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

We present B- and R-band optical imaging and photometry, Hα narrowband imaging, near-infrared H-band imaging, and H I 21 cm spectroscopy of the nearby (Vh = 407 km s-1), Sd spiral galaxy UGC 7321. UGC 7321 exhibits a remarkably thin stellar disk with no discernible bulge component. The galaxy has a very diffuse, low surface brightness disk, which appears to suffer relatively little internal extinction in spite of its nearly edge-on geometry (i ≈ 88°). If seen face-on, UGC 7321 would have an observed central B-band surface brightness of only ∼23.4 mag arcsec-2. The UGC 7321 disk shows significant B-R color gradients in both the radial and vertical directions: Δ(B-R) ≥ 0.80 mag along the galaxy major axis, and Δ(B-R) as large as 0.45 mag is observed parallel to the galaxy minor axis. These color gradients cannot be explained solely by dust and are indicative of changes in the mix of stellar ages and/or metallicity as a function of both radius and height above the galaxy plane. The outer regions of the UGC 7321 disk are too blue to be explained by low metallicity alone (B-R ≤ 0.6) and must be relatively young. However, the galaxy also contains stellar populations with B-R > 1.1, indicating it is not a young or recently formed galaxy. The disk of UGC 7321 is not a simple exponential but exhibits a light excess at small radii, as well as distinct surface brightness zones. Despite its organized disk structure, many of the global properties of UGC 7321 (MB = -17.0; ℳ = 1.1 × 109; ℳ/LB = 1.1 M⊙/L⊙; W20 = 233 km s-1; hr = 2.1 kpc) are reminiscent of a dwarf galaxy. Together the properties of UGC 7321 imply that it is an underevolved galaxy in both a dynamical and in a star formation sense.