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 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. 
Julianne J. Dalcanton (U. of Washington), Rebecca A. Bernstein (U. of   Michigan)