The taxonomy of blue amorphous galaxies. II. Structure and evolution

Dwarf galaxies play an important role in our understanding of galaxy format ion and evolution, and starbursts are believed to affect the structure and evolution of dwarf galaxies strongly. We have therefore embarked on a syste matic study of 12 of the nearest dwarf galaxies thought to be undergoing bu rsts of star formation. These were selected primarily by their morphologica l type (blue "amorphous" galaxies). We show that these blue amorphous galax ies are not physically distinguishable from dwarfs selected as starbursting by other methods, such as blue compact dwarfs (BCDs) and H II galaxies. Al l these classes exhibit surface brightness profiles that are exponential in the outer regions (r greater than or similar to 1.5r(e)) but often have a predominantly central blue excess, suggesting a young burst in an older, re dder galaxy. Typically, the starbursting "cores" are young(similar to 10(7) -10(8) yr) events compared to the older (similar to 10(9)-10(10) yr) underl ying galaxy (the "envelope"). The ratio of the core to envelope in blue lig ht ranges from essentially zero to about 2. These starbursts are therefore modest events involving only a few percent of the stellar mass. The envelop es have surface brightnesses that are much higher than typical dwarf irregu lar (dI) galaxies, so it is unlikely that there is a straightforward evolut ionary relation between typical dIs and dwarf starburst galaxies. Instead w e suggest that amorphous galaxies may repeatedly cycle through starburst an d quiescent phases, corresponding to the galaxies with strong and weak/abse nt cores, respectively. Once amorphous galaxies use up the available gas (e ither through star formation of galactic winds) so that star formation is s hut off, the faded remnants would strongly resemble dwarf elliptical galaxi es. However, in the current cosmological epoch, this is evidently a slow pr ocess that is the aftermath of a series of many weak, recurring bursts. Pre sent-day dE's must have experienced more rapid and intense evolution than t his in the distant past.