Neutral Hydrogen and Star Formation in the Irregular Galaxy NGC 2366

We present UBVJHKHalpha and HI data of the irregular galaxy NGC 2366. It is anormal boxy-shaped disk seen at high inclination angle. We do not see anyunambiguous observational signature of a bar. There is an asymmetricalextension of stars along one end of the major axis of the galaxy, and this iswhere the furthest star-forming regions are found, at 1.3R_Holmberg. The HI isnormal in many respects but shows some anomalies: 1) The integrated HI showstwo ridges running parallel to the major axis that deproject to a large ring.2) The velocity field exhibits several large-scale anomalies superposed on arotating disk. 3) The inclination and position angles derived from thekinematics differ from those dervied from the optical and HI mor- phology. 4)There are regions in the HI of unusually high velocity dispersion thatcorrelate with deficits of HI emission in a manner suggestive of long-range,turbulent pressure equilibrium. Star-forming regions are found where the gasdensities locally exceed 6 Msolar/pc^2. NGC 2366, like other irregulars, haslow gas densities relative to the critical gas densities of gravitationalinstability models. Because of the lack of shear in the optical galaxy, thereis little competition to the slow gravitational contraction that follows energydissipation. However, the peak gas densities in the star-forming regions areequal to the local tidal densities for gravitational self-binding of a rotatingcloud. Evidently the large scale gas concentrations are marginally boundagainst background galactic tidal forces. This condition for self-binding maybe more fundamental than the instability condition because it is local,three-dimensional, and does not involve spiral arm generation as anintermediate step toward star formation.Comment: To be published in ApJ; better figures available ftp.lowell.edu, cd pub/dah/n2366pape