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Nearly every massive galaxy harbors a supermassive black hole (SMBH) in itsnucleus. SMBH masses are millions to billions $M_{\odot}$, and they correlatewith properties of spheroids of their host galaxies. While the SMBH growthchannels, mergers and gas accretion, are well established, their origin remainsuncertain: they could have either emerged from massive "seeds" ($10^5-10^6M_{\odot}$) formed by direct collapse of gas clouds in the early Universe orfrom smaller ($100 M_{\odot}$) black holes, end-products of first stars. Thelatter channel would leave behind numerous intermediate mass black holes(IMBHs, $10^2-10^5 M_{\odot}$). Although many IMBH candidates have beenidentified, none is accepted as definitive, thus their very existence is stilldebated. Using data mining in wide-field sky surveys and applying dedicatedanalysis to archival and follow-up optical spectra, we identified a sample of305 IMBH candidates having masses $3\times10^4<M_{\mathrm{BH}}<2\times10^5M_{\odot}$, which reside in galaxy centers and are accreting gas that createscharacteristic signatures of a type-I active galactic nucleus (AGN). Weconfirmed the AGN nature of ten sources (including five previously knownobjects which validate our method) by detecting the X-ray emission from theiraccretion discs, thus defining the first bona fide sample of IMBHs in galacticnuclei. All IMBH host galaxies possess small bulges and sit on the low-massextension of the $M_{\mathrm{BH}}-M_{\mathrm{bulge}}$ scaling relationsuggesting that they must have experienced very few if any major mergers overtheir lifetime. The very existence of nuclear IMBHs supports the stellar massseed scenario of the massive black hole formation.

A Population of Bona Fide Intermediate Mass Black Holes Identified as Low Luminosity Active Galactic Nuclei