Imaging Observations of Superthin Galaxies . I . Ugc 7321

We have used the Very Large Array to image the isolated “superthin” galaxy UGC 7321 in the H i line with a spatial resolution of 16′′ and a spectral resolution of 24 kHz (5.2 km s−1). We have reached a sensitivity of (0.36 – 0.40) mJy/beam per channel, which correspond to a column density of (8 – 9)×1018 atoms cm−2 (1σ). UGC 7321 has a gas-rich disk with MHI= (1.06± 0.01)× 10 d10 M and MHI/LB= 1.0 (d10 is the distance to UGC 7321 in units of 10 Mpc, the value adopted in this paper), and no detectable radio continuum emission (FCONT = 0.41 ± 0.25 mJy). The global H i distribution of UGC 7321 is rather symmetric and extends to ∼1.5 times the optical radius (DHI = 8′ .65 ± 0′ .15 at nHI = 3 × 10 atoms cm−2). An “integral sign” warp is observed in the H i disk, commencing near the edge of the stellar distribution, and twisting back toward the equatorial plane in the outermost regions. In addition, the position-velocity diagram suggests the presence of a bar or inner arm within ∼40” from the center. The rotation curve of UGC 7321 is slowly rising; it reaches its asymptotic velocity of ∼110 km s−1 at ∼ 2′ .5 from the center (about 0.9 optical radii) and declines near the edge of the H i disk. The ratio of the inferred dynamical mass to the mass in gas and stars is ∼12 d−1 10 , implying that UGC 7321 is a highly dark-matter dominated galaxy. Subject headings: galaxies: spiral—galaxies: kinematics and dynamics—galaxies: individual (UGC 7321)—galaxies: ISM—ISM: H i