Star Formation Histories of the LEGUS Dwarf Galaxies (I): recent History of NGC1705, NGC4449 and Holmberg II
M. Cignoni (1, 2, 3), E. Sacchi (3, 4), A. Aloisi (5), M., Tosi (3), D. Calzetti (6), J. C. Lee (5, 7), E. Sabbi (5), A. Adamo (8),, D. O. Cook (9), D. A. Dale (10), B. G. Elmegreen (11), J.S. Gallagher III, (12), D. A. Gouliermis (13, 14), K. Grasha (6), E. K. Grebel (13)

TL;DR
This study reconstructs recent star formation histories of three dwarf galaxies using HST UV data, revealing consistent SFRs over the past 100-200 Myr and highlighting discrepancies in stellar models and evolutionary tracks.
Contribution
It provides the first detailed SFHs of NGC4449, Holmberg II, and NGC1705 using two stellar models, and discusses the implications of model uncertainties and observed data-model discrepancies.
Findings
All three dwarfs have had nearly constant SFRs in the last 100-200 Myr.
NGC1705 and Holmberg II have a Salpeter IMF slope down to 5 M_sun.
NGC4449's IMF is slightly flatter, with s=-2.0.
Abstract
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 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 SFRarea. The Initial Mass Function (IMF) of NGC1705 and Holmberg II is consistent with a Salpeter slope down to…
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