Demographics of Isolated Galaxies Along the Hubble Sequence
  Isolated galaxies in low-density regions are significant in the sense that
they are least affected by the hierarchical pattern of galaxy growth, and
interactions with perturbers, at least for the last few Gyr. To form a
comprehensive picture of the star formation history of isolated galaxies, we
constructed a catalog of isolated galaxies and their comparison sample in
relatively denser environments. The galaxies are drawn from the SDSS DR7 in the
redshift range of $0.025<z<0.044$. We performed a visual inspection and
classified their morphology following the Hubble classification scheme. For the
spectroscopic study, we make use of the OSSY catalog. We confirm most of the
earlier understanding on isolated galaxies. The most remarkable additional
results are as follows. Isolated galaxies are dominantly late type with the
morphology distribution (E: S0: S: Irr) = (9.9: 11.3: 77.6: 1.2)\%. The
frequency of elliptical galaxies among isolated galaxies is only a third of
that of the comparison sample. Most of the photometric and spectroscopic
properties are surprisingly similar between isolated and comparison samples.
However, early-type isolated galaxies are less massive by 50\% and younger (by
H$\beta$) by 20\% than their counterparts in the comparison sample. This can be
explained as a result of different merger and star formation histories for
differing environments in the hierarchical merger paradigm. We provide an
on-line catalog for the list and properties of our sample galaxies.
