Invisible Active Galactic Nuclei. II Radio Morphologies & Five New HI 21 cm Absorption Line Detections

Abstract

We have selected a sample of 80 candidates for obscured radio-loud active galactic nuclei and presented their basic optical/near-infrared (NIR) properties in Paper 1. In this paper, we present both high-resolution radio continuum images for all of these sources and HI 21cm absorption spectroscopy for a few selected sources in this sample. A-configuration 4.9 and 8.5 GHz VLA continuum observations find that 52 sources are compact or have substantial compact components with size <0.5" and flux density >0.1 Jy at 4.9 GHz. The most compact 36 sources were then observed with the VLBA at 1.4 GHz. One definite and 10 candidate Compact Symmetric Objects (CSOs) are newly identified, a detection rate of CSOs ~3 times higher than the detection rate previously found in purely flux-limited samples. Based on possessing compact components with high flux densities, 60 of these sources are good candidates for absorption-line searches. Twenty seven sources were observed for HI 21cm absorption at their photometric or spectroscopic redshifts with only 6 detections made (one detection is tentative). However, five of these were from a small subset of six CSOs with pure galaxy optical/NIR spectra and for which accurate spectroscopic redshifts place the redshifted 21cm line in a RFI-free spectral window. It is likely that the presence of ubiquitous RFI and the absence of accurate spectroscopic redshifts preclude HI detections in similar sources (only one detection out of the remaining 22 sources observed, 14 of which have only photometric redshifts). Future searches for highly-redshifted HI and molecular absorption can easily find more distant CSOs among bright, blank field' radio sources but will be severely hampered by an inability to determine accurate spectroscopic redshifts for them due to their lack of rest-frame UV continuum.

AJ accepted

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• A Giant Metrewave Radio Telescope survey for associated HI 21-cm absorption in the Caltech-Jodrell Flat-spectrum sample
• A Search for Intrinsic HI 21 cm and OH 18 cm Absorption Toward Compact Radio Sources
• A comparative study of intervening and associated HI 21-cm absorption profiles in redshifted galaxies
• The evolution of neutral hydrogen over the past 11 Gyr via HI 21 cm absorption
• Ionisation of the atomic gas in redshifted radio sources
• The HI absorption zoo: JVLA extension to $z\sim 0.4$
• Atomic and Molecular Absorption in Redshifted Radio Sources
• The mid-infrared properties and gas content of active galaxies over large look-back times
• FLASH Pilot Survey: Detections of associated 21 cm HI absorption in GAMA galaxies at 0.42 < z <1.00
• Further Observational Evidence for a Critical Ionising Luminosity in Active Galaxies
• uGMRT search for cold gas at z~1-1.4 towards red quasars
• Redshift evolution of the HI detection rate in radio-loud active galactic nuclei
• The time delay of CLASS B1600+434 from VLA multi-frequency and polarization monitoring
• Intervening or associated? Machine learning classification of redshifted H I 21-cm absorption
• The FAST HI 21-cm absorption blind survey. II. -- Statistic Exploration for Associated and Intervening systems