Radio spectra of bright compact sources at z>4.5

Abstract

High-redshift quasars are important to study galaxy and active galactic nuclei (AGN) evolution, test cosmological models, and study supermassive black hole growth. Optical searches for high-redshift sources have been very successful, but radio searches are not hampered by dust obscuration and should be more effective at finding sources at even higher redshifts. Identifying high-redshift sources based on radio data is, however, not trivial. Here we report on new multi-frequency Giant Metrewave Radio Telescope (GMRT) observations of eight z > 4.5 sources previously studied at high angular resolution with very long baseline interferometry (VLBI). Combining these observations with those from the literature, we construct broad-band radio spectra of all 30 z > 4.5 sources that have been observed with VLBI. In the sample we found flat, steep and peaked spectra in approximately equal proportions. Despite several selection effects, we conclude that the z > 4.5 VLBI (and likely also non-VLBI) sources have diverse spectra and that only about a quarter of the sources in the sample have flat spectra. Previously, the majority of high-redshift radio sources were identified based on their ultra-steep spectra (USS). Recently a new method has been proposed to identify these objects based on their megahertz-peaked spectra (MPS). Neither method would have identified more than 18 per cent of the high-redshift sources in this sample. More effective methods are necessary to reliably identify complete samples of high-redshift sources based on radio data.

The space density ofz > 4 blazars

“…These are intrinsically small radio-sources, possibly because of their young age. Interestingly, radio-loud AGN at high redshift often show these characteristics (Frey et al 2003;Frey et al 2005;Coppejans et al 2016;Coppejans et al 2017). At m.a.s.…”

“…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 This table includes other z>4 QSO present in the CLASS survey that have not been selected mostly because they have a low Galactic latitude or because they have magnitude below the adopted limit. Only one object (in bold face) is missed by the selection because of a low dropout value.…”

“…The most striking one is GB6J160608+312504 whose limit on α 1.4 0.15 is significantly different from the slope measured between 1.4 GHz and 8.4 GHz. This object is known to have a spectrum peaking at 2.6 GHz (Coppejans et al 2017), corresponding to 14.4 GHz in the source rest-frame. VLBI observations suggest a Compact-Symmetric Objects (CSO) morphology (Coppejans et al 2017).…”

“…This object is known to have a spectrum peaking at 2.6 GHz (Coppejans et al 2017), corresponding to 14.4 GHz in the source rest-frame. VLBI observations suggest a Compact-Symmetric Objects (CSO) morphology (Coppejans et al 2017). Another object that lies below the 1:1 relation in Fig.…”

Evolving parsec-scale radio structure in the most distant blazar known

“…I " (5) where β⊥ is the transverse jet speed, a = (1 + z) −1 and DA is the angular size distance. This can be approximated in terms of the Hubble distance c/H0 as 47 Using equations (6), (7) and β⊥ ≈ Γβ (for an extremely beamed jet) in equation (5)…”