Star Formation Histories of the LEGUS Dwarf Galaxies (I): recent History of NGC1705, NGC4449 and Holmberg II

We use HST observations from the Legacy Extragalactic UV Survey to reconstruct the recent star formation histories (SFHs) of three actively star-forming dwarf galaxies, NGC4449, Holmberg II and NGC1705, from their UV color-magnitude diagrams (CMDs). We apply a CMD fitting technique using two independent sets of stellar isochrones, PARSEC-COLIBRI and MIST, to assess the uncertainties related to stellar evolution modelling. Irrespective of the adopted stellar models, all the three dwarfs are found to have had almost constant star formation rates (SFRs) in the last 100-200 Myr, with modest enhancements (a factor of $\sim$2) above the 100 Myr-averaged-SFR. Significant differences among the three dwarfs are found in the overall SFR, the timing of the most recent peak and the SFR$/$area. The Initial Mass Function (IMF) of NGC1705 and Holmberg II is consistent with a Salpeter slope down to $\approx$ 5 M$_{\odot}$, whereas it is slightly flatter, s$=-2.0$, in NGC4449. The SFHs derived with the two different sets of stellar models are consistent with each other, except for some quantitative details, attributable to their input assumptions. They also share the drawback that all synthetic diagrams predict a clear separation in color between upper main sequence and helium burning stars, which is not apparent in the data. Since differential reddening, significant in NGC4449, or unresolved binaries don't appear to be sufficient to fill the gap, we suggest this calls for a revision of both sets of stellar evolutionary tracks.