VLA Observations of HI 1225+ 01

Abstract

H I 1225+01 [Giovanelli & Haynes, ApJ, 346, L5 (1989)] is a gas-rich system consisting of two major components, a Northeast clump (NE clump), a Southwest clump (SW clump), and a 'Bridge' component between the two [Giovanelll et al. AJ, 101,1242(1991) hereafter referred to as (GWH)]. The NE clump has an optical counterpart, a magellanic-type dwarf irregular galaxy located at the center of the clump, while the SW clump has no detected stellar emission to a surface magnitude limit of 27 mag arcsec^2^ in V and 26.5 mag arcsec^-2^ in J [Salzer et al. AJ, 101, 1258 (1991)]. Here we present VLA D array maps of the HI emission from this object. The angular resolution of the VLA data is 40", considerably better than the 3.3' resolution of the best previously available Arecibo map. All three components of H I 1225+01 are clearly seen in the higher resolution maps, however, the SW clump appears to have a diffuse component that is resolved out at the VLA. The integrated HI emission map for the NE clump shows large spiral arm-like features and the channel maps also resemble those for galaxies with pronounced spiral arms. Despite the presence of substantial kinematical disturbances, the NE clump can clearly be seen to be rotating. Accurate determination of the rotation curve is, however, hampered both by these kinematical disturbances and the uncertain inclination of the clump. The adopted rotation curve gives a total dynamical mass of M_dyn_ ~ 1.0 x 10^10^ d_20_M_sun_, compared to the total gas mass off M_gas_ ~ 3 x 10^9^d_20_^2^ M_sun_. Star formation in the NE clump has occurred only above a (inclination corrected) threshold HI column density of ~4.6 x 10^20^ atoms cm^-2^, which agrees within the uncertainties to the theoretically expected gravitational instability threshold density. The SW clump has an observed peak column density that is higher than that of the NE clump, despite having a H I mass that is a nearly a factor of 2 smaller. This, combined with its high axial ratio, suggest that it is in fact a highly inclined disk. The inferred rotation velocity of the SW clump is, however, quite small, ~13 km s^-1^. This geometry also implies that the 'face-on' HI column density is smaller than the gravitational instability threshold density, consistent with no stellar emission being detected from the clump. From the radial velocity field across H I 1225+01, it can be seen that the SW component is undergoing a `direct' encounter, this is consistent with the considerable tidal damage of this component. The Bridge is then material that has been pulled out from the SW clump and will presumably be captured by the NE clump. The spiral arm-like features of the NE clump are also characteristic of two sided tidal responses. The morphology and kinematics of H I 1225+01 are thus qualitatively well explained as a consequence of tidal interaction between the main two components, consistent with the relative isolation of the object [GWH; Salzer, AJ, 103,385(1992)].