Kavli Affiliate: Mark Vogelsberger
| First 5 Authors: Aklant K Bhowmick, Laura Blecha, Paul Torrey, Rachel S Somerville, Luke Zoltan Kelley
| Summary:
The first "seeds" of supermassive black holes (BHs) continue to be an
outstanding puzzle, and it is currently unclear whether the imprints of early
seed formation survive today. Here we examine the signatures of seeding in the
local Universe using five $[18~mathrm{Mpc}]^3$ BRAHMA simulation boxes run to
$z=0$. They initialize $1.5times10^5~M_{odot}$ BHs using different seeding
models. The first four boxes initialize BHs as heavy seeds using criteria that
depend on dense & metal-poor gas, Lyman-Werner radiation, gas spin, and
environmental richness. The fifth box initializes BHs as descendants of lower
mass seeds ($sim10^3~M_{odot}$) using a new stochastic seed model built in
our previous work. We find that strong signatures of seeding survive in
$sim10^5-10^6~M_{odot}$ local BHs hosted in $M_*lesssim10^{9}~M_{odot}$
dwarf galaxies. The signatures survive due to two reasons: 1) there is a
substantial population of local $sim10^5~M_{odot}$ BHs that are ungrown
relics of early seeds from $zsim5-10$; 2) BH growth up to $sim10^6~M_{odot}$
is dominated by mergers all the way down to $zsim0$. As the contribution from
gas accretion increases, the signatures of seeding start to weaken in more
massive $gtrsim10^6~M_{odot}$ BHs, and they eventually disappear for
$gtrsim10^7~M_{odot}$ BHs. This is in contrast to high-z ($zgtrsim5$) BH
populations wherein the BH growth is fully merger dominated, which causes the
seeding signatures to persist at least up to $sim10^8~M_{odot}$. The
different seed models predict abundances of local $sim10^6~M_{odot}$ BHs
ranging from $sim0.01-0.05~mathrm{Mpc}^{-3}$ with occupation fractions of
$sim20-100%$ in $M_*sim10^{9}~M_{odot}$ galaxies. Our results highlight the
potential for local $sim10^5-10^6~M_{odot}$ BH populations in dwarf galaxies
to serve as a promising probe for BH seeding models.
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