Evidence for heavy seed origin of early supermassive black holes from a z~10 X-ray quasar

Abstract

Observations of quasars reveal that many supermassive black holes (BHs) were in place less than 700 million years after the Big Bang. However, the origin of the first BHs remains a mystery. Seeds of the first BHs are postulated to be either light (i.e., $10-100~\rm{M_{\odot}})$, remnants of the first stars or heavy (i.e., $10^4-10^5~\rm{M_{\odot}})$, originating from the direct collapse of gas clouds. Harnessing recent data from the Chandra X-ray Observatory, we report the detection of an X-ray-luminous massive BH in a gravitationally-lensed galaxy identified by JWST at $z\approx10.3$ behind the cluster lens Abell 2744. This heavily-obscured quasar with a bolometric luminosity of $L_{\rm bol}\sim5\times10^{45}~\rm{erg\ s^{-1}}$ harbors a $M_{\rm BH}\sim10^7-10^8~\rm{M_{\odot}}$ BH assuming accretion at the Eddington limit. This mass is comparable to the inferred stellar mass of its host galaxy, in contrast to what is found in the local Universe wherein the BH mass is $\sim0.1\%$ of the host galaxy's stellar mass. The combination of such a high BH mass and large BH-to-galaxy stellar mass ratio just $\sim$500 Myrs after the Big Bang was theoretically predicted and is consistent with a picture wherein BHs originated from heavy seeds.

Figures (6)

• Figure 1: A $4.2\sigma$Chandra X-ray detection of a source cospatial with UHZ1: Panel (a) 1.25 Msec Chandra X-ray image of $0.5-7$ keV emission associated with the galaxy cluster lens Abell 2744. The image has been smoothed with an adaptive Gaussian kernel, and a logarithmic stretch has been applied. The $15" \times 15"$ white box is centered on the location of UHZ1 and shows the zoomed-in field of view of the adjacent panel. Panel (b) Chandra$2-7$ keV band X-ray image of the $15" \times 15"$ region surrounding UHZ1. Black contours show the JWST/NIRCam F200W morphology of the UHZ1 galaxy candidate at $z\approx10.32$. The solid white circle has a $1"$ radius, corresponding to both the off-axis Chandra PSF ($\approx88\%$ encircled counts fraction) at this location on the image, as well as the source spectral extraction region as described in the text. The dashed white circle has an inner and outer radius of $3"$ and $6"$ respectively, marking the background spectral extraction region. North is up and East is left.
• Figure 2: JWST and Chandra images of UHZ1: Panel (a) The JWST NIRCam image of the surroundings of UHZ1, and a zoom-in NIRCam image of UHZ1 in Panels (b and c). Panel (d) JWST images of UHZ1 in seven filters. The galaxy is detected in all JWST bands except for F115W. The non-detection in the bluest F115W band clearly indicates the dropout nature of the galaxy and suggests that it is located at $z\approx10$. The source is extended, with a potentially disturbed morphology evocative of late-stage mergers at lower redshift. A bright nuclear region is apparent in the F150W and F200W bands, and the contrast of this nucleus against the galaxy outskirts decreases for the redder bands. (e) A JWST/Chandra overlay showing a $4.2\sigma$ excess of X-ray counts cospatial with UHZ1. (f) The same Chandra$2-7$ keV Chandra image, this time with UHZ1 represented as black contours. The size of the X-ray source is consistent with a point source. The location, luminosity, and spectral characteristics of the source suggest that it is a heavily obscured quasar residing in the $z=10.3$ galaxy, UHZ1. North is up and East is left.
• Figure 3: Chandra X-ray spectral energy distribution and model fits: Observed-frame X-ray photons extracted from the merged Chandra ACIS data in the source aperture containing UHZ1 and the foreground lensing cluster, Abell 2744 (gray crosses; binned to a minimum of 2 counts for plotting purposes). The dotted gray line provides the thermal plasma model (APEC with $kT\sim10.9$ keV) established from a surrounding background annulus and rescaled to match the source aperture area. The solid gray curve provides the best-fit total model of the combined cluster+AGN X-ray emission constructed utilizing the MyTorus library. The observed X-ray spectrum is inconsistent with a pure plasma model - solid blue line shows the best-fit pure AGN model after subtraction of the cluster emission; this model reproduces the background-subtracted broad-band X-ray photometry extracted using forced photometry at the position of UHZ1 ($0.5-1.5$ keV photometric point is a $3\sigma$ upper limit due to the non-detection of X-ray emission after background subtraction). For illustration purposes, we further provide the best-fit pure AGN components for a range of assumed column densities ($N_{\rm H}=${0.1,0.3,1.0,3.0}$\times 10^{24}$ cm$^{-2}$).
• Figure 4: Sketch of the growth of BHs with different initial seed masses and accretion rates: BHs formed via the light seed scenario with $10-100 \,M_{\odot}$ mass can only reach $10^4-10^5 \,M_{\odot}$ by $z=10.3$ if they accrete at their Eddington limit (blue shaded region), which falls short by $2-4$ orders of magnitude of the BH mass estimated for UHZ1. Implausibly high sustained accretion at a rate of at least twice the Eddington limit would be required for light seeds to reach the BH mass close to that of UHZ1 (blue-hatched region). However, for light seeds continuous accretion at the Eddington limit or above for several hundred of million years is highly unlikely as noted by Willott+10. Heavy seed models with $10^4-10^5 \,M_{\odot}$ initial BH masses can grow to the mass of the BH powering UHZ1 by $z=10.3$ assuming accretion at the Eddington limit (tan shaded region). All over-plotted models assume a radiative efficiency of $10\%$ and continuous accretion. We also show the location of the three previously known highest redshift quasars at $z\sim7.5$, which were identified in large-area optical surveysmortlock11banados18wang21. The systematic uncertainty (not shown) on the BH mass of these quasars is $\sim\,0.5$ dex. The mass range shown for UHZ1 corresponds to the derived estimate as noted in the text.
• Figure 5: Best-fit statistical uncertainties and posterior draws of the demagnified intrinsic X-ray luminosity ($L_{\rm X,int}$) and gas column density ($N_{\rm H}$) of UHZ1. One, 2, and 3$\sigma$ (solid, dashed, and dotted lines, respectively) contours are constructed from the X-ray spectral fits to the Chandra data. Black lines show the statistical fits for a fixed AGN spectral slope of $\Gamma=1.9$ and assuming fixed best-fit (rescaled) normalization and plasma temperature for the background APEC component. Light-gray color represents a fixed slope of $\Gamma=2.3$. Blue lines are the posterior draws from a Bayesian fit employing broad informative Gaussian priors for $\Gamma$, $n_{\rm APEC}$ and kT. The mode of the posterior is shown with a blue cross. For values of $N_{\rm H} \gtrsim 2\times 10^{24}$ (equivalent to the lower $1\sigma$ contours), a strong steep degeneracy between $L_{\rm X,int}$ and $N_{\rm H}$ is clearly evident, we thus adopt this threshold to mitigate these degeneracies, resulting in an adopted $L_{\rm X} \sim 1.9 \times 10^{44}$ erg/s.