Chandra detects low-luminosity AGN with MBH=10⁴-10⁶~M_ in nearby ($z<0.5$), dwarf and star-forming galaxies

We searched the Chandra and XMM archives for observations of 900 green pea galaxies to find AGN signatures. Green peas are low-mass galaxies with prominent emission lines, similar in size and star formation rate to high-redshift dwarf galaxies. Of the 29 observations found, 9 show X-ray detections with $S/N>3$. The 2-10 keV X-ray luminosity for these 9 sources exceeds 10⁴⁰~erg~s⁻¹, with 2 sources exceeding 10⁴¹~erg~s⁻¹, suggesting the presence of intermediate-mass black holes (IMBH) or low-luminosity AGN (LLAGN) with BH masses between 100-10⁶M_. All X-ray detected sources (plus 6 additional sources) show He~IIλ4686 emission and a broad component of the Hα emission line, indicating winds. The line widths of the broad Hα and He IIλ4686 emitting gas clouds are weakly correlated (R²=0.15), suggesting He IIλ4686 emission is inconsistent with winds from super-Eddington accretors. However, the ratio of X-ray luminosity to star formation rate shows an anti-correlation with metallicity in 5 out of 9 X-ray detected sources, implying ultraluminous X-ray sources are key contributors to the observed X-ray luminosity. This could be due to super-Eddington accretors or IMBH. The X-ray emission is much higher than that produced by Wolf-Rayet stars and supernovae-driven winds. Thus, the X-ray luminosity in these 9 sources can only be explained by black holes with masses over 100~M_. Our findings suggest the presence of LLAGN in these galaxies, with broad Hα line widths implying BH masses of 10⁴-10⁶M_. Given Green Peas' role as significant Lyman Continuum leakers, LLAGN in these galaxies could have contributed significantly to cosmic reionization.