Markarian 421's Unusual Satellite Galaxy
Gorham, P. W., van Zee, L., Unwin, S. C., & Jacobs, C. S. 2000, AJ, 119, 1677
Abstract
We present Hubble Space Telescope (HST) imagery and photometry of
the active galaxy Markarian 421 and its companion galaxy 14 arcsec
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 MK 421 mass within the
companion orbit (R ~ 10 kpc) of 5.9 x 10^11 solar masses, and a
mass-to-light ratio of > 17. Our spectroscopy also shows for the first time
the presence of H alpha and [NII] emission lines from the nucleus of MK 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 dispersionn of
several thousand kilometers per second evident in the broad lines.
ADS abstract