Kavli Affiliate: Robert A. Simcoe
| First 5 Authors: Jorryt Matthee, Rohan P. Naidu, Gabriel Brammer, John Chisholm, Anna-Christina Eilers
| Summary:
Characterising the prevalence and properties of faint active galactic nuclei
(AGN) in the early Universe is key for understanding the formation of
supermassive black holes (SMBHs) and determining their role in cosmic
reionization. We perform a spectroscopic search for broad H$alpha$ emitters at
$zapprox4-6$ using deep JWST/NIRCam imaging and wide field slitless
spectroscopy from the EIGER and FRESCO surveys. We identify 20 H$alpha$ lines
at $z=4.2-5.5$ that have broad components with line widths from $sim1200-3700$
km s$^{-1}$, contributing $sim30-90$ % of the total line flux. We interpret
these broad components as being powered by accretion onto SMBHs with implied
masses $sim10^{7-8}$ M$_{odot}$. In the UV luminosity range M$_{rm UV}=-21$
to $-18$, we measure number densities of $approx10^{-5}$ cMpc$^{-3}$. This is
an order of magnitude higher than expected from extrapolating quasar UV
luminosity functions. Yet, such AGN are found in only $<1$ % of star-forming
galaxies at $zsim5$. The SMBH mass function agrees with large cosmological
simulations. In two objects we detect narrow red- and blue-shifted H$alpha$
absorption indicative, respectively, of dense gas fueling SMBH growth and
outflows. We may be witnessing early AGN feedback that will clear dust-free
pathways through which more massive blue quasars are seen. We uncover a strong
correlation between reddening and the fraction of total galaxy luminosity
arising from faint AGN. This implies that early SMBH growth is highly obscured
and that faint AGN are only minor contributors to cosmic reionization.
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