Feeding the Black Hole (2) ...
Even when optical images do not indicate that galaxies have interacted, information at other
wavelengths may tell a different story.
The adjacent right figure shows a Very Large Array (VLA) radio observation of neutral hydrogen
in a cluster of galaxies containing a quasar.
Because this is a relatively nearby quasar
(its redshift is a little larger than 0.05, implying a distance of about 240 Mpc by the Hubble
law), its
parent galaxy and surrounding galaxies of the cluster can be seen clearly.
Neutral hydrogen is invisible except to radio
telescopes. In the image the optical images of the galaxies are shown in blue and white and
the radio frequency signal indicating neutral hydrogen in orange. The optical
image gives little evidence that anything unusual has happened in this cluster.
However, the
extension and distortion
of the neutral hydrogen envelopes suggests that the quasar and the galaxy have
interacted in the past. It is possible that this interaction is related to the
present activity of the quasar.
