NGC-5291 - IMPLICATIONS FOR THE FORMATION OF DWARF GALAXIES

The possible formation and evolution of dwarf irregular galaxies from material derived from perturbed evolved galaxies is addressed via an H I study of a likely example, the peculiar system NGC 5291. This syste m, located in the western outskirts of the cluster Abell 3574, contain s the lenticular galaxy NGC 5291 which is in close proximity to a dist urbed companion and is flanked by an extensive complex of numerous kno ts extending roughly 4' north and 4' south of the galaxy, In an initia l optical and radio study, Longmore et al. (1979, MNRAS, 188, 285) sho wed that these knots have the spectra of vigorous star-forming regions , and suggested that some may in fact be young dwarf irregular galaxie s. High resolution 21-cm line observations taken with the VLA are pres ented here and reveal that the H I distribution associated with this s ystem encompasses not only the entire N-S complex of optical knots, bu t also forms an incomplete ring or tail that extends approximately 3' to the west. The HI associated with NGC 5291 itself shows a high veloc ity range; the Seashell is not detected. The formation mechanism for t his unusual system is unclear and two models-a large, low-luminosity r am-swept disk, and a ram-swept interaction-are discussed. The H I in t he system contains numerous concentrations, mostly along the N-S are o f the star-forming complexes, which generally coincide with one or mor e optical knots; the larger H I features contain several X10(9) M-. of gas. Each of the knots is compared to a set of criteria designed to d etermine if these objects are bound against their own internal kinetic energy and are tidally stable relative to the host galaxy. An analysi s of the properties of the H I concentrations surrounding the optical star-forming complexes indicates that at least the largest of these is a bound system; it also possesses a stellar component. It is suggeste d that this object is a genuinely young dwarf irregular galaxy that ha s evolved from the material associated with the system and that this e ntire complex contains several proto or young dwarf irregular galaxies in various stages of development. We are therefore witnessing the ear ly evolution of a number of genuinely young galaxies. Given the eviden t importance of the NGC 5291 system as a ''nursery'' for young galaxie s, careful modeling is required if we are to understand this remarkabl e galaxy. (C) 1997 American Astronomical Society.