The remarkable Seyfert galaxy Markarian 231

Spectra, scans and a direct electronograph of Markarian 231 are presented. The continuum can be represented by a $${f}_{\nu }\propto {\nu }^{-2.5}$$ power law but the presence of optical and infrared absorption features favours interpretation as either a reddened $${f}_{\nu }\sim $$ constant spectrum or a reddened 10 000 K blackbody distribution from hot stars. Either of these models has intrinsic absolute magnitude, $${M}_{v}\,\sim \,-\,25.1,$$ placing Markarian 231 among the quasars in optical luminosity. The observed emission spectrum can be synthesized from blended features of HI, He I, [O II], Ca II, Fe II, [Fe II] and Ni II at a redshift of 12 600±200km/s. Except for the [O II], which must be emitted from a separate low-density region, these lines indicate a high-density (n_e ~ 10⁷ cm^-3) emission region. The presence of Ca II triplet emission and no corresponding H and K emission shows that the optical depths in H and K in the emission region are large $$({\tau }_{H}\sim 100)$$. There are three absorption line redshifts – System I containing resonance lines of Ca II and Na I and λ 3889 of He I at a redshift of ~ 8000 km/s, System II represented by D-lines and λ 3889 at 6250 km/s and System III featuring the K-line and members of the Balmer series at 12 900 km/s. If the observed continuum is emitted by hot stars the System III lines represent the absorption spectra of those stars. The lines are much broader than the absorption lines seen in high-redshift quasars. The doublet ratio for H and K in System I indicates a low optical depth $$({\tau }_{H}\sim 0.6)$$ along our line of sight to the nucleus in contrast to the result from the emission lines. The calcium and iron emission lines can be produced by fluorescence if the ions experience the unreddened radiation of the continuum source. Two possible configurations for the nuclear components of Markarian 231 are presented.