Redshift-Independent Distances in the NASA/IPAC Extragalactic Database:
  Methodology, Content and Use of NED-D
  Estimates of galaxy distances based on indicators that are independent of
cosmological redshift are fundamental to astrophysics. Researchers use them to
establish the extragalactic distance scale, to underpin estimates of the Hubble
constant, and to study peculiar velocities induced by gravitational attractions
that perturb the motions of galaxies with respect to the Hubble flow of
universal expansion. In 2006 the NASA/IPAC Extragalactic Database (NED) began
making available a comprehensive compilation of redshift-independent
extragalactic distance estimates. A decade later, this compendium of distances
(NED-D) now contains more than 100,000 individual estimates based on primary
and secondary indicators, available for more than 28,000 galaxies, and compiled
from over 2,000 references in the refereed astronomical literature. This
article describes the methodology, content, and use of NED-D, and addresses
challenges to be overcome in compiling such distances. Currently, 75 different
distance indicators are in use. We include a figure that facilitates comparison
of the indicators with significant numbers of estimates in terms of the
minimum, 25th percentile, median, 75th percentile, and maximum distances
spanned. Brief descriptions of the indicators, including examples of their use
in the database, are given in an Appendix.
