Star formation in and evolution of the blue compact dwarf galaxy UGC 6456 determined from Hubble Space Telescope images

Photometry on the UV1 system has been performed on the resolved stellar content of the blue compact dwarf galaxy UGC 6456 using Wide Field Planetary Camera 2 (WFPC2) images obtained with the Hubble Space Telescope. The resulting color-magnitude diagram (CMD) goes to about V = 27.5 and reveals not only a young population of blue main-sequence stars and blue and red supergiants, but also an older evolved population of red giants and a fairly well represented asymptotic giant branch. The distance to the galaxy is estimated from the tip of the red giant branch to be 4.5 Mpc, placing it about 1.5 Mpc farther away than the major members of the M81 Group, with which it is usually associated. The youngest stars are generally associated with H II regions shown on our H alpha image and are largely confined to the 745 pc field of our PC images. A comparison of their distribution in the CMD with theoretical isochrones suggests ages from 4 to 10 Myr. The population of older stars is found throughout all WFPC2 camera fields and seems to show an elliptical distribution with an aspect ratio of about 2.4 and an exponential falloff in surface density with distance from a center of symmetry that is not far from the centroid of the youngest stars. Theoretical modeling of the CMD at a metallicity of Z = 0.001 suggests star formation in the age interval 1-2 Gyr, a strong burst in the interval 600-800 Myr, and a lower rate of star formation up to the present. The evidence is compatible with a scenario beginning with the formation of a population of low-metallicity stars, enriching a major residual of prestellar material that subsequently fueled an active episode of star formation. That burst of star formation must have been particularly spectacular and may be related to the activity we now see in the distant blue dwarf galaxies revealed in deep imaging.