Evidence for an intermediate mass black hole in a low-mass star-forming galaxy

Abstract Intermediate Mass Black Holes (IMBHs) are the bridge of the gap between stellar mass black holes, which are around ten solar masses, and supermassive black holes, which can have millions to billions of solar masses. However, it is still challenging to detect and identify the existence of them with respect to supermassive black holes. We report the evidence of a hidden IMBH for a low-mass galaxy, MaNGA 9885-9102. This galaxy is originally selected from the MaNGA survey with distinctive bipolar Hα blobs at the minor axis. The bipolar feature can be associated with AGN activity, while the two blobs are classified as the HII regions on the BPT diagram, making the origins confusing. The Swift UV continuum shows that the two blobs do not have UV counterparts, suggesting that the source of ionization is out of the blobs. Consistent with this, the detailed photoionization models prefer to AGN rather than star-forming origin with a significance of 5.8σ. The estimated black hole mass is MBH ∼7.2× 105M⊙ from the central velocity dispersion of stars. Not only identifying an active IMBH, this work presents a new method to search for the light echo of IMBH in low-mass and dwarf galaxies of low metallicity, where the traditional BPT diagram fails.