An Intermediate-mass Black Hole Hidden behind Thick Obscuration

Abstract

Recent models suggest approximately half of all accreting supermassive black holes (SMBHs; M _BH ≳ 10⁵ M _⊙) are expected to undergo intense growth phases behind Compton-thick (N _H > 1.5 × 10²⁴ 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 ∼ 10²–10⁵ 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 <10⁵ M _⊙. We find the equivalent width of neutral 6.4 keV Fe Kα to be 1.9‑1.0+2.2 keV via phenomenological modeling of the coadded 177 ks Chandra spectrum. Such large equivalent widths are seldom produced by processes other than fluorescence from dense obscuration. We fit three physically motivated X-ray spectral models to infer a range of possible intrinsic 2–10 keV luminosity posteriors that encompass the systematic uncertainties associated with a choice of model. Despite a wide range of predicted intrinsic 2–10 keV luminosities between ∼ 10⁴¹ and 10⁴³ erg s^‑1, all three models agree that IC 750 has a Compton-thick line-of-sight column density to >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.