Testing Intermediate Mass Black Hole Selection Diagnostics for the JWST Era Using the Chandra Archives

Understanding the demographics of intermediate mass black holes (IMBHs) is critical to unraveling the seeding mechanism behind the formation of supermassive black holes. However, the search for IMBHs has proved difficult, in part because black holes in this mass range may be located preferentially in dwarf galaxies, which are typically more metal-poor and have more recent bursts of star formation than more massive galaxies. For IMBH searches at X-ray wavelengths, luminous X-ray binaries (XRBs) can be a contaminating factor, as XRBs form efficiently in low-metallicity, star-forming galaxies. Nebular emission line diagnostics can help with the search for IMBHs, but the selection diagnostics must be calibrated from photoionization simulations that carefully consider the time- and metallicity-dependent impact of stars and XRBs on the observed emission lines. To that end, we present emission line diagnostics for selecting candidate IMBHs in metal-poor, star-forming galaxies based on state-of-the-art photoionization simulations that include ionizing contributions from IMBHs, XRBs, and massive stars that vary with time and metallicity. These simulations suggest that while XRBs can contaminate line searches for IMBHs in low-metallicity star-forming galaxies, select emission line diagnostics, such as those detectable with JWST, can effectively distinguish between IMBH or XRB induced photoionization. We apply these emission line diagnostics to a sample of galaxies from SDSS for which there are archival Chandra data, and report the X-ray properties of this new candidate IMBH sample. Finally, we discuss these results in the context of prospects for IMBH searches locally and at high redshift with JWST.