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[abridged] Aims. We observed the outer parts of NGC 1569 and NGC 4449, two ofthe closest and strongest dwarf starburst galaxies in the local universe, tocharacterize their stellar density and populations, and obtain new insightsinto the structure, formation, and evolution of starburst galaxies and galaxyhalos. Methods. We obtained HST/WFPC2 images between 5 and 8 scale radii fromthe center, along the intermediate and minor axes. We performed point-sourcephotometry to determine color magnitude diagrams of I vs. V-I. We compared theresults at different radii, including also our prior HST/ACS results for morecentrally located fields. Results. We detect stars in the RGB and TP-AGB(carbon star) phases in all outer fields, but not younger stars such as thosepresent at smaller radii. The RGB star density profile is well fit by either ade Vaucouleurs profile or a power-law profile, but has more stars at largeradii than a single exponential. To within the uncertainties, there are noradial gradients in the RGB color or carbon-to-RGB-star ratio at large radii.Conclusions. The galaxies have faint outer stellar envelopes that are nottidally truncated within the range of radii addressed by our study. The densityprofiles suggest that these are not outward extensions of the inner disks, butare instead distinct stellar halos. This agrees with other work on galaxies ofsimilar morphology. The presence of such halos is consistent with predictionsof hierarchical galaxy formation scenarios. The halos consist ofintermediate-age/old stars, confirming the results of other studies that haveshown the starburst phenomenon to be very centrally concentrated. There is noevidence for stellar-population age/metallicity gradients within the halosthemselves.

Hubble Space Telescope study of resolved red giant stars in the outer halos of nearby dwarf starburst galaxies