We present B - and R -band optical imaging and photometry , H \alpha narrow-band imaging , near-infrared H -band imaging , and H i 21-cm spectroscopy of the nearby ( V _ { h } =407 km/s ) , 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 \approx 88 ^ { \circ } ) .
If seen face-on , UGC 7321 would have an observed central B -band surface brightness of only \sim 23.4 mag arcsec ^ { -2 } .

The UGC 7321 disk shows significant B - R color gradients in both the radial and vertical directions : \Delta ( B - R ) \geq 0.80 magnitudes along the galaxy major axis , and \Delta ( B - R ) as large as 0.45 magnitudes is observed parallel to the galaxy minor axis .
These color gradients can not 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 \leq 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 ( M _ { B } = -17.0 ; { \cal M } _ { HI } =1.1 \times 10 ^ { 9 } { \cal M } _ { \odot } ; \frac { { \cal M } _ { HI } } { L _ { B } } =1.1 { \cal M } _ { \odot } / { \cal L } _ { \odot } ; W _ { 20 } =233 km s ^ { -1 } ; h _ { r } =2.1 kpc ) are reminiscent of a dwarf galaxy .
Together the properties of UGC 7321 imply that it is an under-evolved galaxy in both a dynamical and in a star-formation sense .