@article{17627,
  abstract     = {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⊙.},
  author       = {Visbal, Eli and Haiman, Zoltán},
  issn         = {2041-8205},
  journal      = {The Astrophysical Journal Letters},
  number       = {1},
  publisher    = {American Astronomical Society},
  title        = {{Identifying direct collapse black hole seeds through their small host galaxies}},
  doi          = {10.3847/2041-8213/aadf3a},
  volume       = {865},
  year         = {2018},
}

@article{17640,
  abstract     = {Gravitational waves (GWs) in the nano-hertz band are great tools for understanding the cosmological evolution of supermassive black holes (SMBHs) in galactic nuclei. We consider SMBH binaries in high-z ultra-luminous infrared galaxies (ULIRGs) as sources of a stochastic GW background (GWB). ULIRGs are likely associated with gas-rich galaxy mergers containing SMBHs that possibly occur at most once in the life of galaxies, unlike multiple dry mergers at low redshift. Adopting a well-established sample of ULIRGs, we study the properties of the GWB due to coalescing binary SMBHs in these galaxies. Since the ULIRG population peaks at z>1.5, the amplitude of the GWB is not affected even if BH mergers are delayed by as long as ∼ 10 Gyrs. Despite the rarity of the high-z ULIRGs, we find a tension with the upper limits from Pulsar Timing Array (PTA) experiments. This result suggests that if a fraction fm,gal of ULIRGs are associated with SMBH binaries, then no more than 20fm,gal(λEdd/0.3)5/3(tlife/30 Myr) % of the binary SMBHs in ULIRGs can merge within a Hubble time, for plausible values of the Eddington ratio of ULIRGs (λEdd) and their lifetime (tlife).},
  author       = {Inayoshi, Kohei and Ichikawa, Kohei and Haiman, Zoltán},
  issn         = {2041-8205},
  journal      = {The Astrophysical Journal Letters},
  number       = {2},
  publisher    = {American Astronomical Society},
  title        = {{Gravitational waves from supermassive black hole binaries in ultraluminous infrared galaxies}},
  doi          = {10.3847/2041-8213/aad8ad},
  volume       = {863},
  year         = {2018},
}