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

TL;DR

This study uses deep optical imaging to estimate redshifts of two BL Lac objects by analyzing their host galaxies, finding z ≈ 0.45 for PKS 0735+178 and a lower limit of z > 0.40 for RGB 0136+391.

Contribution

First optical imaging-based redshift estimates for RGB 0136+391 and PKS 0735+178 using host galaxy analysis during low nucleus activity.

Findings

PKS 0735+178 host galaxy has I = 18.64 and r_eff = 1.8 arcsec.

Estimated redshift of PKS 0735+178 is z = 0.45 ± 0.06.

RGB 0136+391 has a lower redshift limit of z > 0.40.

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.