Emission Lines from Companion Galaxies of QSOs

Abstract

Anticipating new spectroscopic studies of quasar companions, we calculate expected emission line spectra for the companion galaxies. Under the assumption of isotropic photoionizing radiation, we predict equivalent widths for Halpha, Hbeta, [O III], [O II], [Ne III], and Mg II lines. We consider 6 quasars, two of which (PKS 2135-14 and PKS 2349-014) have very close companions with projected separation < 4 h^{-1} kpc. For these companions the calculated emission lines should be easily observable, with W_lambda > 20 AA for the [O III], [O II], and Halpha lines. Presence of strong emission lines from the companions occupying only a tiny solid angle would suggest that 1) quasar radiation is isotropic; 2) companion galaxies have measurable amount of gas, which can be assessed using the photoionization models; 3) host galaxies, if present, have been depleted of gas or do not interact with the companions.