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 Hα, Hβ, [O III] λ5007, [O II] λ3727, [Ne III] λ3869, and Mg II λ2798 lines. We consider six quasars, two of which (PKS 2135-14 and PKS 2349-014) have very close companions with projected separation less than 4 h^-1 kpc. For these companions the calculated emission lines should be easily observable, with W_λ >~ 20 Å for the [O III], [O II], and Hα lines. The 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 amounts 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.