Astron. Astrophys. 354, L45-L48 (2000)

3. Results

The continuum image obtained of NGC 4261 is shown in Fig. 1. The noise level in this image is 0.35 mJy/beam. We find that the Western jet is somewhat brighter and more extended than the Eastern one, consistent with the VLBA maps of Jones & Wehrle (1997). From these VLBA maps it is clear that the flat spectrum core lies close to the peak of Fig. 1. We were able to fit a three component Gaussian model to the continuum visibility data, consisting of one compact core component and two jet components 18 and 14 mas (2.5 and 2 pc) to the East and West respectively.

Fig. 1. Continuum image of NGC 4261 at 21cm obtained with the EVN. The beam size is mas. The contours are plotted at levels 1, 2, 4, 8, 16, 32 and 64 mJy/beam, with a map peak intensity of 79 mJy/beam. The Eastern counterjet side is only slightly weaker at this frequency.

In order to detect the weak H I absorption the spectral line data was self-calibrated with the continuum image, and then the continuum was subtracted using the AIPS task UVLIN. The spectral absorption was unambiguously detected on the Jodrell Bank - Westerbork baseline (Fig. 2). Other baselines did not have sufficient sensitivity to give any detections. From the phase information (not shown) it is clear that the absorption is not centred on the reference position of the self-cal process; but is offset from the core. The sign of the phase on the Jodrell Bank - Westerbork baseline suggests that the absorption is preferentially on the Eastern (counterjet) side. Fig. 2 shows the absorbed flux density integrated at the supposed position of the main counterjet component located 18 mas to the East of the core (see below).

Fig. 2. Absorbed flux density for the Jodrell Bank - Westerbork baseline integrated 18 mas to the East of the map centre, i.e. at the counterjet side. The continuum has been subtracted. One spectral channel corresponds to 27 km/s.

From the VLA spectrum presented in the Jaffe & McNamara (1994) paper, we estimate a total integrated absorbed flux density of mJy km/s, compared to the corresponding number for our VLBI spectrum; mJy km/s. The amount of VLBI scale absorption is therefore consistent with the VLA observations. Although we cannot exclude the possibility of additional H I absorbing gas on scales larger than sampled by the VLBI observations, we feel confident that we detect the bulk of the absorbing gas.

In making quantitative estimates of the opacity toward different source components we applied a model-fitting technique based on the three component model used to fit the continuum data. We first averaged the Jodrell - Westerbork spectral absorption data in frequency over the line width and then fitted the resulting phase and amplitude versus time using a three component Gaussian model based on the continuum model. Each Gaussian had the same fixed shape and position as that fitted to the continuum data, only the amplitude of each component was allowed to vary. The minimised is achieved when most of the absorption is on the counterjet, a possible small absorption at the core and no absorption against the jet component. Fixing the jet absorption at zero we obtained the -landscape shown in Fig. 3 for different combinations of counterjet and core absorption. From this we estimate the absorbed counterjet and core flux densities averaged over the line to be mJy and mJy respectively.

Fig. 3. The -landscape achieved when varying the amount of absorbed flux over the counterjet and the core. The best fitting point has . Number of degrees of freedom=35.

Dividing by the continuum flux densities of each component from the absorbed fluxes we can estimate line-averaged opacities of and against the counterjet and core respectively. It therefore appears that virtually all of the absorbing gas is against the counterjet. Integrating the opacity over the line we estimate a total H I column density towards the counterjet of and towards the core.

© European Southern Observatory (ESO) 2000

Online publication: January 31, 2000
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