Extraplanar HII Regions in the edge-on Spiral Galaxies NGC 3628 and NGC
  4522
  Gas infall and outflow are critical for determining the star formation rate
and chemical evolution of galaxies but direct measurements of gas flows are
diffcult to make. Young massive stars and HII regions in the halos of galaxies
are potential tracers for accretion and/or outflows of gas. Gas phase
abundances of three HII regions in the lower halos of the edge-on galaxies NGC
3628 and NGC 4522 are determined by analysing optical long-slit spectra. The
observed regions have projected distances to the midplane of their host from
1.4 to 3 kpc. With the measured flux densities of the optical nebular emission
lines, we derive the oxygen abundance 12 + log(O/H) for the three extraplanar
HII regions. The analysis is based on one theoretical and two empirical
strong-line calibration methods. The resulting oxygen abundances of the
extraplanar HII regions are comparable to the disk HII regions in one case and
a little lower in the other case. Since our results depend on the accuracy of
the metallicity determinations, we critically discuss the difference of the
calibration methods we applied and confirm previously noted offsets. From our
measurements, we argue that these three extraplanar HII regions were formed in
the disk or at least from disk material. We discuss the processes that could
transport disk material into the lower halo of these systems and conclude that
gravitational interaction with a companion galaxy is most likely for NGC 3628
while ram pressure is favoured in the case of NGC 4522.
