2016ApJ...826...59W

We present results from deep, wide-field surface photometry of three nearby (D=4-7 Mpc) spiral galaxies: M94 (NGC 4736), M64 (NGC 4826), and M106 (NGC 4258). Our imaging reaches a limiting surface brightness of µ_{B} ∼ 28-30 mag arcsec^–2 and probes colors down to µ_{B} ∼ 27.5 mag arcsec^–2. We compare our broadband optical data to available ultraviolet and high column density H i data to better constrain the star-forming history and stellar populations of the outermost parts of each galaxy's disk. Each galaxy has a well-defined radius beyond which little star formation occurs and the disk light appears both azimuthally smooth and red in color, suggestive of old, well-mixed stellar populations. Given the lack of ongoing star formation or blue stellar populations in these galaxies' outer disks, the most likely mechanisms for their formation are dynamical processes such as disk heating or radial migration, rather than inside-out growth of the disks. This is also implied by the similarity in outer disk properties despite each galaxy showing distinct levels of environmental influence, from a purely isolated galaxy (M94) to one experiencing weak tidal perturbations from its satellite galaxies (M106) to a galaxy recovering from a recent merger (M64), suggesting that a variety of evolutionary histories can yield similar outer disk structure. While this suggests a common secular mechanism for outer disk formation, the large extent of these smooth, red stellar populations-which reach several disk scale lengths beyond the galaxies' spiral structure-may challenge models of radial migration given the lack of any nonaxisymmetric forcing at such large radii.