The space density of z>4 blazars

Abstract

High redshift blazars are an important class of Active Galactic Nuclei (AGN) that can provide an independent estimate of the supermassive black-hole mass function in high redshift radio-loud AGN without the bias due to absorption along the line-of-sight. Using the Cosmic Lens All Sky Survey (CLASS) we built a complete radio flux-limited sample of high redshift (z>4) blazars suitable for statistical studies. By combining dedicated optical observations and the SDSS spectroscopic database, we obtained a sample of 26 blazar candidates with a spectroscopic redshift above 4. On the basis of their radio spectrum we distinguish between blazars and QSO with a Gigahertz Peaked Spectrum (GPS) like spectrum. Out of the 18 confirmed blazars 14 constitute a completely identified, flux-limited sample down to a magnitude of 21 (AB). Using this complete sample we derive a space density of blazars with 4<z<5.5 of rho=0.13 (+0.05,-0.03) Gpc^-3. This is the first actual estimate of the blazar space density in this range of redshift. This value is in good agreement with the extrapolation of the luminosity function and cosmological evolution based on a sample of flat-spectrum radio quasars selected at lower redshifts and it is consistent with a cosmological evolution peaking at z$\sim$2 similar to radio-quiet QSO. We do not confirm, instead, the presence of a peak at z~4 in the space density evolution, recently suggested using an X-ray selected sample of blazars. It is possible that this extreme peak of the evolution is present only among the most luminous blazars.

14 pages, accepted for publication on MNRAS (https://doi.org/10.1093/mnras/sty3526)

References in corpus (11)

• An Observational Determination of the Bolometric Quasar Luminosity Function
• GaLactic and Extragalactic All-sky Murchison Widefield Array (GLEAM) survey I: A low-frequency extragalactic catalogue
• CGRaBS: An All-Sky Survey of Gamma-Ray Blazar Candidates
• High-redshift AGN in the Chandra Deep Fields: the obscured fraction and space density of the sub-$L_{\ast }$ population
• Two bright z > 6 quasars from VST ATLAS and a new method of optical plus mid-infra-red colour selection
• Constraining the nature of High Frequency Peakers. I. The spectral variability
• Blazar candidates beyond redshift 4 observed by Swift
• On the nature of bright compact radio sources at z>4.5
• High-resolution double morphology of the most distant known radio quasar at z=6.12
• High-resolution radio imaging of the most distant radio quasar SDSS J0836+0054
• Radio spectra of bright compact sources at z>4.5

Cited by in corpus (8)

• Evolving parsec-scale radio structure in the most distant blazar known
• Blazars at the Cosmic Dawn
• High-redshift quasars at $z\ge 3$ -- I. Radio spectra
• The evolution of the heaviest super-massive black-holes in jetted AGNs
• The central engine of the highest redshift blazar
• The Impact of the CMB on the Evolution of high-z Blazars
• Flux-density measurements of the high-redshift blazar PSO J047.4478+27.2992 at 4.7 and 8.2 GHz with RATAN-600
• Extragalactic astrophysics with next-generation CMB experiments