An Intermediate-mass Black Hole Hidden behind Thick Obscuration

Abstract

Recent models suggest approximately half of all accreting supermassive black holes (SMBHs; M _BH ≳ 10 ^5 M _⊙ ) are expected to undergo intense growth phases behind Compton-thick ( N _H > 1.5 × 10 ^24 cm ^−2 ) veils of obscuring gas. However, despite being a viable source for the seeding of SMBHs, there are currently no examples known of a Compton-thick accreting intermediate-mass black hole (IMBH; M _BH ∼ 10 ^2 –10 ^5 M _⊙ ). We present a detailed X-ray spectral analysis of IC 750—the only active galactic nuclei (AGN) to date with a precise megamaser-based intermediate mass 99% confidence. Compton-thick obscuration is well-documented to impinge substantial bias on the pursuit of SMBH AGN. Our results thus provide the first indication that Compton-thick obscuration should also be properly considered to uncover and understand the IMBH population in an unbiased manner.

Keywords

• Active galaxies
• X-ray active galactic nuclei
• Intermediate-mass black holes