The Discovery of Host Galaxy HI Absorption in CTA 21
  We report the discovery of HI 21-cm absorption towards the well-studied GHz
Peaked-Spectrum source CTA 21 (4C 16.09) using the Arecibo Telescope on 2009
September 20 and 21. Recently, the frequency band between 700 and 800 MHz was
temporarily opened up to radio astronomy when US TV stations were mandated to
switch from analog to digital transmissions, with new frequency allocations.
The redshifted HI frequency for CTA 21 falls within this band. CTA 21 has a
complex radio structure on a range of scales. The innermost prominent
components are separated by ~12 mas while weak diffuse emission extends for up
to ~300 mas. The HI absorption profile that we find has two main components,
one narrow, the other wider and blue-shifted. The total HI column density is
7.9 x 10^20 cm^-2, assuming a covering factor of unity and a spin temperature
of 100 K. This HI absorption confirms the recently determined optical redshift
of this faint galaxy of z ~ 0.907. We discuss this new detection in the light
of HI absorption studies towards compact radio sources, and also the
possibility that CTA 21 may be exhibiting multiple cycles of nuclear activity.
This new detection in CTA 21 is consistent with a strong trend for detection of
HI absorption in radio galaxies with evidence of episodic nuclear/jet activity.
