Intermediate-mass black holes in dwarf galaxies out to redshift=2 in the Chandra COSMOS Legacy Survey

How supermassive black holes form is still one of the long-standing questions in astronomy. In order to reach 10 ^{9} solar masses when the Universe was less than 1 Gyr old, they should have started as seed intermediate-mass black holes (IMBHs) of 100-10 ^{6} Msun. The presence of IMBHs at z>7 is difficult to prove; however, those seed IMBHs that did not grow into supermassive black holes should be found in local dwarf galaxies resembling the first galaxies formed at early epochs. I will present observational evidence that a population of IMBHs exists in dwarf galaxies based on the X-ray stacking analysis of low-mass galaxies in the Chandra COSMOS-Legacy survey and on the finding of AGN X-ray emission in 40 dwarf galaxies at redshifts ≤2. One of the dwarf galaxies has a stellar mass of ∼7e7 Msun and is the least massive galaxy found so far to host an AGN. Unlike massive galaxies, the AGN fraction of low-mass galaxies is found to decrease with redshift, suggesting that AGN in dwarf galaxies evolve differently than those in high-mass galaxies. The future large, deep, multiwavelength surveys that will result from the synergy of next decade missions such as Athena with major optical/IR facilities will allow us to detect IMBHs in fainter (less massive) and more distant galaxies and thus to better understand how supermassive black holes in the early Universe formed.