An old stellar population or diffuse nebular continuum emission discovered in green pea galaxies

TL;DR

This study uses HST imaging to analyze the structure and color of Green Pea Galaxies, revealing a significant diffuse, older stellar component alongside younger star-forming regions, with implications for understanding galaxy evolution and reionization.

Contribution

It provides the first detailed analysis of the resolved stellar populations and diffuse emission in Green Pea Galaxies, highlighting the presence of old stars and the potential role of nebular continuum.

Findings

Diffuse emission indicates old stellar populations (>1 Gyr).

Star-forming regions are younger (<500 Myr) and bluer.

Diffuse red emission could be from nebular continuum or older stars.

Abstract

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.