HiImaging Observations of Superthin Galaxies. I. UGC 7321

Abstract

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-1 channel-1, which correspond to a column density of (8–9) × 1018 atoms cm-2 (1 σ). UGC 7321 has a gas-rich disk, with MH I = (1.06 ± 0.01) × 109 d M⊙ and MH I/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 (DH I = 865 ± 015 at nH I = 3 × 1019 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 ~25 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 ~12d, implying that UGC 7321 is a highly dark-matter–dominated galaxy.