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We investigate the utility of the asymptotic giant branch (AGB) and the redgiant branch (RGB) as probes of the star formation history (SFH) of the nearby(D=2.5 Mpc) dwarf irregular galaxy, KKH 98. Near-infrared (IR) Keck Laser GuideStar Adaptive Optics (AO) images resolve 592 IR bright stars reaching over 1magnitude below the Tip of the Red Giant Branch. Significantly deeper optical(F475W and F814W) Hubble Space Telescope images of the same field contain over2500 stars, reaching to the Red Clump and the Main Sequence turn-off for 0.5Gyr old populations. Compared to the optical color magnitude diagram (CMD), thenear-IR CMD shows significantly tighter AGB sequences, providing a good probeof the intermediate age (0.5 - 5 Gyr) populations. We match observed CMDs withstellar evolution models to recover the SFH of KKH 98. On average, the galaxyhas experienced relatively constant low-level star formation (5 x 10^-4 Moyr^-1) for much of cosmic time. Except for the youngest main sequencepopulations (age < 0.1 Gyr), which are typically fainter than the AO data fluxlimit, the SFH estimated from the the 592 IR bright stars is a reasonable matchto that derived from the much larger optical data set. Differences between theoptical and IR derived SFHs for 0.1 - 1 Gyr populations suggest that currentstellar evolution models may be over-producing the AGB by as much as a factorof three in this galaxy. At the depth of the AO data, the IR luminous stars arenot crowded. Therefore these techniques can potentially be used to determinethe stellar populations of galaxies at significantly further distances.

The Asymptotic Giant Branch and the Tip of the Red Giant Branch as Probes of Star Formation History: The Nearby Dwarf Irregular Galaxy KKH 98