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

TL;DR

This study analyzes radio spectra of heavily obscured luminous quasars, revealing that most have peaked or curved spectra indicative of young, compact radio jets with sizes of 3-100 pc, and explores their physical properties and implications.

Contribution

It introduces a comprehensive analysis of radio spectra for obscured quasars, identifying a high prevalence of GPS-like sources and providing insights into their jet properties and absorption mechanisms.

Findings

61% of quasars have peaked or curved radio spectra

36% are classified as GPS sources

Inferred source sizes are 3-100 pc with magnetic fields of 6-100 mG

Abstract

We present radio spectra spanning $0.1 - 10$ GHz for the sample of heavily obscured luminous quasars with extremely red mid-infrared-optical colors and compact radio emission. The spectra are constructed from targeted 10 GHz observations and archival radio survey data, which together yield $6-11$ flux density measurements for each object. Our suite of Python tools for modeling the radio spectra is publicly available on Github. Our primary result is that most (61%) of the sample have peaked or curved radio spectra and many (36%) could be classified as Gigahertz Peaked Spectrum (GPS) sources. This 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 young ages ($30 - 10^4$ years). Conversely, free-free absorption (FFA) could also create peaks due to the 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. The high-frequency spectral indices are steep ($\alpha \approx -1$) 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.