On the redshift of the very high-energy gamma-ray BL Lac object S2 0109+22. Issue 3 (1st March 2016)

• Record Type: Journal Article
• Authors: Paiano, S.; Landoni, M.; Falomo, R.; Scarpa, R.; Treves, A.
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 458:Issue 3(2016)
• Page Start: 2836
• Page End: 2839
• Publication Date: 2016-03-01
• Subjects: BL Lacertae objects: individual -- galaxies: distances and redshifts; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/stw472
• Languages: English
• ISSNs: 0035-8711

Abstract

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, active galactic nuclei 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 4000–10 000 Å, 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 Å. 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 arcsec from S2 0109. This galaxy could belong to a group of faint galaxies located at z ∼ 0.26.