The Extraordinary “Superthin” Spiral Galaxy UGC 7321. II. The Vertical Disk Structure

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Abstract
We explore the vertical light distribution as a function of galactocentric radius in the edge-on (i = 88°) Sd "superthin" galaxy UGC 7321. UGC 7321 is a low-luminosity spiral (MB,i = -17.0) with a diffuse, low surface brightness stellar disk and no discernible bulge component. Within ~ 025 (~725 pc) of the disk center the global luminosity profile of UGC 7321 can be reasonably characterized by an exponential function with a scale height hz ~ 29 (~140 pc) in H and hz ~ 31 (~150 pc) in R, making this among the thinnest galaxy disks known. Near the disk center we derive a ratio of disk scale length to global disk scale height hr/hz ~ 14 in both H and R; near the edge of the disk, hr/hz ~ 10. At intermediate galactocentric radii (025 ≤ |r| ≤ 15), the disk of UGC 7321 becomes less peaked than an exponential near the galactic plane. At these radii the vertical luminosity profiles can be well reproduced by a linear combination of two isothermal disk components of differing scale heights. These fits, together with the strong disk color gradients by Matthews, Gallagher, & van Driel, suggest that UGC 7321 has multiple disk subcomponents comprised of stellar populations with different ages and velocity dispersions. Thus even examples of the thinnest pure disk galaxies exhibit complex structure and signatures of dynamical heating.
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