A near-infrared stellar census of blue compact dwarf galaxies: the Wolf-Rayet galaxy I Zw 36

We report the results of near-IR imaging in J and H, of I Zw 36 (approximate toZ./14) with the Hubble Space Telescope. Whereas imaging with the pre-COSTAR Faint Object Camera (FOC) previously resolved hot and massive stars in the near-UV, the NICMOS data furnish a census of the cool, intermediate- and low-mass stars. There clearly was star formation in I Zw 36 prior to the activity which earned it its blue compact dwarf/Wolf-Rayet galaxy classification. The detection of luminous, asymptotic giant branch stars requires that stars formed vigorously several hundred megayears ago. The well-populated red giant branch indicates stars with ages of at least 1-2 Gyr (and possibly older than 10 Gyr). We use the tip-of-the-red-giant-branch method to derive a distance of greater than or equal to5.8 Mpc. This is the third in a series of papers on near-IR-resolved blue compact dwarf galaxies. We notice that the color-magnitude diagrams of VII Zw 403, Mrk 178, and I Zw 36 do not exhibit the gaps expected from an episodic mode of star formation. Using simulated color-magnitude diagrams we demonstrate for I Zw 36 that star formation did not stop for more than a few 10(8) yr over the past 10(9) yr, and we discuss the implications of this result.