Massive black holes in low-mass galaxies: what happened to the X-ray Corona?

Finding massive black holes (MBHs, with mass ≈ 10⁴–10⁷ M⊙) in the nuclei of low-mass galaxies is crucial to constrain seeding and growth of black holes over cosmic time, but it is particularly challenging due to their low accretion luminosities. Variability selection via long-term photometric variability, e.g. in the optical, or infrared has proved effective in identifying lower-Eddington ratios compared to broad and narrow optical spectral lines searches. I will present puzzling results from my recent study on the X-ray emission of MBHs selected via optical and infrared variability: despite being selected as accreting MBHs from other wavebands, X-rays are suspiciously weak and inconsistent with predictions based on scaling relations from more massive AGN. A possibility is that a canonical X-ray corona might be lacking in the majority of this population of low-mass galaxies or that unusual accretion modes and spectral energy distributions are in place for MBHs in dwarf galaxies.