The peculiar optical spectrum of Mrk 231

• Published in: Astronomy and astrophysics (Berlin) Vol. 690; p. A155
• Main Authors: Misquitta, Persis, Dünnebier, Benita, Eckart, Andreas, Bhat, Harshitha
• Format: Journal Article
• Language: English
• Published: 01.10.2024
• ISSN: 0004-6361; 1432-0746
• DOI: 10.1051/0004-6361/202347589

Markarian 231 (Mrk 231) is one of the closest and brightest ultra-luminous infrared galaxies that is considered to be in a late galaxy merger stage. The optical spectrum of Mrk 231 presents peculiar features and has been the subject of detailed studies. In this paper we study the optical long-slit spectroscopy of the central arcsecond of Mrk 231 as well as six off-nuclear spectral positions observed using the multi-object double spectrographs at the Large Binocular Telescope. Additionally, we carefully examine acquisition images taken before the spectroscopic observations. We find that both the nuclear and off-nuclear spectra of Mrk 231 exhibit broad H α emission lines, which could be attributed to high-velocity outflows from the nucleus of Mrk 231, an extreme circumnuclear starburst, or a disturbed circumnuclear disc and broad line region associated with the merger of galaxies. The narrow H α emission line has three components in every spectrum, which could be indicative of low-velocity outflows or a disturbed structure of the narrow-line region. The region to the south of the nucleus exhibits weak H β emission, and the regions to the north show no or extremely faint H β emission. The continuum has a stronger blue component in the south compared to the north. The central spectrum has prominent broad absorption lines at a redshift of ∼0.0262, which is distinct from the redshift of the emission line system, ∼0.0422. Broad absorption lines belonging to at least three systems are present in all of the off-nuclear spectra, but the regions to the south have their most prominent absorption lines at a redshift of ∼0.0262, and all three systems have absorption lines of similar strengths in the regions to the north. A careful examination of the acquisition images of Mrk 231 shows that there is diffuse emission in the region adjacent to the northern tail, extending in the north-east direction almost orthogonally, which might be an indication of a more complex interaction history.