HI 21cm absorption in Cygnus A

These results came from an observation of Cygnus A by John Conway and myself in the Spring of 1994, at the Very Large Array radio telescope near Socorro, New Mexico.

Compare these two radio images of Cygnus A:

Now you see it... 22cm continuum radio image.

This was made from wavelengths on either side of the atomic hydrogen line. The radio core of Cygnus A is faint but easily visible in this image, midway between between the two giant lobes.

...now you don't! Redshifted 21cm HI line image.

Although the radio lobes appear similar to the image above, the compact radio core has almost vanished - a clear sign of absorption by neutral gas around the nucleus of Cygnus A.

(Due to the narrower frequency band used, this image is more noisy than the continuum image above).

See an animated version of this comparison.

How does the HI absorption change with wavelength?

The absorption spectrum below shows how the radio intensity from the compact nucleus is reduced by the intervening neutral hydrogen as we scan through the redshifted HI line frequency (here expressed as a velocity relative to the redshift measured in the visible, 0.0565). We found a double-peaked structure to the absorption, which suggests that the neutral hydrogen may be in a disk of neutral gas rotating around a central radio jet.

HI 21cm spectrum of the nucleus of Cygnus A, taken with the VLA in March 1994. Synthesized beam is 1.3 arcsec, spectral sampling is 11.5 km/s. The absorption due to atomic hydrogen in front of the radio core is clearly visible. Dashed and solid lines show single and double gaussian model profiles fitted to the data.

For more information:

Cygnus A

Philip Blanco's WWW page