The cometary blue compact dwarf galaxies Mkn 59 and Mkn 71 - A clue to dwarf galaxy evolution?

"Cometary" Blue Compact Dwarf Galaxies (iI,C BCDs) are characterized by an off-center starburst close to the end of their elongated stellar bodies. Th is rare phenomenon may carry some clues on how collective star formation ig nites and propagates in gas-rich low-mass stellar systems. This off-center burst may be a fortuitous enhancement of the otherwise moderate star-formin g activity of a dwarf irregular (dI), or may be caused by a set of special properties of such systems or their environment. We attempt here a first in vestigation of this issue by analysing two prototypical examples of cometar y dwarf galaxies, the nearby II,C BCDs Markarian 59 and Markarian 71, both containing an extraordinarily luminous HII region in the outskirts of a dI- like host. Using deep ground-based spectrophotometric data(1,2) and HST ima ges(3), we study the physical state of the starburst regions and the struct ural properties of the underlying irregular galaxies. We find that the aver age metallicities show small scatter in the vicinity of the star-forming re gions and along the major axis of Mkn 59 which suggests that mixing of heav y elements must have been efficient on scales of several kpc. The azimuthal ly averaged radial intensity distributions of the underlying host galaxies in either iI,C BCD can be approximated by an exponential law with a central surface brightness and scale length that is intermediate between typical i E/nE BCDs and dwarf irregulars. Spectral population synthesis models in com bination with colour magnitude diagrams and colour profiles yield a most pr obable formation age of similar to 2 Gyr for the low surface brightness (LS B) host galaxies in both LI,C BCDs, with upper age limits of similar to 4 G yr for Mkn 59 and similar to 3 Gyr for Mkn 71, i.e. significantly lower tha n the typical age of several Gyr derived for the LSB component of iE/nE BCD s. These findings raise the question whether iI,C systems form a distinct p hysical class within BCDs with respect to the age and structural properties of their hosts, or whether they represent an evolutionary stage connecting young i0 BCDs and "classical" iE/nE BCDs. In spite of the scarcity of avai lable data, a review of the properties of analogous objects studied in the local universe and at medium redshifts provides some support for this evolu tionary hypothesis.