The Structure of the Cold Neutral ISM on 10-100 Astronomical Unit Scales

We have used the Very Long Baseline Array (VLBA) and the Very Large Array (VLA) to image Galactic neutral hydrogen in absorption towards four compact extragalactic radio sources with 10 milliarcsecond resolution. Previous VLBA data by Faison et al (1998) have shown the existence of prominent structures in the direction of the extragalactic source 3C~138 with scale sizes of 10-20 AU with changes in HI optical depth in excess of 0.8 $\pm$ 0.1. In this paper we confirm the small scale \hi optical depth variations toward 3C~147 suggested earlier at a level up to 20 % $\pm$ 5% . The sources 3C~119, 2352+495 and 0831+557 show no significant change in \hi optical depth across the sources with one sigma limits of 30%, 50%, and 100%. Of the seven sources recently investigated with the VLBA and VLA, only 3C~138 and 3C~147 show statistically significant variations in HI opacities. Deshpande (2000) have attempted to explain the observed small-scale structure as an extension of the observed power spectrum of structure on parsec size scales. The predictions of Deshpande (2000) are consistent with the VLBA HI data observed in the directions of a number of sources, including 3C~147, but are not consistent with our previous observations of the HI opacity structure toward 3C~138

Changes in the measured image separation of the gravitational lens system, PKS 1830-211

We present eight epochs of 43 GHz, dual-polarisation VLBA observations of the gravitational lens system PKS 1830-211, made over fourteen weeks. A bright, compact ``core'' and a faint extended ``jet'' are clearly seen in maps of both lensed images at all eight epochs. The relative separation of the radio centroid of the cores (as measured on the sky) changes by up to 87 micro arcsec between subsequent epochs. A comparison with the previous 43 GHz VLBA observations (Garrett et al. 1997) made 8 months earlier show even larger deviations in the separation of up to 201 micro arcsec. The measured changes are most likely produced by changes in the brightness distribution of the background source, enhanced by the magnification of the lens. A relative magnification matrix that is applicable on the milliarcsecond scale has been determined by relating two vectors (the ``core-jet'' separations and the offsets of the polarised and total intensity emission) in the two lensed images. The determinant of this matrix, -1.13 +/-0.61, is in good agreement with the measured flux density ratio of the two images. The matrix predicts that the 10 mas long jet, that is clearly seen in previous 15 and 8.4 GHz VLBA observations (Garrett et al. 1997, Guirado et al. 1999), should correspond to a 4 mas long jet trailing to the south-east of the SW image. The clear non-detection of this trailing jet is a strong evidence for sub-structure in the lens and may require more realistic lens models to be invoked, e.g. Nair & Garrett (2000).Comment: 8 pages, 5 figure

Self-association of complement factor H in the presence and absence of metals

In the complement system of innate immunity, factor H (FH) is a major regulator of its activation, and is comprised of 20 short complement regulator (SCR) domains. FH is related to age-related macular degeneration (AMD) through a Tyr402His polymorphism, and occurs in drusen deposits that are a key feature of early AMD. High concentrations of zinc are also present in drusen, and the function of FH is inhibited by zinc. In this thesis, FH self-association and its interaction with zinc were investigated. Using X-ray solution scattering and analytical ultracentrifugation, pooled heterozygous FH in physiological conditions self-associated to form 10-15 % of dimer and higher oligomers. Titrations of FH with zinc induced uncontrolled oligomerisation of FH when the zinc concentration was above 20 micromolar, and this correlated with the reduction of FH activity. Structurally distinct large oligomers were also observed for Cu, while Ni, Cd and Fe showed low amounts of oligomers, and Mg and Ca showed no change. These experiments were repeated for the native Tyr402 allotype and the disease-related His402 allotype of FH. X-ray scattering combined with constrained modelling showed that the homozygous Tyr402 and His402 allotypes of FH have indistinguishable foldedback structures in solution. Both homozygous allotypes of full-length FH exhibit similar self-association properties in solution, showing no dependence on heterozygosity. Surface plasmon resonance confirmed these results, but showed that the His402 allotype of SCR-6/8 self-associates more than the Tyr402 allotype. Zinc titrations of the two FH allotypes showed that each allotype formed similar oligomers with zinc. While the major interaction sites of FH with zinc were located within the SCR-6/8 fragment, the surface exposed His402 residue in SCR-6/8 showed no preferential interactions with zinc. Overall, these findings provide insight on the development of drusen deposits in Bruch’s membrane and the uncontrolled inflammation associated with AMD

Helical motions in the jet of blazar 1156+295

The blazar 1156+295 was observed by VLBA and EVN + MERLIN at 5 GHz in June 1996 and February 1997 respectively. The results show that the jet of the source has structural oscillations on the milliarcsecond scale and turns through a large angle to the direction of the arcsecond-scale extension. A helical jet model can explain most of the observed properties of the radio structure in 1156+295.Comment: 6 pages, 2 figures, to appear in New Astronomy Reviews (EVN/JIVE Symposium No. 4, special issue