Proximity effect (astrophysics)

The quasar proximity effect is a reduction of nearby Lyman-alpha forest absorption caused by extra photoionization from the quasar. At fixed density and temperature in photoionization equilibrium, an added rate ${\Gamma }_Q$ changes the neutral fraction by the factor ${\Gamma }_{\mathrm{bg}}/({\Gamma }_{\mathrm{bg}}+{\Gamma }_Q)$. A host environment with positive density contrast can mask this reduction, and a variable or anisotropic source complicates its interpretation.

In photoionization equilibrium at fixed gas density and temperature, the extra quasar photoionization rate reduces neutral-hydrogen column density by the displayed factor. Its local rate is calculated from the quasar flux, while the isotropic background rate is ${\Gamma }_{\mathrm{bg}}=4\pi \int J_{\nu }{\sigma }_{\mathrm{HI}}(\nu )/(h\nu )d\nu$. Fitting the absorption suppression versus proper distance constrains the cosmic ionizing background. For an unperturbed column-density distribution proportional to $N^{-p}$ with $p>1$, the line abundance above a fixed threshold is reduced by $(1+\omega {)}^{1-p}$. Density enhancement, quasar age, anisotropy and redshift errors must be distinguished from this ionization effect.