The Surprising Absence of Absorption in the Far-Ultraviolet Spectrum of Mrk 231

TL;DR

This study presents a far-ultraviolet spectrum of Mrk 231 revealing unexpected absence of broad absorption features, suggesting the AGN dominates the emission and challenging previous assumptions about the dusty outflow's filtering effects.

Contribution

The paper provides the first FUV spectrum of Mrk 231 showing no broad absorption features, indicating a different geometric or physical configuration of the outflow and emission regions.

Findings

FUV spectrum shows faint, broad, blueshifted Ly-alpha emission.

No unambiguous broad absorption features detected in FUV.

Ly-alpha emission likely produced in outflowing BAL clouds.

Abstract

Mrk 231, the nearest (z = 0.0422) quasar, hosts both a galactic-scale wind and a nuclear-scale iron low-ionization broad absorption line (FeLoBAL) outflow. We recently obtained a far-ultraviolet (FUV) spectrum of this object covering ~1150 - 1470 A with the Cosmic Origins Spectrograph on board the Hubble Space Telescope. This spectrum is highly peculiar, highlighted by the presence of faint (~< 2% of predictions based on H-alpha), broad (>~ 10,000 km/s at the base), and highly blueshifted (centroid at ~ -3500 km/s) Ly-alpha emission. The FUV continuum emission is slightly declining at shorter wavelengths (consistent with F_lambda ~ lambda^1.7) and does not show the presence of any obvious photospheric or wind stellar features. Surprisingly, the FUV spectrum also does not show any unambiguous broad absorption features. It thus appears to be dominated by the AGN, rather than hot stars, and virtually unfiltered by the dusty FeLoBAL screen. The observed Ly-alpha emission is best explained if it is produced in the outflowing BAL cloud system, while the Balmer lines arise primarily from the standard broad emission line region seen through the dusty (A_V ~ 7 mag.) broad absorption line region. Two possible geometric models are discussed in the context of these new results.