Local Relics: The Hunt for Intermediate Mass Black Holes



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While supermassive black holes (SMBHs) up to a billion times the mass of the Sun are found in the centers of all massive galaxies and stellar mass black holes of $\sim 10$ solar masses (M$_{\odot}$) are formed from supernovae, there is currently no direct evidence for intermediate mass black holes (IMBHs) with masses between $\approx 100-10^4$M$_{\odot}$. Black holes in this ``mass desert" are of significant astrophysical interest, because they could provide insight into SMBH ``seeds", formed at high redshift and observationally undetectable. With the discovery of over $200$ SMBHs with masses more a billion times the mass of our Sun found when the Universe was less than a billion years old, there has been increased interest in determining the origins of these mysterious giants. As SMBHs have been found much earlier in cosmic time than previously believed possible given radiation pressure limits on the growth rate of stellar mass black holes, this requires new thought as to their initial formation. Were they formed from the end products of massive primordial stars, resulting in numerous black hole seeds of mass $\approx100$ M$_{\odot}$? Or were they formed from the direct collapse of massive clouds of gas that may have formed in the low angular momentum, metal-free environment of the early universe, forming rarer seeds up to $10^4$ M$_\odot$ in size? Finding a population of IMBHs in dwarf galaxies, which have not undergone significant growth and evolution over cosmic history, as indicated by their low stellar masses and metallicities, can provide insight into which path is more likely to have occurred. While there is now a growing number of black holes found with masses between $10^5 - 10^6$ M$_{\odot}$, very little is known about the existence, properties, host galaxy demographics, and scaling relations of black holes with masses less than $\approx10^5$M$_{\odot}$. Determining the mass function and occupation fraction of this elusive population will prove pivotally important to disentangling the history of these objects. Most searches for IMBHs have been done using traditional optical spectroscopic, X-ray, and radio diagnostics, however, these diagnostics fail in the low mass and low metallicity regimes, causing a large population of IMBHs to remain elusive. The work of this thesis focuses on theoretically determining, and observationally validating, the most effective ways to search for IMBHs in these significant environments. In this thesis, I highlight the deficiencies of traditional optical diagnostics in finding IMBHs and provide an alternative solution using infrared coronal lines, fine structure emission lines with ionization potentials greater than 60~eV. These emission lines are particularly significant as stars do not produce enough high energy radiation to excite these ions, meaning that the detection of a coronal line can provide robust evidence for the presence of an AGN. As the emission structure of a black hole is dependent on its mass, these coronal lines could also provide a diagnostic to estimate the mass of the black hole. In order to test the efficacy of coronal lines in finding AGNs in low metallicity galaxies, a multi-wavelength study was done on J1056+3138, showing the power of infrared diagnostics in the search for black holes in extreme environments.