An Old Stellar Population or Diffuse Nebular Continuum Emission Discovered in Green Pea Galaxies

Leonardo Clarke,Matthew Hayes,Hugh Dickinson,Chris Lintott,Claudia Scarlata,Brooke Simmons,Vihang Mehta,Lucy Fortson,Sandor Kruk,William C Keel,Carolin Cardamone,Nico Adams

doi:10.3847/2041-8213/abf7cc

Abstract

Abstract We use new Hubble Space Telescope (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 ∼ 0.25), minimizes the contribution of the nebular [O iii] and Hα emission lines to the broadband 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 500 Myr. While a redder stellar component is perhaps the most plausible explanation for these results, the limitations of our current data set lead us to examine possible alternative mechanisms, particularly recombination emission processes, which are unusually prominent in systems with such strong line emission. 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 allowing large escape fractions in these local galaxies may be different from those at play during the reionization epoch.

Highlights

Green Pea galaxies (GPGs) are low-redshift (z) objects first discovered by citizen scientists in the GalaxyZoo project (Lintott et al 2008; Cardamone et al 2009)
In most galaxies we identify multiple stellar clusters, likely star-forming regions responsible for the characteristic emission line spectra, in addition to a diffuse and extended continuum
E.g., J1004+2017 and J1020+2937, we see a single prominent star cluster that lies in the galaxy center, giving rise to a relatively symmetric morphology, while in J1336+6255 the cluster is off-centered with respect to the diffuse continuum

Summary

Introduction

Green Pea galaxies (GPGs) are low-redshift (z) objects first discovered by citizen scientists in the GalaxyZoo project (Lintott et al 2008; Cardamone et al 2009). They have low stellar masses (≈109 Me), high specific star formation rates (≈10−8 yr−1), small sizes ( 1 kpc, completely unresolved at ground-based resolution), and their optical spectra are characterized by extreme equivalent widths (EWs) in the [O III]λ5007 and Hα emission lines (e.g., Yang et al 2017b; Brunker et al 2020) These galaxies have been the focus of intense research activity since their discovery, as they are thought to be among the best-known analogs of high-redshift galaxies and those that were responsible for the reionization of the Universe at z > 6.

Results

Discussion

Conclusion