The possible accretion discs of GN-z11 at redshift z = 10.6, MoM-z14 at z = 14.44 and other high redshift objects

Abstract

The JWST has enabled the discovery of Active Galactic Nuclei at high redshifts. The intrinsic UV spectrum of GN-z11 at redshift z=10.6 has a spectral slope compatible with a standard accretion disc. By fitting a disc model to its spectrum, we find that the mass of the black hole must be above 1.12 x 10^7 Msun in order that it lies below the Eddington limit. We define this mass as the Eddington mass of the black hole. We note that the spectral shape is consistent with that of accreting stellar mass black holes sources in their soft state, for which no variability is expected. Mom-z14 is a more distant object at $z=14.44$ and has a similar UV slope. Disc model-fitting gives a similar result but lower mass accretion rate. We also examine3 further high redshift objects: GS z14-1, GHZ2 and PAN-z14-1 at z=13.86, 12.34 and 13.53, again obtaining similar results. If sub-Eddington accretion discs are indeed the origin of much of the UV emission from these objects, then the existence of massive black holes less than 304 and 290 Myr after the Big Bang point either to exceptional black hole seeds or to primordial black holes. The observed spread of UV spectral slopes in high redshift objects suggests that our approach may be relevant to about half of that population.

Figures (5)

• Figure 1: The NIRSpec spectrum of GN-z11 fitted with a relativistic multi-colour disc thermal emission model assuming different combinations of BH spin and inclination angle. The solid, dashed, dotted, dash-dotted and dash-dot-dotted lines assume $\log(m_{\rm BH}) =$ 6, 6.5, 7, 7.5 and 8 respectively. The $\log(m_{\rm BH}) =$7.5 model deviates from the observed spectrum below 20,000 Å in the top left panel for a low-spin and low-inclination angle model. In the bottom right panel, for the maximum BH spin and a high inclination angle, the difference between these models for different BH masses is very small in the observed wavelength range. A high-spin and high-inclination angle model would allow a wider range of possible BH masses to fit the data.
• Figure 2: The inferred bolometric luminosity of the multi-colour disc thermal emission assuming different combinations of BH spin and disc inclination angle in comparison with Eddington luminosity (dash-dot-dotted line). The pink horizontal dashed line shows the bolometric luminosity of GN-z11 calculated by applying a correction factor to the rest-frame 1400 Å luminosity of the object maiolino24. (Note: $L_{\rm Bol}=\epsilon\dot{M}c^{2}$ in this calculation, with $\epsilon=0.057$. The Eddington BH mass is $\log(m_{\rm BH})=7.05$ (or $1.12\times10^{7}M_{\odot}$) for $a=0$ and $i=30$.)
• Figure 3: Monochromatic luminosity at rest-frame 1400 Å in erg s$^{-1}$ for different black hole masses $m_{\rm BH}$ inferred by the zkerrbb model. Lines with different styles show different parameter sets. The calculations assume the bolometric luminosity of the accretion disc equals the Eddington limit. The vertical dashed line marks the monochromatic luminosity of several JWST-observed, high-redshift objects at rest-frame 1400 Å. This figure can be used to estimate black hole mass from a photometric luminosity, without prior knowledge of the spectral shape.
• Figure 4: Black hole mass ($m_{\rm BH}$) plotted against redshift for 3 values of Eddington fraction ($\lambda_{\rm Edd}= 0.1, 1\ {\rm and}\ 10$ red, blue and green respectively and radiative efficiency of $6\%$ dashed and $10\%$ solid). The lines are anchored at $\log(m)=7.05$, the Eddington mass of GN-z11.
• Figure 5: Top: the same best-fit disc thermal emission model as in the bottom right panel of Fig. \ref{['pic_jwst_fit3']}. The solid line shows the best-fit model for a range of BH masses from $10^{6}$ to $10^{8}$ solar masses. The grey line is the average $z=0$ intrinsic quasar spectrum calculated using QSOGEN Temple21. Note that it compares well to the accretion disc. Bottom: the HST (UV) and UKST (optical) spectra of a local NLS1 AGN 1H 0707$-$495 ($z=0.04$). Its UV and optical continuum is consistent with an absorbed disc thermal emission ($a_{*}=0.99$, $i=60^{\circ}$, $m_{\rm BH}=10^{7}$, dash-dotted line), after removing the effects of dust extinction.