Near-Infrared Detection of a Super-Thin Disk in NGC 891

Abstract

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.