High-resolution radio imaging of two luminous quasars beyond redshift 4.5

Abstract

Context. Radio-loud active galactic nuclei in the early Universe are rare. The quasars J0906+6930 at redshift z=5.47 and J2102+6015 at z=4.57 stand out from the known sample with their compact emission on milliarcsecond (mas) angular scale with high (0.1-Jy level) flux densities measured at GHz radio frequencies. This makes them ideal targets for very long baseline interferometry (VLBI) observations. Aims. By means of VLBI imaging we can reveal the inner radio structure of quasars and model their brightness distribution to better understand the geometry of the jet and the physics of the sources. Methods. We present sensitive high-resolution VLBI images of J0906+6930 and J2102+6015 at two observing frequencies, 2.3 and 8.6 GHz. The data were taken in an astrometric observing programme involving a global five-element radio telescope array. We combined the data from five different epochs from 2017 February to August. Results. For one of the highest redshift blazars known, J0906+6930, we present the first-ever VLBI image obtained at a frequency below 8 GHz. Based on our images at 2.3 and 8.6 GHz, we confirm that this source has a sharply bent helical inner jet structure within ∼3 mas from the core. The quasar J2102+6015 shows an elongated radio structure in the east-west direction within the innermost ∼2 mas that can be described with a symmetric three-component brightness distribution model at 8.6 GHz. Because of their non-pointlike mas-scale structure, these sources are not ideal as astrometric reference objects. Our results demonstrate that VLBI observing programmes conducted primarily with astrometric or geodetic goals can be utilized for astrophysical purposes as well.

The space density ofz > 4 blazars

“…This is an indication that the relativistic jet is mis-aligned with respect to the observer. Examples are J2102+6015, at z=4.57 (Frey et al 2018), J1606+3124, at z=4.56 (Coppejans et al 2017) and J1427+3312, at z=6.12 (Frey et al (Liodakis et al 2018) and of the SED (An & Romani 2018). These results suggest that J0906+6930 is a GPS source likely oriented towards the observer.…”

“…It is important to note, however, that this is not always true: the source J0906+6930 at z=5.47 shows a clearly peaked radio spectrum(Coppejans et al 2017) with a turnover frequency of 6.4 GHz corresponding to 41.4 GHz in the source rest-frame. At the same time, Very Long Baseline Interferometry (VLBI) data found evidence of Doppler boosting(Zhang et al 2017, Frey et al 2018, which was further supported by the study of radio variabilty…”

An extremely X-ray weak blazar at z = 5

“…The first one is a complex multicomponent ultra steep spectrum radio source (Parijskij et al 2014). The second one is classified as a Giga-Hertz Peaked Source (GPS, O'Dea 1998), meaning a very young and compact radio galaxy resolved only at VLBI scale with a peaked radio spectrum above 1 GHz (Frey et al 2018). Therefore, although the high radio-loudness supports the blazar nature of DES0141-54, we cannot exclude the possibility that it is an intrinsically powerful misaligned RL AGN, like J2102+60 and J0311+0507.…”

VLBI monitoring of two distant quasars as a showcase for 'EVN Lite'

“…Here we show the results of an imaging analysis based on observations conducted at 5 different epochs spanning nearly 7 months, from February to August 2017. More details of the observations, data analysis and results are presented in [3]. The participating radio telescopes were Badary, Svetloe, Zelenchukskaya (Russia), Yebes (Spain), and Sheshan (China), providing interferometer baselines longer than 9000 km, i.e.…”

“…2). Based on the imaging and model-fitting, there is no evidence for Doppler-boosting in this powerful gigahertz-peaked spectrum source [3].…”