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Radio Spectra of Luminous, Heavily Obscured WISE-NVSS Selected Quasars with Young Radio Jets

Presentation #129.04 in the session AGN and Quasars II.

Published onJun 29, 2022
Radio Spectra of Luminous, Heavily Obscured WISE-NVSS Selected Quasars with Young Radio Jets

Young, compact radio sources associated with accreting supermassive black holes represent an essential phase in the life cycles of jetted AGN for understanding AGN triggering and duty cycles. In this talk, we present radio spectra for a sample of distant (z~0.5-3) heavily obscured quasars with sub-galactic, young radio jets. Our sample selection identified optically faint but MIR-bright WISE quasars with a luminous (L1.4 GHz≲1025 W Hz-1) radio source that is unresolved by NVSS (<45”). A follow-up high-resolution VLA imaging has revealed compact, sub-arcsecond-scale radio morphologies (<1.7 kpc at z~2) for about 55% of our sources. The spectra presented here are carefully constructed from our own 10 GHz observations and archival radio survey data, which together yield 6-11 flux density measurements spanning 0.1-10 GHz frequencies. Our analysis shows that 63% of the sample exhibit either peaked or curved radio spectra, and 37% are classified as Peaked Spectrum (PS) sources. This strongly indicates compact emission regions likely arising from recently triggered radio jets. Assuming synchrotron self-absorption (SSA) generates the peaks, we infer compact source sizes (3–100 pc) with strong magnetic fields (6–100 mG) and very young ages (30–104 years). Conversely, free-free absorption (FFA) could also create peaks due to the very high column densities associated with the deeply embedded nature of the sample. However, we find no correlations between the existence or frequency of the peaks and any parameters of the MIR emission, casting doubt on the importance of FFA associated with the high MIR columns. The high-frequency spectral indices are steep (α ~ -1) and correlate, weakly, with the ratio of MIR photon energy density to magnetic energy density, suggesting that the spectral steepening could arise from inverse Compton scattering off the intense MIR photon field. This study provides a foundation for combining multi-frequency and mixed-resolution radio survey data for understanding the impact of young radio jets on the ISM and star formation rates of their host galaxies.

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