Spectroscopic confirmation of hydrogen alpha-selected satellite galaxies
  We present a spectroscopic test confirming the potential of narrow-band
optical imaging as a method for detecting star-forming satellites around nearby
galaxies. To date the efficiency of such methods, and particularly the fraction
of false detections resulting from its use, has not been tested. In this paper
we use optical spectroscopy to verify the nature of objects that are apparently
emission-line satellites, taken from imaging presented elsewhere. Observations
of 12 probable satellites around 11 host galaxies are presented and used to
compare the recession velocities of the host and satellite. This test confirms,
in all cases, that there is genuine line emission, that the detected line is
hydrogen alpha, and that the satellites have similar recession velocities to
their hosts with a maximum difference of ~ 250 km/s, consistent with their
being gravitationally bound companions. We conclude that the spectroscopy has
confirmed that narrow-band imaging through H alpha filters is a reliable method
for detecting genuine, star-forming satellites with low contamination from
galaxies seen in projection along the line-of-sight.
