Cosmological investigation of multi-frequency VLBI observations of ultra-compact structure in $z\sim 3$ radio quasars

TL;DR

This study demonstrates that multi-frequency VLBI measurements of high-redshift quasars can serve as standard rulers for cosmology, providing constraints on cosmological models through analysis of their angular sizes and jet physics.

Contribution

It introduces a method to use multi-frequency VLBI angular size data of quasars as standard rulers for cosmological inference, accounting for synchrotron self-absorption effects.

Findings

Angular sizes inversely proportional to frequency up to 43 GHz

Jet opacity governed by synchrotron self-absorption

Constraints on $\

Abstract

In this paper, we use multi-frequency angular size measurements of 58 intermediate-luminosity quasars reaching the redshifts $z\sim 3$ and demonstrate that they can be used as standard rulers for cosmological inference. Our results indicate that, for the majority of radio-sources in our sample their angular sizes are inversely proportional to the observing frequency. From the physical point of view it means that opacity of the jet is governed by pure synchrotron self-absorption, i.e. external absorption does not play any significant role in the observed angular sizes at least up to 43 GHz. Therefore, we use the value of the intrinsic metric size of compact milliarcsecond radio quasars derived in a cosmology independent manner from survey conducted at 2 GHz and rescale it properly according to predictions of the conical jet model. This approach turns out to work well and produce quite stringent constraints on the matter density parameter $\Omega_m$ in the flat $\Lambda$CDM model and Dvali-Gabadadze-Porrati braneworld model. The results presented in this paper pave the way for the follow up engaging multi-frequency VLBI observations of more compact radio quasars with higher sensitivity and angular resolution.