A Structural and Dynamical Study of Late-Type, Edge-on Galaxies. II. Vertical Color Gradients and the Detection of Ubiquitous Thick Disks

(abridged) We present an analysis of B-R and R-K_s color maps for 47late-type, edge-on, unwarped, bulgeless disk galaxies spanning a wide range ofmass. The color maps show that the thin disks of these galaxies are embeddedwithin a low surface brightness red envelope that is substantially thicker thanthe thin disk (a/b~4:1 vs a/b>8:1), extends to at least 5 vertical disk scaleheights above the galaxy midplane, and has a radial scale length that appearsto be uncorrelated with that of the embedded thin disk. The color of the redenvelope is similar from galaxy to galaxy and is consistent with a relativelyold (>6Gyr) stellar population that is not particularly metal-poor. The colordifference between the thin disk and the envelope varies systematically withrotation speed, indicating a younger thin disk relative to the red envelope inlower mass galaxies. The red stellar envelopes are similar to the MW thickdisk, having common surface brightnesses, spatial distributions, mean ages, andmetallicities. The ubiquity of the red stellar envelopes implies that theformation of the thick disk is a nearly universal feature of disk formation andneed not be associated with bulge formation. Furthermore, our data suggest thatthe thick disk forms early, even in cases where the majority of star formationwas recent. Finally, we find that our data and the observed properties of theMW 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 majormerging, and the age difference between the thin and thick disks can become astrong constraint on cosmological constants and models of galaxy and/orstructure formation.Comment: accepted to the September 2002 Astronomical Journal, LaTeX, 31 pages including figure