BL Lacertae objects beyond redshift 1.3 - UV-to-NIR photometry and photometric redshift for Fermi/LAT blazars

TL;DR

This study enhances the understanding of high-redshift BL Lac objects by providing photometric redshifts for 103 Fermi/LAT blazars, significantly increasing the confirmed high-redshift BL Lac sample and identifying sources beyond previous redshift records.

Contribution

It offers the first large-scale photometric redshift estimates for high-redshift BL Lac objects using UV-to-NIR data, quadrupling the known high-redshift BL Lac sample.

Findings

Confirmed 8 BL Lac objects at z>1.3, including the highest redshift record.

Provided redshift constraints for 75 sources, with 67 upper limits.

Discovered three sources with redshifts around 1.9, surpassing previous records.

Abstract

Observations of the gamma-ray sky with Fermi led to significant advances towards understanding blazars, the most extreme class of Active Galactic Nuclei. A large fraction of the population detected by Fermi is formed by BL Lacertae (BL Lac) objects, whose sample has always suffered from a severe redshift incompleteness due to the quasi-featureless optical spectra. Our goal is to provide a significant increase of the number of confirmed high-redshift BL Lac objects contained in the 2 LAC Fermi/LAT catalog. For 103 Fermi/LAT blazars, photometric redshifts using spectral energy distribution fitting have been obtained. The photometry includes 13 broad-band filters from the far ultraviolet to the near-IR observed with Swift/UVOT and the multi-channel imager GROND at the MPG/ESO 2.2m telescope. Data have been taken quasi-simultaneously and the remaining source-intrinsic variability has been corrected for. We release the UV-to-near-IR 13-band photometry for all 103 sources and provide redshift constraints for 75 sources without previously known redshift. Out of those, eight have reliable photometric redshifts at z>1.3, while for the other 67 sources we provide upper limits. Six of the former eight are BL Lac objects, which quadruples the sample of confirmed high-redshift BL Lac. This includes three sources with redshifts higher than the previous record for BL Lac, including CRATES J0402-2615 with the best-fit solution at z~1.9.