Identifying direct collapse black hole seeds through their small host galaxies

Observations of high-redshift quasars indicate that supermassive black holes (SMBHs) with masses greater than ∼109 M⊙ were assembled within the first billion years after the Big Bang. It is unclear how such massive black holes (BHs) formed so early. One possible explanation is that these SMBHs were seeded by "heavy" direct collapse black holes (DCBHs) with masses of MBH ≈ 105 M⊙, but observations have not yet confirmed or refuted this scenario. In this Letter, we utilize a cosmological N-body simulation to demonstrate that before they grow roughly an order of magnitude in mass, DCBHs will have BH mass to halo mass ratios that are much higher than expected for BH remnants of Population III (Pop III) stars that have grown to the same mass (∼106 M⊙). We also show that when Tvir ≈ 104 K halos (the potential sites of DCBH formation) merge with much larger nearby halos (Mh > 1010 M⊙), they almost always orbit their larger host halos with a separation of a few kpc, which is sufficient to be spatially resolved with future X-ray and infrared telescopes. Thus, we propose that a future X-ray mission such as Lynx, combined with infrared observations, will be able to distinguish high-redshift DCBHs from smaller BH seeds due to the unusually high BH mass to stellar mass ratios of the faintest observed quasars, with inferred BH masses below ∼106 M⊙.