The Apparent "Angular Size - Frequency" Dependence in ~3000 Parsec-scale Extragalactic Jets

TL;DR

This study analyzes VLBI observations of approximately 3000 parsec-scale extragalactic jets, revealing a power-law relationship between apparent angular size and frequency, likely caused by synchrotron self-absorption.

Contribution

It demonstrates that the angular size-frequency dependence follows a specific power-law, supporting the synchrotron self-absorption model over other absorption mechanisms.

Findings

Angular size scales with frequency as a power law with index ~ -0.95.

Synchrotron self-absorption is the likely cause of the observed dependence.

The dependence is inconsistent with free-free absorption or scatter broadening.

Abstract

The upper envelope of the amplitude of the VLBI visibility function usually represents the most compact structural pattern of extragalactic radio sources, in particular, the "core-jet" morphologies. By fitting the envelope to a circular Gaussian model in ~3000 parsec-scale core-jet structures, we find that the apparent angular size shows significant power-law dependence on the observing frequency (power index n = -0.95 pm 0.37). The dependence is likely to result from synchrotron self-absorption in the inhomogeneous jet and not the free-free absorption (n = -2.5), nor the simple scatter broadening (n leq -2).