2010ApJ...715L.117S

We report the discovery of H I 21 cm absorption toward the well-studied Gigahertz 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 H I 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 H I absorption profile that we find has two main components, one narrow and the other wider and blueshifted. The total H I column density is 7.9x10²⁰/cm2, assuming a covering factor of unity and a spin temperature of 100 K. This H I absorption confirms the recently determined optical redshift of this faint galaxy of z ∼ 0.907. We discuss this new detection in light of H I absorption studies toward 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 H I absorption in radio galaxies with evidence of episodic nuclear/jet activity.