In the classical unified model, blazars constitute a class of active galactic nuclei (AGNs) viewed at small angles from the jet axis (Blandford & Rees 1978; Antonucci 1993; Urry & Padovani 1995). Traditionally, blazars have been further splitted into two subclasses based on the strength of the features present in their optical spectra. While flat-spectrum radio quasars (FSRQs) show emission lines with equivalent width ≳5 Å, in BL Lacertae objects (BL Lacs) the non-thermal synchrotron radiation of the jet completely dominates the optical/UV emission, ending up in a typical featureless power-law spectrum. This makes the determination of their redshift via the detection of absorption/emission lines from the nuclear emission and/or from the host galaxy particularly challenging, even with 8–10 m class telescopes (e.g. Sbarufatti et al. 2005a, 2006, 2009; Landoni et al. 2013; Sandrinelli et al. 2013; Shaw et al. 2013; Falomo, Pian & Treves 2014; Pita et al. 2014; Paiano et al. 2016; Rosa-Gonzalez et al. 2017 for a review). In past years, several alternatives have been proposed to constrain the redshift of BL Lac objects, including the detection of intervening absorption features either from the halo of lower redshift galaxies (e.g. Shaw et al. 2013; Landoni et al. 2014) or from the neutral hydrogen in the intergalactic medium (e.g. Danforth et al. 2010; Furniss et al. 2013); the spectroscopy of galaxies in the environment where the blazars are embedded (e.g. Muriel et al. 2015; Farina et al. 2016); the detection of molecular emission lines from the host galaxy (e.g. Fumagalli et al. 2012); the study of the effect of the interaction with the extragalactic background light (EBL) in the blazar emission in the GeV and TeV domain (e.g. Prandini et al. 2010; Prandini, Bonnoli & Tavecchio 2012). In particular, the narrow distribution in luminosity of BL Lac host galaxies (Urry et al. 2000; Sbarufatti, Treves & Falomo 2005b) opened the possibility to use them as standard candles, and thus to measure their distance via broad-band imaging (e.g. Nilsson et al. 2008; Meisner & Romani 2010; Kotilainen et al. 2011). The main challenge of this approach is to accurately remove the bright central emission that typically outshine the host galaxy. Given the average 1.0 arcsec (or 3.2 kpc in the considered cosmology) effective radius calculated from the collection of z ≲ 0.6 BL Lac hosts observed with HST by Scarpa et al. (2000), it is clear that images with an exquisite spatial resolution and high contrast are necessary to unveil the faint and diffuse starlight emission around the bright, point-like emission from the active nucleus. In this paper, we exploit the capabilities of the new Advanced Rayleigh guided Ground layer adaptive Optics System (ARGOS; Rabien et al. 2010) mounted on the Large Binocular Telescope (LBT; Hill & Salinari 2004; Hill et al. 2012) to collect high-resolution near-infrared (NIR) LUCI 1 (i.e. LBT Utility Camera in the Infrared; Seifert et al. 2003; Ageorges et al. 2010) observations of HESS J1943+213. This blazar was detected by HESS in the very high energy (VHE) domain (i.e. at E >100 GeV) during a VHE galactic survey (H.E.S.S. Collaboration et al. 2011), making it the only BL Lac object known located in the Galactic plane. Broad Ks-band images gathered with the 3.5 m CAHA telescope revealed the presence of an extended emission that has been attributed to the host galaxy of HESS J1943+21 (Peter et al. 2014). A comparison with the typical size of blazar host derived by Cheung et al. (2003) allowed Peter et al. (2014) to set a lower limit on the redshift of z > 0.03. This is consistent with the z > 0.14 derived from the fit of the spectral energy distribution of HESS J1943+21 (Cerruti 2011; H.E.S.S. Collaboration et al. 2011) and with the z  0.45 limit obtained via modelling the attenuation of the VHE emission by the EBL (Peter et al. 2014). A tighter constraint on the redshift is however necessary to understand the nature of the VHE emission and to derive the EBL properties.
Citation: Danforth et al. 2010
Citation function: Background; Background