The Extraordinary “Superthin” Spiral Galaxy UGC 7321. I. Disk Color Gradients and Global Properties from Multiwavelength Observations

We present B- and R-band imaging and photometry, H-alpha narrow-band imaging,NIR H-band imaging, and HI 21-cm spectroscopy of the nearby Sd spiral galaxyUGC7321. UGC7321 exhibits a remarkably thin stellar disk with no bulgecomponent. The galaxy has a very diffuse, low surface brightness disk, whichappears to suffer little internal extinction in spite of its edge-on geometry.The UGC7321 disk shows significant B-R color gradients in both the radial andvertical directions. These color gradients cannot be explained solely by dustand are indicative of changes in the mix of stellar ages and/or metallicity asa function of both radius and height above the galaxy plane. The outer regionsof the UGC7321 disk are too blue to be explained by low metallicity alone(B-R<0.6), and must be relatively young. However, the galaxy also containsstellar populations with B-R>1.1, indicating it is not a young orrecently-formed galaxy. The disk of UGC7321 is not a simple exponential, butexhibits a light excess at small radii, as well as distinct surface brightnesszones. Together the properties of UGC7321 imply that it is an under-evolvedgalaxy in both a dynamical and in a star-formation sense. (Abridged)Comment: Accepted to the Astronomical Journal; 28 pages, 1 table and 21 figures (GIF and postscript