On the nature of bright compact radio sources atz> 4.5

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 per cent. 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.

Radio spectra of bright compact sources at z>4.5

“…However, because there is an equal probability that the flux density of the source is at any value below (including only slightly below) the upper limits, additional observations are required to confirm or refute this. In the 1.7 GHz EVN observations, J1548+3335 was found to have two components that are separated by 812 ± 3 mas, which translates to a projected linear size of 5267 ± 17 pc (Coppejans et al 2016). The second (fainter) component is not detected in the 5 GHz EVN observations (Coppejans et al 2016).…”

“…In the 1.7 GHz EVN observations, J1548+3335 was found to have two components that are separated by 812 ± 3 mas, which translates to a projected linear size of 5267 ± 17 pc (Coppejans et al 2016). The second (fainter) component is not detected in the 5 GHz EVN observations (Coppejans et al 2016). The primary component coincides with the SDSS position and no jet was detected between the two components.…”

“…The primary component coincides with the SDSS position and no jet was detected between the two components. It is, therefore, possible that the second component is a lobe or hotspot of the first component, an unrelated AGN at the same redshift, a foreground or background source that is unrelated to J1548+3335, or that the two components are gravitationally lensed images of the same source (Coppejans et al 2016). From the spectrum it is clear that some of the source's flux density was resolved out in the 1.7 GHz CFC2016(V) observations, or the source is variable.…”

“…However, the predicted flux density at 4850 MHz would then be ∼ 27 mJy, which is well above the 4.9 GHz GB6 upper limit of 18 mJy. It is therefore most likely that the spectrum flattens towards higher frequencies, and considering that the spectral index between the 1.7 and 5 GHz of the FPG2010(V) VLBI points is −0.53 ± 0.06 (Coppejans et al 2016), it appears to turn over. While care should be taken when comparing non-VLBI and VLBI spectral indices, we believe it is justified in this case, as the 1.4 GHz FIRST and 1.6 GHz FPG2010(V) flux densities are comparable (12.4 ± 0.6 and 15.5 ± 0.8 mJy, respectively).…”

“…As discussed in CFC2016, this allows us to distinguish between emission from AGN and star formation and is critical to explain the spectra of J1429+5447 and J1205−0742 in Sections 4.2.6 and 4.4.2. (2) The z > 4.5 VLBI sources can be seen as forming a flux density limited sample since all z > 4.5 sources with 1.4 GHz flux densities above ∼ 5 mJy in the Very Large Array (VLA) Faint Images of the Radio Sky at Twenty-centimeter (FIRST) survey (White et al 1997) have been systematically observed with VLBI in published (Coppejans et al 2016, and references therein) and ongoing VLBI campaigns. We do however note that some authors have specifically targeted fainter sources.…”