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We have mapped the molecular clouds of Sgr B2 in the 110 → 101 ortho-transition of C3H2 at 18.3 GHz, using the 70-m NASA telescope at Tidbinbilla (beamwidth 55 arcsec). Three clouds show absorption against the Sgr B2 continuum emission at radial velocities of 50, 65 and 80 km s−1. The 65-km s−1 cloud covers most of the observed area (4 × 6 arcmin in right ascension and declination), has a peak optical depth of 2.7 and a corresponding C3H2 column density of 7.6 × 1015 cm−2. The C3H2 fractional abundance relative to H2 is 1.5 × 10−9. The 80-km s−1 cloud, located north of the Sgr B2 continuum peak, has a peak optical depth of 0.9 and a C3H2 column density of 1.9 × 1015 cm−2. The 50-km s−1 cloud is centred 2 arcmin south of the continuum peak; here the minimum optical depth of 0.5 yields a column density of 5.3 × 1014cm−2.
We have measured the 2.3 GHz total and correlated flux densities on a baseline of 275 km of all sources in the Parkes catalogue which
(i)are south of declination +10°,
(ii)have a catalogued 2.7 GHz total flux density exceeding 0.5 Jy, and
(iii)have a 2.7/5.0 GHz spectral index flatter than −0.5.
More than 14% of the sample showed visibility amplitudes greater than 0.9, and more than 72% showed visibility amplitudes greater than 0.5. Of the sources with optical or other identifications 79% were quasars. In this paper we briefly summarise the results of this survey.
NGC 253 is the major member of the Sculptor group of galaxies. It is classified SAB(s)c by de Vaucouleurs, de Vaucouleurs and Corwin (1976) and is almost edge-on. Previous studies have revealed a peculiar velocity field near the nucleus (e.g. Ulrich 1978). The nucleus is also a moderately strong radio and infrared source (Beck et al. 1979, Klein and Emerson 1981, Elias et al. 1978, Telesco and Harper 1980, Rieke et al. 1980).
Young-of-the-year bass (Dicentrarchus labrax) were sampled from ten Bristol Channel sites during 1996, 1997 and 1998 to investigate whether the pattern of recruitment to south Wales nurseries observed following the ‘Sea Empress’ oil spill in spring 1996 was anomalous. Back-calculated egg fertilization (spawned) dates indicated that early spawning events (February–March) at the Trevose Head spawning area provide recruits to south Bristol Channel nurseries, and that the majority of recruits to south Wales nurseries arise from subsequent spawning events. Recruitment of post-larvae to all nurseries in the Bristol Channel starts in late June–early July. The late recruitment of 0-group bass to all nurseries in 1996 was attributed to lower water temperatures in February and March than in 1997 and 1998, and was not restricted to south Wales. However, 0-group bass were relatively less abundant in 1996 at sites nearest the ‘Sea Empress’ oil spill, suggesting a possible effect of the latter on survival of post-larvae in the nurseries, rather than during the offshore egg and larval stage.
Wurzite GaN epilayers on sapphire substrates usually suffer from biaxial compressive strain due to the mismatches of the thermal expansion coefficients and the lattice constants between GaN layers and sapphire substrates. We have investigated the layer thickness effects on strain and transition energies by photoluminescence (PL), photoreflectance (PR) and X-ray diffraction (XRD). Samples used in this study are grown by hydride vapor phase epitaxy (HVPE) and have the layer thickness of 0.76, 2.6, 5.3 and 48 m. The PL and PR spectra showed the redshift of the transition energies with increasing layer thickness. This is attributed to strain-induced energy shift. The layer thickness dependence of strains is directly observed by XRD. The strain along the c -axis (εzz) decreased with increasing layer thickness. This indicates the strain is relaxed with layer thickness. From strain variation with layer thickness, we suggest that strain relaxation process is rapid at the initial stage of growth and becomes slower as the layer grows. The full width at half maximum (FWHM) of PL spectra and theta rocking curves decrease with increasing layer thickness. This indicates the crystal quality improves as the strain is reduced. Since the strain effect is very small at the layer thickness of 48 μm, we expect zero strain for thicker layers that can potentially be used as substrates for homoepitaxy.
The first VLBI images of H2O maser emission in the Circinus Galaxy AGN show both a warped, edge-on accretion disk and an outflow 0.1 to 1 pc from the central engine. The inferred central mass is 1.3 × 106 M⊙, while the disk mass may be on the order of 105 M⊙, based on a nearly Keplerian rotation curve. The bipolar, wide-angle outflow appears to contain “bullets” ejected from within < 0.1 pc of the central mass. The positions of filaments and bullets observed in the AGN ionization cone on kpc-scales suggest that the disk channels the flow to a radius of ∼ 0.4 pc, at which the flow appears to disrupt the disk.
We describe the expected distribution of intensity for a scintillating source of finite size observed through a scattering medium, including systematic and instrumental effects. We describe measurements of the size of the Vela pulsar, using this technique.
The link of the Hipparcos and VLBI extragalactic reference frames has been achieved with a precision of 0.0005″ in global orientation at the epoch of the catalogue (1991.25) and of 0.0003″/yr in rate of rotation by VLBI observations of 12 radio-emitting stars.
