The probable binary galaxy system MKN 421: Kinematics & structure from optical observations.

We present Hubble Space Telescope (HST) imagery, and ground-based spectroscopy and CCD photometry of the active galaxy Markarian 421 and its companion galaxy 14 arcsec to the ENE. The HST images indicate that the companion is a morphological spiral rather than elliptical as previous ground-based imaging has concluded. The companion has a bright, compact nucleus, appearing unresolved in the HST images. This is suggestive of Seyfert activity, or possibly a highly luminous compact star cluster. We also report the results of high dynamic range long-slit spectroscopy with the slit placed to extend across both galaxies and nuclei. Velocities derived from a number of absorption lines visible in both galaxies indicate that the two systems are probably tidally bound and thus in close physical proximity. Using the measured relative velocities, we derive a lower limit on the MKN 421 mass within the companion orbit (R∼10 kpc) of 5.9×10[sup 11] solar masses, and a mass-to-light ratio of >=17. Our spectroscopy also shows for the first time the presence of Hα and [NII] emission lines from the nucleus of MKN 421. We see both broad and narrow line emission, with a velocity dispersion of several thousand km s-1 evident in the broad lines. Based on the imagery and broad-line velocity dispersion we find evidence for a black hole with mass of order 10[sup 9] M[sub &beye;] at the center of MKN 421. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]