VLBI observations of four radio quasars at z > 4: blazars or not?
Artikel i vetenskaplig tidskrift, 2017

Blazars are active galactic nuclei (AGN) whose relativistic jets point nearly to the line of sight. Their compact radio structure can be imaged with very long baseline interferometry (VLBI) on parsec scales. Blazars at extremely high redshifts provide a unique insight into the AGN phenomena in the early Universe. We observed four radio sources at redshift z > 4 with the European VLBI Network (EVN) at 1.7 and 5 GHz. These objects were previously classified as blazar candidates based on X-ray observations. One of them, J2134–0419 is firmly confirmed as a blazar with our VLBI observations, due to its relativistically beamed radio emission. Its radio jet extended to ∼10 milli-arcsec scale makes this source a promising target for follow-up VLBI observations to reveal any apparent proper motion. Another target, J0839+5112 shows a compact radio structure typical of quasars. There is evidence for flux density variability and its radio “core” has a flat spectrum. However, the EVN data suggest that its emission is not Doppler-boosted. The remaining two blazar candidates (J1420+1205 and J2220+0025) show radio properties totally unexpected from radio AGN with small-inclination jet. Their emission extends to arcsec scales and the Doppler factors of the central components are well below 1. Their structures resemble that of double-lobed radio AGN with large inclination to the line of sight. This is in contrast with the blazar-type modeling of their multi-band spectral energy distributions. Our work underlines the importance of high-resolution VLBI imaging in confirming the blazar nature of high-redshift radio sources.

radio continuum: galaxies

galaxies: active

galaxies: high-redshift


H.M. Cao

Chinese Academy of Sciences

Shangqiu Normal University

S. Frey

Magyar Tudomanyos Akademia

K.E. Gabanyi

Magyar Tudomanyos Akademia

Z. Paragi

Joint Institute for VLBI in Europe (JIVE)

Jun Yang

Chalmers, Rymd- och geovetenskap, Onsala rymdobservatorium

Chinese Academy of Sciences

D. Cseh

Radboud Universiteit

X.-Y. Hong

Chinese Academy of Sciences

T. An

Chinese Academy of Sciences

Monthly Notices of the Royal Astronomical Society

0035-8711 (ISSN) 1365-2966 (eISSN)

Vol. 467 1 950-960


Astronomi, astrofysik och kosmologi


Grundläggande vetenskaper


Onsala rymdobservatorium



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