The Discovery of Host Galaxy HI Absorption 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.

Accepted for publication in ApJL

References in corpus (10)

• Probing radio source environments via HI and OH absorption
• GPS radio sources: new optical observations and an updated master list
• Is cold gas fuelling the radio galaxy NGC 315?
• Unification scheme and the distribution of neutral gas in compact radio sources
• Multiwavelength study of Cygnus A II. X-ray inverse-Compton emission from a relic counterjet and implications for jet duty-cycles
• A circumnuclear disk of atomic hydrogen in Centaurus A
• HI gas in rejuvenated radio galaxies: GMRT observations of the DDRG J1247+6723
• Multiwavelength study of Cygnus A III. Evidence for relic lobe plasma
• HI gas in the rejuvenated radio galaxy 4C29.30
• VLBA polarimetric observations of the CSS quasar 3C147

Cited by in corpus (16)

• Compact steep-spectrum and peaked-spectrum radio sources
• The interstellar and circumnuclear medium of active nuclei traced by HI 21-cm absorption
• A sample of small size compact steep-spectrum radio sources. VLBI images and VLA polarization at 5 GHz
• A comparative study of intervening and associated HI 21-cm absorption profiles in redshifted galaxies
• Jets in radio galaxies and quasars: an observational perspective
• A radio study of the double-double radio galaxy 3C293
• Ionisation of the atomic gas in redshifted radio sources
• The X-ray and radio-emitting plasma lobes of 4C23.56: further evidence of recurrent jet activity and high acceleration energies
• Atomic and Molecular Absorption in Redshifted Radio Sources
• A search for HI and OH absorption in z > 3 CO emitters
• The mid-infrared properties and gas content of active galaxies over large look-back times
• H I absorption towards radio Active Galactic Nuclei of different accretion modes
• Further Observational Evidence for a Critical Ionising Luminosity in Active Galaxies
• Broad, weak 21 cm absorption in an early type galaxy: spectral-line finding and parametrization for future surveys
• Redshift evolution of the HI detection rate in radio-loud active galactic nuclei
• HI gas in the rejuvenated radio galaxy PKS 2014-55