Kavli Affiliate: Kohei Inayoshi
| First 5 Authors: Kohei Inayoshi, Kohei Inayoshi, , ,
| Summary:
The birth of seeds of massive black holes (BHs) and nascent galaxies at
cosmic dawn takes place in dense gaseous environments, which play a crucial
role in shaping their coevolution and radiation spectra. We investigate gas
accretion during the assembly of massive halos with $M_rm hgtrsim
10^10-11~M_odot$ at redshifts $zsimeq 4-10$, driving both rapid BH feeding
and concurrent nuclear starbursts. As the BH grows to $sim 10^6-7~M_odot$
via super-Eddington accretion, the accretion power inflates a dense envelope
whose effective temperature approaches the Hayashi limit at $T_rm effsimeq
5000~rm K$, producing red optical emission, while a coeval young stellar
population of $sim 10^7~M_odot$ provides blue UV emission. This early
coevolving system naturally reproduces the characteristic spectral features of
the so-called little red dots (LRDs), a population of broad-line active
galactic nuclei (AGNs), including the V-shaped UV-to-optical spectra and
weakness of X-ray, infrared, and radio emission. Massive stars in the nuclear
starburst soon explode as supernovae, injecting energy and momentum that expel
gas from the nucleus, quench gas supply to the BH envelope, and ultimately
drive a transition into normal AGN phases. For individual LRDs, the
optical-to-UV luminosity ratio remains nearly constant at $L_rm opt/L_rm
UVsimeq 2-10$ from the onset of accretion bursts for $simeq 15~rm Myr$,
one-third of the Salpeter time, until quenching by stellar feedback. While this
ratio is sustained for the LRD population at $zsimeq 4-8$, it declines toward
lower redshifts as BHs can no longer maintain red envelopes, thereby losing the
LRD characteristics.
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