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 $sim 30 – 90$ % 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$_{rm
UV, AGN+host}$ = -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 number density discrepancy is
much lower when compared to the broad H$alpha$ luminosity function. The SMBH
mass function agrees with large cosmological simulations. In two objects we
detect complex H$alpha$ profiles that we tentatively interpret as caused by
absorption signatures from 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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