Research Note

Redshift constraints for RGB 0136+391 and PKS 0735+178 from deep optical imaging^⋆

¹ Finnish Centre for Astronomy with ESO (FINCA), University of Turku, Väisäläntie 20, 21500 Piikkiö, Finland
e-mail: kani@utu.fi
² Nordic Optical Telescope, Apartado 474, 38700 Santa Cruz de La Palma, Spain
³ Department of Astronomy, University of Florida, 211 Bryant Space Science Center, Gainesville, FL 32611-2055, USA
⁴ Tuorla Observatory, Department of Physics and Astronomy, University of Turku, Väisäläntie 20, 21500 Piikkiö, Finland

Received: 20 June 2012
Accepted: 13 September 2012

Abstract

We present the results of deep I-band imaging of two BL Lacerate objects, RGB 0136+391 and PKS 0735+178, during an epoch when the optical nucleus was in a faint state in both targets. In PKS 0735+178 we find a significant excess over a point source, which, if fitted by the de Vaucouleurs model, corresponds to a galaxy with I = 18.64 ± 0.11 and r_eff = 1.8 ± 0.4 arcsec. Interpreting this galaxy as the host galaxy of PKS 0735+178 we derive z = 0.45 ± 0.06 using the host galaxy as a “standard candle”. We also discuss the immediate optical environment of PKS 0735+178 and the identity of the MgII absorber at z = 0.424. Despite of the optimally chosen epoch and deep imaging we find the surface brightness profile of RGB 0136+391 to be consistent with a point source. By determining a lower limit for the host galaxy brightness by simulations, we derive z > 0.40 for this target.