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

We present an analysis of optical (B-R) and optical-infrared (R-K_s) color maps for 47 extremely 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 in a low surface brightness red envelope. This component is substantially thicker than the thin disk (a:b~4:1 vs. >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, even when the thin disk is extremely blue, and is consistent with a relatively old (>6 Gyr) stellar population that is not particularly metal-poor. The color difference between the embedded thin disk and the red stellar envelope varies systematically with rotation speed, reflecting an increasing age difference between the thin and thick components in lower mass galaxies, driven primarily by changes in the age of the thin disk. The red stellar envelopes are similar to the thick disk of the Milky Way, having common surface brightnesses, spatial distributions, mean ages, and metallicities. We argue that the ubiquity of the red stellar envelopes implies that the formation of the thick disk is a nearly universal feature of disk formation and is not necessarily connected to the formation of a bulge. Our data suggest that the thick disk forms early (>6 Gyr ago), even within galaxies where the bulk of the stars formed very recently (<2 Gyr). We argue that several aspects of our data and the observed properties of the Milky Way 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 younger thin disk and the older thick disk can become a strong constraint on cosmological constants and models of galaxy and/or structure formation.