"Blueberries and Green Peas"

Abstract:

Blueberries and Green Peas are compact, intensely star-forming galaxies whose low metallicities and extreme star-formation properties make them close local analogues of the first galaxies in the early Universe. Some Green Peas were reported to be unusually X-ray bright, exceeding standard X-ray–SFR relations and hinting at the presence of an active galactic nucleus (AGN). Motivated by this, we obtained new XMM–Newton observations of Green Pea and Blueberry samples and find that most sources are actually X-ray underluminous for their star-formation rates, which further strengthens the case for the rare X-ray–bright outliers as compelling AGN candidates. We highlight the most recent AGN confirmation in a Green Pea galaxy based on X-ray spectral variability, supported by broad Hα line modelling, and place these results in the context of the JWST Little Red Dots at high redshift, which suggest AGN activity in similarly compact, low-metallicity systems. Finally, we present new radio constraints showing trends analogous to the X-ray domain, and results from a clustering analysis indicating that these galaxies preferentially reside in low-density environments—potentially explaining their delayed onset of star formation and rapid growth.