On the nature of bright compact radio sources at<i>z</i>&gt; 4.5

Abstract

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