the determination of the cosmological neutral gas mass density, arise in low surface brightness dwarf galaxies. Zwaan et al. have shown that only 15% of the neutral gas at the present epoch is contained in low surface brightness galaxies. Thus, if the low-redshift DLA galaxy trends hold up with larger samples, it would indicate that a different population of objects is responsible for the bulk of the neutral hydrogen gas in the universe at z ≈ 0.5.

We report results from a program aimed at investigating the temperature of neutral gas in highredshift damped Lyman-α absorbers (DLAs). This involved (1) HI 21cm absorption studies of a large sample of DLAs towards radio-loud quasars, (2) very long baseline interferometric studies to measure the low-frequency quasar core fractions, and (3) optical/ultraviolet spectroscopy to determine DLA metallicities and the velocity widths of low-ionization metal lines.Including literature data, our sample consists of 37 DLAs with estimates of the harmonicmean spin temperature T s . We find a statistically significant (4σ) difference between the T s distributions in the high-z (z > 2.4) and low-z (z < 2.4) DLA samples. The high-z sample contains more systems with high spin temperature, T s 1000 K. The T s distributions in DLAs and the Galaxy are also significantly (≈ 6σ) different, with more high-T s sightlines in DLAs than in the Milky Way. The high T s values in the high-z DLAs of our sample arise due to low fractions of the cold neutral medium (CNM). Only two of 23 DLAs at z > 1.7 have T s values indicating CNM fractions > 20%, comparable to the median value (≈ 27%) in the Galaxy.We tested whether the HI column density measured towards the optical quasar might be systematically different from that towards the radio core by comparing the HI column densities inferred from HI 21cm emission studies at different spatial resolutions (≈ 15 pc −1 kpc) in the Large Magellanic Cloud. The high-resolution N HI values are, on average, larger than the smoothed ones for N HI > 10 21 cm −2 , but lower than the smoothed N HI estimates for N HI < 10 21 cm −2 . Since there are far more DLAs with low N HI values than high ones, the use of the optical N HI value for the radio sightline results in a statistical tendency to under-estimate DLA spin temperatures.For 29 DLAs with metallicity estimates, we confirm the presence of an anti-correlation between T s and metallicity [Z/H], at 3.5σ significance via a non-parametric Kendall-tau test. This result was obtained with the assumption that the DLA covering factor is equal to the core fraction. However, Monte Carlo simulations show that the significance of the result is only marginally decreased if the covering factor and the core fraction are uncorrelated, or if there is a random error in the inferred covering factor.We also find statistically significant evidence for redshift evolution in DLA spin temperatures even for the DLA sub-sample at z > 1. Since all DLAs at z > 1 have angular diameter distances comparable to or larger than those of their background quasars, they have similar efficiency in covering the quasars. We conclude that low covering factors in high-z DLAs cannot account for the observed redshift evolution in spin temperatures.