2002ApJ...570..514D

A Keck echelle spectrum of the quasar FBQS 0840+3633 reveals outflowing gas that gives rise to blueshifted absorption lines of many low-ionization species. The gas covers a range of velocities from -700 to -3500 km.s^–1, with two main components centered at -900 and -2800 km.s^–1. The physical conditions in the two main velocity components are found to be significantly different and can be attributed to a difference of a factor of ∼100 in the distance from the central continuum source. The low-velocity gas shows absorption lines from excited states with relative strengths that indicate a low density. The level populations of low-lying Ni II, Si II, and Fe II states cannot be explained with a model based on collisional excitation and a single electron density. The lines of Si II provide an upper limit on the electron density of n_e<500 cm^–3, and another excitation mechanism must be responsible for the observed excitation of Fe II and Ni II. Assuming that this mechanism is UV fluorescence leads to an estimate of the distance between the low-velocity gas and the active nucleus of ∼230 pc. Absorption lines from excited states formed in the high-velocity gas indicate a much higher density. This gas gives rise to Fe III and strong Al III absorption, which indicates that it contains the hydrogen ionization front on our line of sight to the active nucleus. The observed Fe III and Al III column densities and the absence of detectable absorption from the He I 2³S state allow us to derive an estimate of the typical distance between the high-velocity gas and the active nucleus of ∼1 pc.