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H I absorption was detected in all three galaxies close to the frequency expected from the optical redshifts. Morganti et al. (2000) have independently detected H I absorption in PKS 1814 -637. No emission was detected (or was expected because of the redshifts). For PKS 1353-341 and PKS 1814-637, the absorption was both deep and of large velocity width. The peak optical depths were 0.12 and 0.21, respectively; the rest-frame velocity dispersions of the main absorption line were 66 and 19 km s-1, respectively; and the FWZI velocity extents were 496 and 372 km s-1, respectively. For PKS 1934-638, the optical depth was about two orders of magnitude lower, $\tau$=0.0022, and the velocity dispersion of the absorption line was low, $\sigma$=8 km s-1. Although very weak, the H I absorption for PKS 1934-638 was confirmed in both the ATCA and LBA observations. The results are summarized in table [*].

Fig. [*] shows the H I absorption spectra of the three sources.

Figure:  HI absorption spectra of the radio sources 1353-341, 1814-637 and 1934-638. The data for 1353-341 and 1814-63 are from the ATCA. The data for 1934-638 are a weighted combination from the ATCA and LBA. The frequency axis is heliocentric. The arrows indicate the position of the HI line corresponding to the optical redshift.

\resizebox {9.4cm}{6cm}{\includegraphics{hiplot.eps}}

The observations suggest that large quantities of H I must be present in 2 of the 3 galaxies: PKS 1353-341 and PKS 1814-637. Assuming a spin temperature T$_{\rm S}\approx$ 102 K, the average column densities in front of the continuum sources are $21\times10^{20}$ and $9\times10^{20}$ atoms cm-2, respectively. For 1934-638, the column density is N$_{\rm H I}$(100)=$0.06\times10^{20}$ atoms cm-2. This could indicate little neutral gas belonging to PKS 1934-638 in our line-of-sight. However, much higher column densities may be present if either: (a) the gas is warm ($T_{\rm S}$ could be $\sim 5000$ K before the thermal linewidth approaches $\sigma$ or more if radiative excitation of the spin levels is important); or (b) if there is a very extensive neutral gaseous halo. For example, Véron-Cetty et al. (1995) found a massive (3.1 1010 M$_{\odot}$) H I disk around the nearby compact flat-spectrum radio galaxy PKS 1718-649, whilst the optical depths of the two absorption components in this galaxy were only 0.003 and 0.005, similar to PKS 1934-638.

The Compact Array observations confirmed that all sources were unresolved to a maximum baseline length of 6 km, implying angular extents less than 5 arcsec. Further, the LBA observations showed that PKS 1934-638 has a similar flux density on all baselines up to 322 km, implying that it is unresolved (<01) to the accuracy of our calibration. This is in agreement with previous observations showing it to be a 42 milliarcsec double radio source (Tzioumis et al. 1989).

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Next: Discussion Up: Neutral Hydrogen Observations Previous: LBA Observations