Mergers Dominate the Host Galaxies of Low-z LoBAL QSOs

The ultrafast outflows characteristic of broad absorption line (BAL) QSOs suggest that, in those systems, we might be observing AGN-driven kinetic feedback capable of affecting the growth of the host galaxy. Particularly relevant to this picture might be low-ionization BAL QSOs (LoBALs) at low redshifts because anecdotal studies find that they might be connected to major mergers and ultra-luminous infrared galaxies. We take a detailed look at the host galaxy morphologies and SEDs of a complete, volume-limited sample of optically-selected LoBALs at 0.5 < z < 0.6. Their infrared luminosities and star formation rates do not suggest they are different from typical type-1 QSOs. After correcting for the AGN contribution to the FIR SED, LoBALs show levels of star formation similar to those of type-1 QSOs. However, the observations of their host galaxies with HST/WFC3 reveal apparent signs of recent or on-going tidal interaction in 68% of the sample, which is in contrast to recent work on QSO hosts showing merger fraction of less than 20-25%. The mergers in our sample represent various stages of the merger process: from settled morphologies with extended low surface-brightness tidal tails to double nuclei (in at least 41% of the sample). Our results favor an evolutionary explanation for the emergence of low-z LoBALs, in which quasar-level accretion during various stages of the merger process is associated with the observed fast outflows, as suggested by some simulation studies. We discuss important differences between LoBALs and FeLoBALs, and demonstrate that the traditional BALs selection criteria would have excluded all but one of the mergers in this sample.