Stellar Populations at the Center of IC 1613

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Abstract
We have observed the center of the Local Group dwarf irregular galaxy IC 1613 with the WFPC2 aboard the Hubble Space Telescope in the F439W, F555W, and F814W filters. We analyze the resulting color-magnitude diagrams (CMDs) using the main-sequence and giant-branch luminosity functions and comparisons with theoretical stellar models to derive a preliminary star formation history for this galaxy. We find a dominant old stellar population (age ≈7 Gyr), identifiable by the strong red giant branch (RGB) and red clump populations. From the (V-I) color of the RGB, we estimate a mean metallicity of the intermediate-age stellar population [Fe/H] = -1.38 ± 0.31. We confirm a distance of 715 ± 40 kpc using the I magnitude of the RGB tip. The main-sequence luminosity function down to I ≈ 25 provides evidence for a roughly constant star formation rate (SFR) of approximately 3.5 × 10-4 M⊙ yr-1 across the WFPC2 field of view (0.22 kpc2) during the past 250–350 Myr. Structure in the blue loop luminosity function implies that the SFR was ≈50% higher 400–900 Myr ago than today. The mean heavy-element abundance of these young stars is around one-tenth solar. The best explanation for a red spur on the main sequence at I ≈ 24.7 is the blue horizontal branch component of a very old stellar population at the center of IC 1613. We have also imaged a broader area of IC 1613 using the 3.5 m WIYN telescope under excellent seeing conditions. The WIYN CMD reveals a prominent sequence of asymptotic giant branch stars and red supergiants that is less prominent in the WFPC2 CMD because of the smaller field of view. The asymptotic giant branch star luminosity function is consistent with a period of continuous star formation over at least the age range 2–10 Gyr. We present an approximate age-metallicity relation for IC 1613, which appears to be similar to that of the Small Magellanic Cloud. We compare the Hess diagram of IC 1613 with similar data for three other Local Group dwarf galaxies and find that IC 1613 most closely resembles the nearby, transition-type dwarf galaxy Pegasus (DDO 216).
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