We present the first J-band spectrum of Mrk 231, which reveals a large \ion{ He}{ 1}*$\lambda 10830$ broad absorption line with a profile similar to that of the well-known \ion{ Na}{ 1} broad absorption line. Combining this spectrum with optical and UV spectra from the literature, we show that the unusual reddening noted by \citet{ veilleux13} is explained by a reddening curve like those previously used to explain low values of total-to-selective extinction in SNe Ia. The nuclear starburst may be the origin and location of the dust. Spatially-resolved emission in the broad absorption line trough suggests nearly full coverage of the continuum emission region. The broad absorption lines reveal higher velocities in the \ion{ He}{ 1}* lines (produced in the quasar-photoionized \ion{ H}{ 2} region) compared with the \ion{ Na}{ 1} and \ion{ Ca}{ 2} lines (produced in the corresponding partially-ionized zone). { \it Cloudy} simulations show that a density increase is required between the \ion{ H}{ 2} and partially-ionized zones to produce ionic column densities consistent with the optical and IR absorption line measurements and limits, and that the absorber lies $\sim 100\rm \, pc$ from the central engine. These results suggest that the \ion{ He}{ 1}* lines are produced in an ordinary quasar BAL wind that impacts upon, compresses, and accelerates the nuclear starburst's dusty effluent (feedback in action), and the \ion{ Ca}{ 2} and \ion{ Na}{ 1} lines are produced in this dusty accelerated gas. This unusual circumstance explains the rarity of \ion{ Na}{ 1} absorption lines; without the compression along our line of sight, Mrk~231 would appear as an ordinary FeLoBAL.
Categories: astro-ph.GA