Keck HIRES observations of the QSO FIRST J104459.6+365605: Evidence for a large-scale outflow

This paper presents an analysis of a Keck HIRES spectrum of the QSO FIRST J 104459.6+365605, covering the rest wavelength range from 2260 to 2900 Angst rom. The line of sight toward the QSO contains two clusters of outflowing c louds that give rise to broad blue-shifted absorption lines. The outflow ve locities of the clouds range from -200 to -1200 km s(-1) and from -3400 to -5200 km s(-1), respectively. The width of the individual absorption lines ranges from 50 to more than 1000 km s(-1). The most prominent absorption li nes are those of Mg II, Mg I, and Fe II, and Mn II is also present. The low -ionization absorption lines occur at the same velocities as the most satur ated Mg II lines, showing that the Fe II, Mg I, and Mg II line-forming regi ons must be closely associated. Many absorption lines from excited states o f Fe II are present, allowing a determination of the population of several low-lying energy levels. The populations of the excited levels are found to be considerably smaller than expected for LTE and imply an electron densit y in the Fe II line-forming regions of n(e) similar to 4 x 10(3) cm(-3). Mo deling the ionization state of the absorbing gas with this value of the ele ctron density as a constraint, we find that the distance between the Fe II and Mg I line-forming region and the continuum source is similar to7 x 10(2 ) pc. From the correspondence in velocity between the Fe II, Mg I, and Mg I I lines we infer that the Mg II lines must be formed at the same distance. The Mg II absorption fulfills the criteria for broad absorption lines defin ed by Weymann and coworkers. Therefore, the distance we find between the Mg II line-forming region and the continuum source is surprising, since BALs are generally thought to be formed in outflows at a much smaller distance f rom the nucleus.