H-alpha Images of Ultra-Flat Edge-On Spiral Galaxies
  We present the H$\alpha$ images of ultra-flat (UF) spiral galaxies seen
practically edge-on. The galaxies have the angular diameter in the $B$ band $a>
1.9^{\prime}$ and the apparent axial ratio $(a/b) >10$. We found that their
H$\alpha$ images look, on average, almost two times thinner than those in the
red continuum. The star-formation rate in the studied objects, determined from
the H$\alpha$ flux, is in good agreement with that calculated from the $FUV$
flux from the GALEX survey if we use the modified Verheijen and Sancisi formula
taking into account the internal extinction in the UF galaxies. The logarithm
of the specific star-formation rate in the UF galaxies shows a small scatter,
$0.19$, with a smooth decrease from $-10.4$ for dwarf spirals to $-10.7$ for
massive ones. The relative amount of the hydrogen mass in UF disks varies from
about 50\% in dwarf disks to about 8\% in massive ones. Structural distortions
are less common in the UF galaxies (about 16\%) than those in thick (less
isolated) disks of edge-on spiral galaxies. On the cosmic time scale, 13.7 Gyr,
large spiral disks are more efficient "engines" for gas processing into stars
than dwarf spirals.
