On the nature of bright compact radio sources at z>4.5

TL;DR

This study expands the sample of high-redshift (z>4.5) radio-loud quasars observed with VLBI, revealing their diverse spectral properties and suggesting classifications into flat-spectrum quasars and GPS/MPS sources, aiding understanding of early universe galaxy evolution.

Contribution

The paper increases the number of VLBI-imaged z>4.5 radio sources by 50%, providing a larger dataset to analyze their spectral and structural properties.

Findings

27 sources classified based on spectral indices and brightness temperatures

13 sources identified as flat-spectrum radio quasars

Steep-spectrum sources are likely off-axis GPS/MPS sources

Abstract

High-redshift radio-loud quasars are used to, among other things, test the predictions of cosmological models, set constraints on black hole growth in the early universe and understand galaxy evolution. Prior to this paper, 20 extragalactic radio sources at redshifts above 4.5 have been imaged with very long baseline interferometry (VLBI). Here we report on observations of an additional ten z>4.5 sources at 1.7 and 5 GHz with the European VLBI Network (EVN), thereby increasing the number of imaged sources by 50%. Combining our newly observed sources with those from the literature, we create a substantial sample of 30 z>4.5 VLBI sources, allowing us to study the nature of these objects. Using spectral indices, variability and brightness temperatures, we conclude that of the 27 sources with sufficient information to classify, the radio emission from one source is from star formation, 13 are flat-spectrum radio quasars and 13 are steep-spectrum sources. We also argue that the steep-spectrum sources are off-axis (unbeamed) radio sources with rest-frame self-absorption peaks at or below GHz frequencies and that these sources can be classified as gigahertz peaked-spectrum (GPS) and megahertz peaked-spectrum (MPS) sources.