Green Peas in X-Rays^∗

Abstract

Green Peas represent a population of compact, highly star-forming dwarf galaxies at redshifts z ∼ 0.2–0.3 that have recently been found to show signatures of ultraviolet ionizing radiation leakage. They are being considered as analogs to high-redshift star-forming galaxies, possibly responsible for cosmic reionization. Despite intensive studies of Green Peas in the ultraviolet and optical domains, their X-ray properties have only so far been probed by nearby analogs. In this paper, we present the first measurements of Green Peas in the X-ray domain to constrain their spectral properties and fluxes at high energies. We analyzed XMM-Newton observations of three Green Pea sources. For two of them, we found an X-ray luminosity exceeding by a half-order of magnitude its predicted value, derived from the star formation rate and metallicity. Only an upper limit of the X-ray luminosity was derived for the third studied galaxy. Our results indicate that at least some Green Peas produce copious amounts of highly energetic photons, larger than detected in other star-forming galaxies. We discuss possible physical scenarios for the measured X-ray excess, including the presence of a hidden active galactic nucleus, a larger population of X-ray binaries, or ultra-luminous X-ray sources. Future spatially resolved X-ray images will discriminate between the models. Larger Green Pea samples will provide a possible link between the X-ray properties and the leaking ultraviolet radiation.

Accretion-driven Sources in Spatially Resolved Lyα Emitters

“…Extrapolating from L X down to 13.6 eV yields predicted Lyα luminosities compatible with those observed, for a typical power-law index Γ = 1.5 − 2.0. Our findings are consistent with those of Bluem et al (2019), who find that 5 of their 8 blue compact galaxies that are Lyman continuum-emitting candidates show significant X-ray excess; and Svoboda et al (2019) also find X-ray excess in 2 of their 3 Green Pea galaxies. As noted in Section 3.2, our sample similarly shows a tendency toward excess L X .…”

X-ray emission from star cluster winds in starburst galaxies

“…In Figure 6, the green points indicate L X as measured for the three Green Pea galaxies in Svoboda et al (2019). As the observed X-ray emission is even higher for the Green Pea galaxies than the relation by Mineo et al (2012b), our model predicts even less of the Xray emission to originate from the hot gas.…”

“…Here, D A (1 + z) 2 corresponds to the luminosity distance D L which we set to the value for SDSS J074936.77+333716.3 (aka 'GP1'); D L = 1437.9 Mpc (Svoboda et al 2019) 2 . Despite step (iii), we applied a representative Green Pea distance to the normalization to obtain appropriate flux levels that could be directly compared to real measurements from Green Peas (Svoboda et al 2019). The number densities of electrons and hydrogen (n e , n H , respectively) result from the stellar models (see Section 2.1.2).…”

“…Unusually bright X-ray emission was recently reported from a class of intermediate redshift (z ∼ 0.2−0.3) compact starburst galaxies called "Green Peas" (Svoboda et al 2019). Green Pea galaxies have attracted attention for their similarity to high-redshift galaxies near the Epoch of Reionisation -namely low mass, low metallicity, vigorous star formation (Cardamone et al 2009), high ionization, and high fractions of escaping ionizing ultraviolet radiation (e.g.…”

Starburst Galaxies

“…Furthermore, quasars set in at some point, providing copious amounts of energetic radiation. Additional sources such as X-ray binaries or cosmic rays are being considered as possible ionization contributors (see references in [55]).…”