The Local Ultraviolet to Infrared Treasury. II. Refining Star Formation Histories of 10 Metal-poor Dwarf Galaxies with Simultaneous UV-optical Two-CMD Fitting

Abstract We present the star formation histories (SFHs) of 10 metal-poor (≲12% Z ⊙ ), star-forming dwarf galaxies from the Local Ultraviolet to Infrared Treasury survey. The derived SFHs exhibit significant variability, consistent with the irregular star formation expected for dwarf galaxies. Using synthetic near-ultraviolet (UV) and optical color–magnitude diagrams (CMDs) with various yet targeted configurations for dust and input SFHs, we quantitatively demonstrate that simultaneous modeling of the UV and optical CMDs (“UVopt” case) improves the precision of SFH measurements in recent time bins up to ∼1 Gyr, compared to the classical single optical CMD modeling (“Opt-only” case). The UVopt case reduces uncertainties relative to the Opt-only case by ∼4%–8% over the past 10 Myr, ∼8%–20% over 100 Myr, and ∼8%–14% over 1 Gyr, across various dust configurations and input SFHs. Additionally, we demonstrate discrepancies in stellar models for blue core helium-burning (BHeB) stars at the low-metallicity regime. This discrepancy can artificially inflate star formation rate (SFR) estimates in younger age bins by misinterpreting the evolved BHeB stars as reddened upper main-sequence (MS) stars. Incorporating UV data improves BHeB-MS separation and mitigates the limitations of current low-metallicity stellar models. Comparisons of the UVopt SFHs with H α and far-UV (FUV)-based SFRs reconfirm that H α is an unreliable tracer over its nominal 10 Myr timescale for low-SFR galaxies, while FUV provides a more reliable tracer but yields SFR FUV values up to twice those of CMD-based 〈SFR〉 100 Myr . Our findings underscore the importance of UV data in refining recent SFHs in low-metallicity environments.