Near-Infrared Detection of a Super-Thin Disk in NGC 891
  We probe the disk structure of the nearby, massive, edge-on spiral galaxy NGC
891 with sub-arcsecond resolution JHKs-band images covering ~+/-10 kpc in
radius and +/-5 kpc in height. We measure intrinsic surface brightness profiles
using realistic attenuation corrections constrained from near- and mid-infrared
(Spitzer) color maps and three-dimensional Monte-Carlo radiative-transfer
models. In addition to the well-known thin and thick disks, a super-thin disk
with 60-80 pc scale-height - comparable to the star-forming disk of the Milky
Way - is visibly evident and required to fit the attenuation-corrected light
distribution. Asymmetries in the super-thin disk light profile are indicative
of young, hot stars producing regions of excess luminosity and bluer
(attenuation-corrected) near-infrared color. To fit the inner regions of NGC
891, these disks must be truncated within ~3 kpc, with almost all their
luminosity redistributed in a bar-like structure 50% thicker than the thin
disk. There appears to be no classical bulge but rather a nuclear continuation
of the super-thin disk. The super-thin, thin, thick, and bar components
contribute roughly 30%, 42%, 13%, and 15% (respectively) to the total Ks-band
luminosity. Disk axial ratios (length/height) decrease from 30 to 3 from
super-thin to thick components. Both exponential and sech^2 vertical SB
profiles fit the data equally well. We find that the super-thin disk is
significantly brighter in the Ks-band than typically assumed in integrated SED
models of NGC 891: it appears that in these models the excess flux, likely
produced by young stars in the super-thin disk, has been mistakenly attributed
to the thin disk.
