Recent star formation in clusters of galaxies:: Extremely compact starbursts in A539 and A634

We report on the detection of two H alpha-emitting extremely compact objects from deep images of the A634 and A539 clusters of galaxies at z similar to 0.03. Follow-up long-slit spectroscopy of these two unresolved sources revealed that they are members of their respective clusters, showing H II-type spectra. The luminosity and the extreme equivalent width of H alpha + [N II] measured for these sources, together with their very compact appearance, has raised a question about the origin of these intense starbursts in the cluster environment. We propose that the compact starburst in A539 resulted from the compression of the interstellar gas of a dwarf galaxy when entering the cluster core, while the starburst galaxy in A634 is likely to be the result of a galaxy-galaxy interaction, illustrating the preprocessing of galaxies during their infall toward the central regions of clusters. The contribution of these compact star-forming dwarf galaxies to the star formation history of galaxy clusters is discussed, as well as a possible link with the recently discovered early-type ultracompact dwarf galaxies. We note that these extreme objects will rarely be detected in normal magnitude-limited optical or NIR surveys, mainly due to their low stellar masses (on the order of 10(6) M-circle dot), whereas they will easily show up in dedicated H alpha surveys given the high equivalent width of their emission lines.