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We present an analysis of the star formation history (SFH) of a field nearthe half light radius in the Local Group dwarf irregular galaxy IC 1613 basedon deep Hubble Space Telescope Advanced Camera for Surveys imaging. Ourobservations reach the oldest main sequence turn-off, allowing a timeresolution at the oldest ages of ~1 Gyr. Our analysis shows that the SFH of theobserved field in IC 1613 is consistent with being constant over the entirelifetime of the galaxy. These observations rule out an early dominant episodeof star formation in IC 1613. We compare the SFH of IC 1613 with expectationsfrom cosmological models. Since most of the mass is in place at early times forlow mass halos, a naive expectation is that most of the star formation shouldhave taken place at early times. Models in which star formation follows massaccretion result in too many stars formed early and gas mass fractions whichare too low today (the "over-cooling problem"). The depth of the presentphotometry of IC 1613 shows that, at a resolution of ~1 Gyr, the star formationrate is consistent with being constant, at even the earliest times, which isdifficult to achieve in models where star formation follows mass assembly.

The ACS LCID project. X. The Star Formation History of IC 1613: Revisiting the Over-Cooling Problem