Neutral hydrogen and star formation in the irregular galaxy NGC 2366

Abstract

We present deep UBV JHK Hα images and H I maps of the irregular galaxy NGC 2366. Optically, NGC 2366 is a boxy-shaped exponential disk seen at high inclination angle. The scale length and central surface brightness of the disk are normal for late-type galaxies. Although NGC 2366 has been classified as a barred Im galaxy, we do not see any unambiguous observational signature of a bar. There is an asymmetrical extension of stars along one end of the major axis of the galaxy, and this is where the furthest star-forming regions are found, at a radius of 1.3 times the Holmberg radius. The star formation activity of the galaxy is dominated by the supergiant H II complex NGC 2363, but the global star formation rate for NGC 2366 is only moderately elevated relative to other Im galaxies. The star formation activity drops off with radius approximately as the starlight in the inner part of the galaxy but it drops faster in the outer part. There are some peculiar features of the H I distribution and kinematics. First, the integrated H I shows two ridges running parallel to the major axis that when deprojected appear as a large ring. Second, the velocity field exhibits several large-scale anomalies superposed on a rotating disk; some of these may be from a weak bar that has no inner Lindblad resonance. Third, the inclination and position angles derived from the kinematics differ from those derived from the optical and H I morphology. Fourth, there are regions in the H I of unusually high velocity dispersion, but these regions are not associated with the optical galaxy nor any obvious H I feature. Instead the velocity dispersion correlates with a deficit of H I emission in a manner suggestive of long-range, turbulent pressure equilibrium. In other respects the H I is fairly normal. The azimuthally averaged surface density of H I is comparable to that of other irregulars in the inner part of the galaxy but drops off slower and extends further in the outer parts. The H I around the star-forming complex NGC 2363 is fairly unremarkable. As in other disk galaxies, the gas in NGC 2366 is lumpy and star-forming regions are associated with these H I complexes. H II regions are found where the gas densities locally exceed 6 M⊙ pc-2. This threshold is required to provide a cool phase of H I as a first step toward star formation. NGC 2366, like other irregulars, has low gas densities relative to the critical gas densities of gravitational instability models, so large-scale gravitational instabilities operate slowly or not at all. Considering the lack of shear in the optical part of this galaxy, the relative slowness of such instabilities may not be a problem - there is little competition to the slow gravitational contraction that follows energy dissipation. This differs from the situation in giant spiral disks where the shear time is short, comparable to the energy dissipation time, and strong self-gravity is required for a condensation to grow and dissipate its turbulent energy before it shears away. The subthreshold surface densities are also not unusual if they are viewed using the critical tidal density for gravitational self-binding of a rotating cloud, rather than the critical surface density from the usual disk instability condition. The peak densities in all regions of star formation are equal to the local tidal densities, giving an agreement between these two quantities that is much better than between the surface density and the critical value. Evidently the large-scale gas concentrations are all marginally bound against background galactic tidal forces. This condition for self-binding may be more fundamental than the instability condition because it is local, three-dimensional, and does not involve spiral arm generation as an intermediate step toward star formation.