Ultra-Flat Galaxies Selected from RFGC Catalog. II. Orbital Estimates of
  Halo Masses
  We used the Revised Flat Galaxy Catalog (RFGC) to select 817 ultra-flat (UF)
edge-on disk galaxies with blue and red apparent axial ratios of $(a/b)_B >
10.0$ and $(a/b)_R > 8.5$. The sample covering the whole sky, except the Milky
Way zone, contains 490 UF galaxies with measured radial velocities. Our
inspection of the neighboring galaxies around them revealed only 30 companions
with radial velocity difference of $\mid\Delta V\mid<500$ km s$^{-1}$ inside
the projected separation of $R_p < 250$ kpc. Wherein, the wider area around the
UF galaxy within $R_p < 750$ kpc contains no other neighbors brighter than the
UF galaxy itself in the same velocity span. The resulting sample galaxies
mostly belong to the morphological types Sc, Scd, Sd. They have a moderate
rotation velocity curve amplitude of about $120$ km s$^{-1}$ and a moderate
K-band luminosity of about $10^{10}L_{\odot}$. The median difference of radial
velocities of their companions is $87$ km s$^{-1}$, yielding the median orbital
mass estimate of about $5\times10^{11}M_{\odot}$. Excluding six probable
non-isolated pairs, we obtained a typical halo-mass-to-stellar-mass of UF
galaxies of about $30$, what is almost the same one as in the principal spiral
galaxies, like M 31 and M 81 in the nearest groups. We also note that
ultra-flat galaxies look two times less "dusty" than other spirals of the same
luminosity.
