  (abridged) We present an analysis of B-R and R-K_s color maps for 47
late-type, edge-on, unwarped, bulgeless disk galaxies spanning a wide range of
mass. The color maps show that the thin disks of these galaxies are embedded
within a low surface brightness red envelope that is substantially thicker than
the thin disk (a/b~4:1 vs a/b>8:1), extends to at least 5 vertical disk scale
heights above the galaxy midplane, and has a radial scale length that appears
to be uncorrelated with that of the embedded thin disk. The color of the red
envelope is similar from galaxy to galaxy and is consistent with a relatively
old (>6Gyr) stellar population that is not particularly metal-poor. The color
difference between the thin disk and the envelope varies systematically with
rotation speed, indicating a younger thin disk relative to the red envelope in
lower mass galaxies. The red stellar envelopes are similar to the MW thick
disk, having common surface brightnesses, spatial distributions, mean ages, and
metallicities. The ubiquity of the red stellar envelopes implies that the
formation of the thick disk is a nearly universal feature of disk formation and
need not be associated with bulge formation. Furthermore, our data suggest that
the thick disk forms early, even in cases where the majority of star formation
was recent. Finally, we find that our data and the observed properties of the
MW thick disk argue in favor of a merger origin for the thick disk population.
If so, then the age of the thick disk marks the end of the epoch of major
merging, and the age difference between the thin and thick disks can become a
strong constraint on cosmological constants and models of galaxy and/or
structure formation.
