First Detection of an Over-Massive Black Hole Galaxy UHZ1: Evidence for Heavy Black Hole Seed Formation from Direct Collapse

TL;DR

Addresses the origin of the first supermassive black holes by testing heavy direct-collapse seeds via the OBG framework. Builds a library of multi-wavelength templates using the GEMS growth model embedded in FiBY cosmology, with metallicity, accretion mode, and stellar population variations, and post-processes with CLOUDY to predict JWST and X-ray signatures. Finds that UHZ1 is best explained by a heavy-seed OBG in standard Eddington-limited growth with low metallicity and moderate stellar age, while light-seed and slim-disk scenarios are disfavored. Concludes that UHZ1 provides empirical support for heavy seed formation and highlights the potential of JWST to reveal a population of OBGs, refining our understanding of early black hole seeding.

Abstract

The recent Chandra-JWST discovery of a quasar in the z = 10.1 galaxy UHZ1 reveals that accreting supermassive black holes (SMBHs) were already in place 470 million years after the Big Bang. The Chandra X-ray source detected in UHZ1 is a Compton-thick quasar with a bolometric luminosity of $L_{\rm bol}\sim5\times10^{45}\ \rm{erg\ s^{-1}},$ which corresponds to an estimated BH mass of $\sim4\times10^7 \ \rm{M_{\odot}}$ assuming accretion at the Eddington rate. JWST NIRCAM and NIRSpec data yield a stellar mass estimate for UHZ1 comparable to its BH mass. These characteristics are in excellent agreement with prior theoretical predictions for a unique class of transient, high-redshift objects, Over-massive Black Hole Galaxies [OBGs] by Natarajan et al. that harbor a heavy initial black hole seed that likely formed from the direct collapse of the gas. Based on the excellent agreement between the observed multi-wavelength properties of UHZ1 with theoretical model template predictions, suggests that UHZ1 is the first detected OBG candidate. Our assertion rests on multiple lines of concordant evidence between model predictions and the following observed properties of UHZ1: its X-ray detection and the estimated ratio of the X-ray flux to the IR flux that is consistent with theoretical expectations for a heavy initial BH seed; its high measured redshift of z = 10.1, as predicted for the transient OBG stage (9 < z< 12); the amplitude and shape of the detected JWST Spectral Energy Distribution (SED) between 1 - 5 microns, which is in very good agreement with simulated template SEDs for OBGs; and the extended JWST morphology of UHZ1 that is suggestive of a recent merger, also expected for the formation of transient OBGs. As the first OBG candidate, UHZ1 provides compelling evidence for the formation of heavy initial seeds from direct collapse in the early Universe.

Figures (4)

• Figure 1: Schematic diagram of the potential assembly history of the OBG candidate UHZ1. The direct collapse of primordial gas disks resulting in the production of heavy initial black hole seeds has been demonstrated to occur feasibly in the setting shown here: satellite DCBH halos that are bound to parent star-forming halos Natarajan+2017. Star formation in the satellite atomic cooling DCBH halo is expected to be suppressed due to the dissociation of molecular hydrogen from the Lyman-Werner radiation produced by the parent galaxy. As a consequence, the OBG is a merger remnant that contains a growing DCBH seed with an initial mass of $\sim 10^4M_{\odot}$ with $M_{\rm bh} \geq {M_*}$
• Figure 2: Selection criteria for OBGs: the predicted location of OBGs in color-color space, with the location of UHZ1 marked with a black star.
• Figure 3: The OBG model match for UHZ1: the model spectrum over-plotted here is obtained by growing an initially heavy seed of $\sim 10^4\,M_{\odot}$ to a final mass of $10^7\,M_{\odot}$ as estimated for UHZ1 and combining with a young stellar population (age of 350 Myr), low metallicity ($10^{-3}\,Z_{M_{\odot}}$) with a column density of $\sim 3 \times 10^{24}\,cm^{-2}$. The observed near-IR JWST SED for UHZ1 points taken from castellano22b are shown in blue. We note that the overplotted SED template from our library generated with the parameters noted above is very similar to UHZ1.
• Figure 4: The template match for the multi-wavelength SED from our library of OBG models on which we over plot the measured fluxes in the IR from JWST and X-ray from Chandra for UHZ1, shows that they are consistent and well matched. The template that provides this optimal match has the following properties: a heavy initial seed with a mass of $10^{4}\,M_{\odot}$; the OBG accreting at the Eddington limit with an age of 350 Myr for the recently merged stellar population with a low metallicity of $\sim\,10^{-3}\,Z_{\odot}$ and a column density of $3 \times 10^{24}\,cm^{-2}$, shown in Figure \ref{['fig:sed']} (in red).