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We present deep HST WFPC2 imaging of the Local Group dwarf irregular galaxyIC 1613. The photometry is the deepest to date for an isolated dwarf irregulargalaxy. The resulting color-magnitude diagram (CMD) is analyzed using threedifferent methods to derive a star formation history (SFH). All three find anenhanced star formation rate (SFR), from 3 to 6 Gyr ago, and similarage-metallicity relationships (AMR). A comparison of the newly observed outerfield with an earlier studied central field of IC 1613 shows that the SFR inthe outer field has been significantly depressed during the last Gyr. Thisimplies that the optical scale length of the galaxy has been decreasing withtime and that comparison of galaxies at intermediate redshift with present daygalaxies should take this effect into account. We find strong similaritiesbetween IC 1613 and the more distant Milky Way dSph companions in that all aredominated by star formation at intermediate ages. In particular, the SFH andAMR for IC 1613 and Leo I are indistinguishable. This implies that dIrrgalaxies cannot be distinguished from dSphs by their intermediate age stellarpopulations. This type of a SFH may also be evidence for slower or suppressedearly star formation in dwarf galaxies due to photoionization after thereionization of the universe by background radiation. Assuming that IC 1613 istypical of a dIrr evolving in isolation, since most of the star formationoccurs at intermediate ages, these dwarf systems cannot be responsible for thefast chemical enrichment of the IGM which is seen at high redshift. There is noevidence for any large amplitude bursts of star formation in IC 1613, and wefind it highly unlikely that analogs of IC 1613 have contributed to the excessof faint blue galaxies in existing galaxy redshift surveys.Comment: 32 pages, including 1 table and 17 figures, accepted for publication  in the Astrophysical Journal October 10, 2003 issu

DeepHubble Space TelescopeImaging of IC 1613. II. The Star Formation History