The Discovery of Host Galaxy HI Absorption in CTA 21

TL;DR

This paper reports the first detection of HI 21-cm absorption in CTA 21, confirming its redshift and suggesting episodic nuclear activity, using observations enabled by new frequency allocations after the analog TV switch.

Contribution

First detection of HI absorption in CTA 21, providing insights into its redshift and nuclear activity cycles, utilizing the newly available frequency band for radio astronomy.

Findings

HI absorption profile with two main components

Total HI column density of 7.9 x 10^20 cm^-2

Supports episodic nuclear activity in CTA 21

Abstract

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.