Christopher John Salter

Compact steep-spectrum sources from the S4 sample

We present the results of 5-GHz observations with the VLA A-array of a sample of candidate compact steep-spectrum (CSS) sources selected from the S4 survey. We also estimate the symmetry parameters of high-luminosity CSS sources selected from different samples of radio sources, and compare these with the larger sources of similar luminosity to understand their evolution and the consistency of the CSS sources with the unified scheme for radio galaxies and quasars. The majority of CSS sources are likely to be young sources advancing outwards through a dense asymmetric environment. The radio properties of CSS sources are found to be consistent with the unified scheme, in which the axes of the quasars are observed close to the line of sight, while radio galaxies are observed close to the plane of the sky.

Search for cold gas in z > 2 damped Lyα systems: 21-cm and H2 absorption

(Abridged) We present the results of a systematic GBT and GMRT survey for 21-cm absorption in a sample of 10 DLAs at 2 2 is predominantly constituted by WNM. The detection rate of 21-cm absorption seems to be higher for systems with higher N(HI) or metallicity. However, no clear correlation is found between the integrated 21-cm optical depth (or spin temperature) and either N(HI), metallicity or velocity spread of the low ionization species. There are 13 DLAs in our sample for which high resolution optical spectra covering the expected wavelength range of H_2 absorption are available. We report the detection of H_2 molecules in the z_abs = 3.3871 21-cm absorber towards J0203+1134 (PKS 0201+113). In 8 cases, neither H_2 nor 21-cm absorption are detected. The lack of 21-cm and H_2 absorption in these systems can be explained if most of the HI in these DLAs originate from low density high temperature gas. In one case we have a DLA with 21-cm absorption not showing H_2 absorption. In two cases, both species are detected but do not originate from the same velocity component. In the remaining 2 cases 21-cm absorption is not detected despite the presence of H_2 with evidence for the presence of cold gas. All this is consistent with the idea that the H_2 components seen in DLAs are compact (with sizes of < 15 pc) and contain only a small fraction (i.e typically <10%) of the total N(HI) measured in the DLAs.

Late-Time Observations of the Afterglow and Environment of GRB 030329

We present Very Long Baseline Interferometry (VLBI) observations 217 days after the {gamma}-ray burst of 2003 March 29. These observations provide further measurements of the size and position of GRB 030329 that are used to constrain the expansion rate and proper motion of this nearby GRB. The expansion rate appears to be slowing down with time, favoring expansion into a constant density interstellar medium, rather than a circumstellar wind with an r{sup -2} density profile. We also present late time Arecibo observations of the redshifted Hi and OH absorption spectra towards GRB 030329. No absorption (or emission) is seen allowing us to place limits on the atomic neutral hydrogen of N{sub H} < 8.5 x 10{sup 20} cm{sup -2}, and molecular hydrogen of N{sub H{sub 2}} < 1.4 x 10{sup 22} cm{sup -2}. Finally, we present VLA limits on the radio polarization from the afterglow of <2% at late times.

Discovery of New Faint Radio Emission on 8° to 3' Scales in the Coma Field, and Some Galactic and Extragalactic Implications

We present a deep, 8 ◦ diameter, 0.4 GHz radio image using a first time combination of the NAIC Arecibo 305-m telescope in Puerto Rico, and the wide-angle interferometer at the Dominion Radio Astrophysical Observatory at Penticton, Canada. Our observations are centered on the Coma Cluster of galaxies in the “Great Wall” of galaxies near the North Galactic Pole. The complementary nature of these two instruments enables us to produce a distortion-free image that is sensitive to radiation on scales from 8 ◦ down to that of an individual galaxy halo at the 100 Mpc distance of the Great Wall. Newly revealed patches of distributed radio “glow” are seen well above the detection limit. One prominent such area coincides with groupings of radio galaxies near the Coma cluster, and indicates intergalactic IGM magnetic fields in the range 0.2 to 0.4 µG on scales of up to ∼ 4Mpc. Other patches of diffuse emission, not previously explored at these high latitudes on arcminute scales, probably contain Galactic “cirrus”. A striking anticorrelation is found between low-level diffuse radio glow and some regions of enhanced optical galaxy surface density, suggesting that cosmological Large Scale Structure (LSS), normally defined by the baryonic (or dark) matter density, is not uniquely traced by faint continuum radio glow. Rather, intergalactic diffuse synchrotron radiation represents IGM magnetic and Cosmic ray energy density, instead of matter density. The diffuse, arcminute-level structures over a large region of sky are potentially important pathfinders to CMB foreground radiation on high multipole scales.

The Repeating Fast Radio Burst FRB 121102 as Seen on Milliarcsecond Angular Scales

The millisecond-duration radio flashes known as fast radio bursts (FRBs) represent an enigmatic astrophysical phenomenon. Recently, the sub-arcsecond localization (∼100 mas precision) of FRB 121102 using the Very Large Array has led to its unambiguous association with persistent radio and optical counterparts, and to the identification of its host galaxy. However, an even more precise localization is needed in order to probe the direct physical relationship between the millisecond bursts themselves and the associated persistent emission. Here, wereport very-long-baseline radio interferometric observations using the European VLBI Network and the 305 m Arecibo telescope, which simultaneously detect both the bursts and the persistent radio emission at milliarcsecond angular scales and show that they are co-located to within a projected linear separation of ≲40 pc (≲12 mas angular separation, at 95% confidence). We detect consistent angular broadening of the bursts and persistent radio source (∼2–4 mas at 1.7 GHz), which are both similar to the expected Milky Way scattering contribution. The persistent radio source has a projected size constrained to be ≲ 0.7 pc (≲0.2 mas angular extent at 5.0 GHz) and a lower limit for the brightness temperature of Tb ≳ 5× 107 K. Together, these observations provide strong evidence for a direct physical link between FRB 121102 and the compact persistent radio source. We argue that a burst source associated with a low-luminosity active galactic nucleus or a young neutron star energizing a supernova remnant are the two scenarios for FRB 121102 that best match the observed data.

RADIOASTRON OBSERVATIONS OF THE QUASAR 3C273: A CHALLENGE TO THE BRIGHTNESS TEMPERATURE LIMIT

Inverse Compton cooling limits the brightness temperature of the radiating plasma to a maximum of

CSSs in a sample of B2 radio sources of intermediate strength

10^{11.5}

Milli-arcsecond scale rotation measure in the CSS quasars 0548+165 and 1524 136

K. Relativistic boosting can increase its observed value, but apparent brightness temperatures much in excess of

THE DISCOVERY OF HOST GALAXY H I ABSORPTION IN CTA 21

10^{13}

A Radio Spectral Line Study of the 2 Jy IRAS-NVSS Sample. I

K are inaccessible using ground-based very long baseline interferometry (VLBI) at any wavelength. We present observations of the quasar 3C273, made with the space VLBI mission RadioAstron on baselines up to 171,000 km, which directly reveal the presence of angular structure as small as 26