We have undertaken VLBI observations of 8 Southern Hemisphere EGRET radio sources. Using our data as well as data obtained from the literature we have examined the difference in radio properties between gamma-ray loud and gamma-ray quiet radio sources. In particular, we find no evidence that gamma-ray loud radio sources lie preferentially in sources with straight radio jets as has been suggested.
We have observed the nucleus of Centaurus A with the VLBA and the SHEVE arrays, individually and in combination. In sensitive experiments at 8.4 GHz which use both arrays together we have detected components in a sub-parsec-scale counterjet. The counterjet is also seen in 2.3 and 22 GHz images. The nuclear region appears to be partially obscured by a free-free absorbing structure of about 1 pc in extent. The jet axis appears to lie at a 50°–70° angle to our line of sight.
We have been observing the sub-parsec-scale radio jet in the nucleus of Centaurus A, the closest active radio galaxy, at 8.4 GHz over the last 6 years with the SHEVE array and most recently with the VLBA at 8.4 and 22.2 GHz. In this paper we will review the results of these observations and give a brief summary of our interpretation, concentrating on the evolution of the sub-parsec-scale jet structure with time; subluminal component motions and rapid short timescale evolution. A full description of this work is soon to be published in a major journal article.
We present high-resolution radio observations of the second Galactic superluminal radio source GRO1655-40, which was detected as an X-ray transient on 1994 July 27. Our radio radio images reveal two components moving away from each other at an angular speed of 65±5 mas/day, corresponding to superluminal motion (υ/c = 1.4 ± 0.4) at the estimated distance of 3–5 kpc. The 12-day delay between the X-ray and radio outbursts suggests that the ejection of material at relativistic speeds occurs during a stable phase of accretion onto a black hole, which follows an unstable phase with a high accretion rate. A complete description and discussion of these observations can be found in Tingay et al 1995 (Nature, 374, pp 141–143).
The remarkably strong radio gravitational lens PKS 1830-211 consists of a one arcsecond diameter Einstein ring with two bright compact components located on opposite sides of the ring. We have obtained 22 GHz VLBA data on this source to determine the intrinsic angular sizes of the compact components. Previous VLBI observations at lower frequencies indicate that the brightness temperatures of these components are significantly lower than 1010 K (Jauncey et al. 1991), less than is typical for compact synchrotron radio sources and less than is implied by flux density variations. A possible explanation is that interstellar scattering is broadening the apparent angular size of the source and thereby reducing the observed brightness temperature. Our VLBA data support this hypothesis.
Here we will describe briefly some of the VLBI observations we are making of low-redshift, compact radio sources in the southern hemisphere, using the Southern Hemisphere VLBI Experiment (SHEVE) array of telescopes (Jauncey et al., 1994).
Centaurus A (NGC 5128) is the nearest giant radio galaxy. It is a Fanaroff-Riley type 1 (low luminosity) radio source, but the compact radio source in the nucleus is strong enough that VLBI imaging has been possible with both the SHEVE array and the VLBA at several frequencies. These observations have detected a sub-parsec scale counterjet. This shows that jet formation in at least some FR I sources is intrinsically two-sided over very small distances and the radio jets in Centaurus A are probably only moderately relativistic. We also find evidence that the center of activity in Centaurus A is partially obscured by a disk or torus of dense plasma.
We have collected multi-waveband (radio, optical and X-ray) data for a complete sample of southern radio sources. The sample includes 88 objects selected from the Wall & Peacock (1985) catalogue that is complete down to S2.7GHz = 2 Jy, δ < 10° and the z < 0.7. This database (Tadhunter et al. (1993), Morganti et ai. (1993), and Siebert et al. these Proceedings) provides an important tool for investigating the nature of anisotropies and orientation effects in AGN and the physical causes of the correlation between their emission at different frequencies.
Centaurus A is the closest active extragalactic radio source, at a distance of approximately 3.5 Mpc, and is identified with the peculiar elliptical galaxy NGC 5128. As such it is a very important target for observations of the small-scale (sub-parsec) and large-scale (kpc) structures in extragalactic jets. Here we present Mk-II VLBI observations made at 8.4 GHz over a 4.3 year period from early 1991 until mid-1995, as well as a 4.8 GHz observation that was co-eval with one of the 8.4 GHz observations. All of the observations were made with the SHEVE array except for the last observation which was made with the VLBA. The dual-frequency observations identify the core of the radio source, while the multi-epoch observations show the complex structural evolution at a resolution of 0.1 pc. Subluminal motion of ≈ 0.15c is evident. Structural changes are observed on time scales shorter than four months.
A sample of 38 southern peaked-spectrum radio sources from the Parkes Catalogue have been observed using single-baseline VLBI. Thirty three objects were successfully detected on baselines of > 30Mλ at 2.3 GHz. For 21 of these sources, the flux density in the compact components contributes more than half the total flux density of the radio emission. Twenty sources showed structure more complex than a point-source.
A comparison of accurate optical and radio positions is presented for 25 southern compact radio objects. There is a systematic “S” shaped trend indicating a zonal bias in the Perth 70 optical catalogue.