Green Peas in X-rays

SVOBODA, JIRI; EHLE, MATTHIAS; ORLITOVÁ, IVANA; DOUNA, VANESA

Lyon

Conferencia; EWASS European Week of Astronomy and Space Science 2019; 2019

European Astronomical Society

Green Pea galaxies represent a class of compact, highly star-forming dwarf galaxies that have recently been found to show signatures of escape of the ultraviolet ionising flux and are thus being considered aspossible analogues to high-redshift star-forming galaxies responsible for the Reionisation of the Universe. Despite the intensive studies of Green Peas in UV and optical domain, their X-ray properties were not known. We performed the first measurements of three Green Pea galaxies in the X-ray domain with the XMM-Newton satellite and constrained their X-ray spectral properties and flux at high energies (Svoboda et al., 2018). We found that two sources have their X-ray luminosity exceeding 10^42  erg/s, which is unusual for a purely star-forming galaxy and is more than half order of magnitude larger than predicted from the known empirical relations between the X-ray flux and the star formation rate, considering also their metallicity. However, the third studied Green Pea galaxy was not detected in X-rays at all, suggesting its X-ray luminosity being even below the empirical relation. Our results thus indicate that some GreenPea galaxies may produce significantly more high-energetic flux than  other similar star-forming galaxies. The measured variety of the X-ray characteristics of Green Pea galaxies challenges our full understandingof the relation between the X-ray luminosity and the star formation rate in compact dwarf galaxies and we will discuss some possible explanations, including the presence of a hidden active nucleus, ultra-luminous X-ray sources, different stellar population or X-ray excess due to hot gas.