An old stellar population or diffuse nebular continuum emission
  discovered in green pea galaxies
  We use new HST images of nine Green Pea Galaxies (GPGs) to study their
resolved structure and color. The choice of filters, F555W and F850LP, together
with the redshift of the galaxies ($z\sim 0.25$), minimizes the contribution of
the nebular [O\thinspace{\sc iii}] and H$\alpha$ emission lines to the
broad-band images. While these galaxies are typically very blue in color, our
analysis reveals that it is only the dominant stellar clusters that are blue.
Each GPG does clearly show the presence of at least one bright and compact
star-forming region, but these are invariably superimposed on a more extended
and lower surface brightness emission. Moreover, the colors of the star forming
regions are on average bluer than those of the diffuse emission, reaching up to
0.6 magnitudes bluer. Assuming that the diffuse and compact components have
constant and single burst Star Formation Histories, respectively, the observed
colors imply that the diffuse components (possibly the host galaxy of the
star-formation episode) have, on average, old stellar ages ($>1$Gyr), while the
star-clusters are younger than 500Myrs. We also discuss the possibility that
the diffuse red emission is due to a varying relative contribution of nebular
continuum, rather than a different stellar population. With the available data,
however, it is not possible to distinguish between these two interpretations. A
substantial presence of old stars would indicate that the mechanisms that allow
large escape fractions in these local galaxies may be different from those at
play during the reionization epoch.
