The Properties of Galaxies in Low Density Regions from the ALFALFA
  Survey
  Galaxies detected in the 21-cm line of neutral hydrogen (HI) from the
on-going Arecibo Legacy Fast ALFA (ALFALFA) blind extragalactic HI survey have
been cross-correlated with Sloan Digital Sky Survey Data Release 7 (Abazajian
et al. 2009) in order to define a reference sample of HI content in regions of
low galactic density. This observational sample will be used in the future to
derive new standards of normal atomic gas content that allow a statistical
investigation of the HI properties of galaxies in differing environments of the
local universe. As a previous step, we compare here morphological indicators,
like color or light concentration index, of ALFALFA detections and
non-detections in low density regions. Our examination is extended also to a
small data set of isolated galaxies. This kind of analysis is necessary in
order to characterize as accurately as possible the type of galaxies that
ALFALFA is detecting.
