N. E. B. Killeen

Analysis of Zeeman effect data in radio astronomy

The analysis of Zeeman effect data in radio astronomy is discussed; in particular, previous techniques are extended to include the case of low signal-to-noise ratios. Three statistical techniques for estimating the line-of-sight magnetic field are considered: maximum likelihood, least-squares, and Wiener filters. For high signal-to-noise ratios, all three estimators are essentially unbiased. It is concluded that, in the poor to moderate signal-to-noise ratio regime, all three estimators are biased; the maximum likelihood technique yields results that are, in general, substantially less biased than least-squares and Wiener filters. However, it is possible to debias the least-squares results and obtain estimates that are as good as maximum likelihood under a restricted set of conditions. 20 refs.

Cosmological Halos: A Search for the Ionized Intergalactic Medium

Standard big bang nucleosynthesis predicts the average baryon density of the universe to be a few percent of the critical density. Only about one-tenth of the predicted baryons have been seen. A plausible repository for the missing baryons is in a diffuse ionized intergalactic medium (IGM). In an attempt to measure the IGM, we searched for Thomson-scattered halos around strong high-redshift radio sources. Observations of the radio source 1935-692 were made with the Australia Telescope Compact Array. We assumed a uniform IGM, and isotropic steady emission of 1935-692 for a duration between 107 and 108 yr. A model of the expected halo visibility function was used in χ2 fits to place upper limits on ΩIGM. The upper limits varied depending on the methods used to characterize systematic errors in the data. The results are 2 σ limits of ΩIGM < 0.65. While not yet at the sensitivity level to test primordial nucleosynthesis, improvements in the technique will probably allow this in future studies.

Faint 1.4-GHz radio sources in the 2dF Galaxy Redshift Survey

We have used the Australia Telescope Compact Array (ATCA) to search for faint radio sources in a ∼3 deg 2 region of sky covered by the 2dF Galaxy Redshift Survey (2dFGRS). Over the region surveyed, the 1σ noise level at 1.4 GHz ranges from 20 μJy to 1 mJy. The survey region includes 365 2dFGRS galaxies, of which 316 have good-quality spectra (176 early-type galaxies or active galactic nuclei, and 140 star-forming galaxies). The fraction of 2dFGRS galaxies detected as radio sources in our survey rises from ∼4 per cent at a 3σ detection limit of 0.3 mJy to 12 per cent at 75 μJy, with roughly equal numbers of star-forming galaxies and active galactic nuclei (AGNs) being detected. The radio luminosity function derived from this sample agrees with earlier determinations, and we find that while the mean redshift of the radio-detected galaxies increases toward lower flux densities for star-forming galaxies, it remains roughly constant for the AGN/early-type radio detections. We discuss possible observing strategies for a submJy radio survey of a larger 2dFGRS region, and show that a survey covering 100 deg 2 could measure the global evolution of the star formation density to z ∼ 0.1 in a way which is less affected by reddening than traditional measurements using optical emission lines and ultraviolet photometry.

HI absorption in Radio Galaxies

Twenty-two powerful radio galaxies have been searched for HI absorption. We find the highest probability of detecting HI in absorption among narrow-line compact (or small) galaxies or galaxies with indication of richer interstellar medium (i.e. with ongoing or recent star-formation). We discuss the difficulty in the interpretation of the origin of the HI absorption due to the uncertainty in the systemic velocity of the galaxies.

A Powerful Jet/Cloud Interaction in the Radio Galaxy PKS 2250-41

Extended emission line regions aligned with the radio axis are a common feature of powerful radio galaxies and there is much interest in the origin of the extended gas and excitation mechanism. One model that can produce this alignment is photoionization by anisotropic nuclear continuum radiation. However, strong evidence exists, especially in high redshift radio galaxies, for powerful interactions between the relativistic radio jets and the ISM/IGM. Here we present the results of our study of the southern radio galaxy PKS 2250-41 (z = 0.308). This object is the most spectacular found in a sample of southern radio sources studied by Tadhunter et al. (1993) and it displays particularly clear evidence for such an interaction (Tadhunter et al. 1994; Dickson et al. 1995).