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
  We investigate the utility of the asymptotic giant branch (AGB) and the red
giant 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 Guide
Star Adaptive Optics (AO) images resolve 592 IR bright stars reaching over 1
magnitude below the Tip of the Red Giant Branch. Significantly deeper optical
(F475W and F814W) Hubble Space Telescope images of the same field contain over
2500 stars, reaching to the Red Clump and the Main Sequence turn-off for 0.5
Gyr old populations. Compared to the optical color magnitude diagram (CMD), the
near-IR CMD shows significantly tighter AGB sequences, providing a good probe
of the intermediate age (0.5 - 5 Gyr) populations. We match observed CMDs with
stellar evolution models to recover the SFH of KKH 98. On average, the galaxy
has experienced relatively constant low-level star formation (5 x 10^-4 Mo
yr^-1) for much of cosmic time. Except for the youngest main sequence
populations (age < 0.1 Gyr), which are typically fainter than the AO data flux
limit, the SFH estimated from the the 592 IR bright stars is a reasonable match
to that derived from the much larger optical data set. Differences between the
optical and IR derived SFHs for 0.1 - 1 Gyr populations suggest that current
stellar evolution models may be over-producing the AGB by as much as a factor
of three in this galaxy. At the depth of the AO data, the IR luminous stars are
not crowded. Therefore these techniques can potentially be used to determine
the stellar populations of galaxies at significantly further distances.
