A reference sample of face-on bulgeless galaxies
  We present a list of 220 face-on, almost bulgeless galaxies assumed to be
counterparts to the objects from the Reference Flat Galaxy Catalog (RFGC). We
selected the Sc, Scd and Sd-type galaxies according to their apparent axial
ratio $log(r_{25}) < 0.05$ and major standard angular diameter $log(d_{25}) >
0.90$ as defined in HyperLEDA. The sample objects are restricted by the radial
velocity $V_{LG} < 10000$ km/s and a declination of above $-30$ deg. The
morphological composition of our sample is quite similar to that of RFGC. We
notice the following common properties of face-on bulgeless galaxies. About
half of them have bar-like structures occurring in the whole range of the
absolute magnitudes of galaxies: from $-17$ to $-22$ mag. An essential part of
our sample (27-50%) exhibit distorted spiral patterns. The galaxies do not show
significant asymmetry in numbers of the "S"- and "Z"-like spin orientation. The
mean (pseudo)bulge-to-total mass ratio for the sample is estimated as 0.11. Due
to a negligible internal extinction, low-light background, and small projection
effect, the face-on Sc-Sd discs are suitable objects to recognize their central
nuclei as moderate-mass BH candidates. About 40-60% of the galaxies have
distinct unresolved nuclei, and their presence steeply depend on the luminosity
of the host galaxy.
