Ha Images of Ultra-Flat Edge-On Spiral Galaxies
2020
Abstract—We present the Hα images of ultra-flat (UF) spiral galaxies seen practically edgeon. The galaxies have the angular diameter in the B band $$a > 1.9' $$ and the apparent axial ratio (a/b) > 10. We found that their Hα 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 Hα 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.
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