Hubble Space Telescope study of resolved red giant stars in the outer
  halos of nearby dwarf starburst galaxies
  [abridged] Aims. We observed the outer parts of NGC 1569 and NGC 4449, two of
the closest and strongest dwarf starburst galaxies in the local universe, to
characterize their stellar density and populations, and obtain new insights
into the structure, formation, and evolution of starburst galaxies and galaxy
halos. Methods. We obtained HST/WFPC2 images between 5 and 8 scale radii from
the center, along the intermediate and minor axes. We performed point-source
photometry to determine color magnitude diagrams of I vs. V-I. We compared the
results at different radii, including also our prior HST/ACS results for more
centrally 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 those
present at smaller radii. The RGB star density profile is well fit by either a
de Vaucouleurs profile or a power-law profile, but has more stars at large
radii than a single exponential. To within the uncertainties, there are no
radial gradients in the RGB color or carbon-to-RGB-star ratio at large radii.
Conclusions. The galaxies have faint outer stellar envelopes that are not
tidally truncated within the range of radii addressed by our study. The density
profiles suggest that these are not outward extensions of the inner disks, but
are instead distinct stellar halos. This agrees with other work on galaxies of
similar morphology. The presence of such halos is consistent with predictions
of hierarchical galaxy formation scenarios. The halos consist of
intermediate-age/old stars, confirming the results of other studies that have
shown the starburst phenomenon to be very centrally concentrated. There is no
evidence for stellar-population age/metallicity gradients within the halos
themselves.
