Markarian 421'S Unusual Satellite Galaxy

We present Hubble Space Telescope (HST) imagery and photometry of the active galaxy Markarian 421 and its companion galaxy 14'' to the east-northeast. 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. We detect no emission lines in the companion nucleus, though there is evidence for recent star formation. 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 Mrk 421 mass within the companion orbit (R ∼ 10 kpc) of 5.9 × 10¹¹ solar masses, and a mass-to-light ratio of ≥ 17. Our spectroscopy also shows for the first time the presence of Hα and [N II] emission lines from the nucleus of Mrk 421, providing another example of the appearance of new emission features in the previously featureless spectrum of a classical BL Lac object. We see both broad and narrow line emission, with a velocity dispersion of several thousand kilometers per second evident in the broad lines.