Chandra detects low-luminosity AGN with $M_{\mathrm{BH}}=10^4-10^6{M}_{\odot }$ in nearby ($z<0.5$), dwarf and star-forming galaxies

Abstract

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 $10^{40}{\mathrm{erg}\mathrm{s}}^{-1}$, with 2 sources exceeding $10^{41}{\mathrm{erg}\mathrm{s}}^{-1}$, suggesting the presence of intermediate-mass black holes (IMBH) or low-luminosity AGN (LLAGN) with BH masses between $100-10^6{M}_{\odot }$. All X-ray detected sources (plus 6 additional sources) show He~II$\lambda 4686$ emission and a broad component of the H$\hat{I}\pm$ emission line, indicating winds. The line widths of the broad H$\hat{I}\pm$ and He II$\lambda 4686$ emitting gas clouds are weakly correlated ($R^2=0.15$), suggesting He II$\lambda 4686$ 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 $100M_{\odot }$. Our findings suggest the presence of LLAGN in these galaxies, with broad H$\hat{I}\pm$ line widths implying BH masses of $10^4-10^6{M}_{\odot }$. Given Green Peas' role as significant Lyman Continuum leakers, LLAGN in these galaxies could have contributed significantly to cosmic reionization.

Submitted to ApJ. 17 pages, 11 figures and 3 tables. Comments welcome