On the redshift of the very high energy {\gamma}-ray BL Lac object S2 0109+22

TL;DR

This study uses optical spectroscopy to constrain the redshift of the gamma-ray BL Lac object S2 0109+22, establishing a lower limit of z>0.35 and clarifying previous redshift estimates.

Contribution

The paper provides the first high signal-to-noise optical spectrum of S2 0109+22, setting a new lower redshift limit and disputing earlier redshift claims based on spectral analysis.

Findings

Redshift lower limit set at z>0.35

No spectral features detected in the optical spectrum

Previous redshift estimate of z=0.265 is likely incorrect

Abstract

The determination of the redshift of extragalactic gamma ray sources is of fundamental importance for the interpretation of their emission models and extragalactic population studies, AGN classification schemes, and to study the interaction with the extragalactic background light. Here we focus on the BL Lac S2 0109+22, recently detected at very high energies. We obtained a high signal-to-noise optical spectrum at Gran Telescopio Canarias for the source over the spectral range 4000-10000 Angstrom, to search for spectral features of the host galaxy and/or of the nuclear source and it was compared with previous observations in order to confirm and/or dispute its redshift determination. We find the S2 0109+22 optical spectrum is well represented by a power law continuum without any absorption or emission feature of Equivalent Width >0.1 Angstrom. Based on the new optical spectrum we are able to set a redshift lower limit z>0.35, assuming the source is hosted by a massive elliptical galaxy typical for this class of sources. The redshift z=0.265 proposed in the literature for this object is most probably referred to a r=18.3 galaxy at 15" from S2 0109. This galaxy could belong to a group of faint galaxies located at z~0.26.