On the connection between metal absorbers and quasar nebulae

We establish a simple model for the distribution of cool (∼10 ⁴ K) gas around L* galaxies using the properties of strong Mg II absorption line systems as observational constraints. Our analysis suggests that the halos of L* galaxies are filled with cool gas clouds having sizes of order 1 kpc and densities of ∼10^-2 cm^-3. We then investigate the physical conditions of a similar ensemble of clouds situated around a central quasar. We show that the flux from the quasar gives rise to (1) extended narrow line emission on ∼100 kpc scales and (2) an anisotropy in the properties of the absorbing gas arising from the geometry of the quasar radiation field. Provided that quasars reside in gaseous halos more massive than those of L* galaxies, our predictions agree with the results from detections of both narrow emission line nebulae and ∼100 kpc Mg II-absorbing halos around quasars, suggesting a common origin for these phenomena. We discuss the implications of our results for understanding quasar absorption line systems, quasar environments at high redshifts, and the quasar unification scheme.