The Taxonomy of Blue Amorphous Galaxies. II. Structure and Evolution

Abstract

Dwarf galaxies play an important role in our understanding of galaxy formation and evolution, and starbursts are believed to aUect the structure and evolution of dwarf galaxies strongly. We have therefore embarked on a systematic study of 12 of the nearest dwarf galaxies thought to be undergoing bursts of star formation. These were selected primarily by their morphological type (blue ii amorphous ˇˇ galaxies). We show that these blue amorphous galaxies are not physically distinguishable from dwarfs selected as starbursting by other methods, such as blue compact dwarfs (BCDs) and H II galaxies. All these classes exhibit surface brightness pro—les that are exponential in the outer regions but often have a (r Z 1.5r e ) predominantly central blue excess, suggesting a young burst in an older, redder galaxy. Typically, the starbursting ii cores ˇˇ are young (D107¨108 yr) events compared to the older (D109¨1010 yr) underlying galaxy (the ii envelope ˇˇ). The ratio of the core to envelope in blue light ranges from essentially zero to about 2. These starbursts are therefore modest events involving only a few percent of the stellar mass. The envelopes have surface brightnesses that are much higher than typical dwarf irregular (dI) galaxies, so it is unlikely that there is a straightforward evolutionary relation between typical dIs and dwarf star- burst galaxies. Instead we suggest that amorphous galaxies may repeatedly cycle through starburst and quiescent phases, corresponding to the galaxies with strong and weak/absent cores, respectively. Once amorphous galaxies use up the available gas (either through star formation or galactic winds) so that star formation is shut oU, the faded remnants would strongly resemble dwarf elliptical galaxies. However, in the current cosmological epoch, this is evidently a slow process that is the aftermath of a series of many weak, recurring bursts. Present-day dEs must have experienced more rapid and intense evolution than this in the distant past. Subject headings: galaxies: evolutiongalaxies: photometrygalaxies: stellar content ¨ galaxies: starburstgalaxies: structure