2,469 research outputs found
The combined effect of AGN and supernovae feedback in launching massive molecular outflows in high-redshift galaxies
We have recently improved our model of active galactic nucleus (AGN) by
attaching the supermassive black hole (SMBH) to a massive nuclear star cluster
(NSC). Here we study the effects of this new model in massive, gas-rich
galaxies with several simulations of different feedback recipes with the
hydrodynamics code RAMSES. These simulations are compared to a reference
simulation without any feedback, in which the cooling halo gas is quickly
consumed in a burst of star formation. In the presence of strong supernovae
(SN) feedback, we observe the formation of a galactic fountain that regulates
star formation over a longer period, but without halting it. If only AGN
feedback is considered, as soon as the SMBH reaches a critical mass, strong
outflows of hot gas are launched and prevent the cooling halo gas from reaching
the disk, thus efficiently halting star formation, leading to the so-called
"quenching". If both feedback mechanisms act in tandem, we observe a non-linear
coupling, in the sense that the dense gas in the supernovae-powered galactic
fountain is propelled by the hot outflow powered by the AGN at much larger
radii than without AGN. We argue that these particular outflows are able to
unbind dense gas from the galactic halo, thanks to the combined effect of SN
and AGN feedback. We speculate that this mechanism occurs at the end of the
fast growing phase of SMBH, and is at the origin of the dense molecular
outflows observed in many massive high-redshift galaxies.Comment: 16 pages, 13 figures, accepted to MNRA
Logical relations for coherence of effect subtyping
A coercion semantics of a programming language with subtyping is typically
defined on typing derivations rather than on typing judgments. To avoid
semantic ambiguity, such a semantics is expected to be coherent, i.e.,
independent of the typing derivation for a given typing judgment. In this
article we present heterogeneous, biorthogonal, step-indexed logical relations
for establishing the coherence of coercion semantics of programming languages
with subtyping. To illustrate the effectiveness of the proof method, we develop
a proof of coherence of a type-directed, selective CPS translation from a typed
call-by-value lambda calculus with delimited continuations and control-effect
subtyping. The article is accompanied by a Coq formalization that relies on a
novel shallow embedding of a logic for reasoning about step-indexing
On the Dynamics of Supermassive Black Holes in Gas-Rich, Star-Forming Galaxies: the Case for Nuclear Star Cluster Coevolution
We introduce a new model for the formation and evolution of supermassive
black holes (SMBHs) in the RAMSES code using sink particles, improving over
previous work the treatment of gas accretion and dynamical evolution. This new
model is tested against a suite of high-resolution simulations of an isolated,
gas-rich, cooling halo. We study the effect of various feedback models on the
SMBH growth and its dynamics within the galaxy.
In runs without any feedback, the SMBH is trapped within a massive bulge and
is therefore able to grow quickly, but only if the seed mass is chosen larger
than the minimum Jeans mass resolved by the simulation. We demonstrate that, in
the absence of supernovae (SN) feedback, the maximum SMBH mass is reached when
Active Galactic Nucleus (AGN) heating balances gas cooling in the nuclear
region.
When our efficient SN feedback is included, it completely prevents bulge
formation, so that massive gas clumps can perturb the SMBH orbit, and reduce
the accretion rate significantly. To overcome this issue, we propose an
observationally motivated model for the joint evolution of the SMBH and a
parent nuclear star cluster (NSC), which allows the SMBH to remain in the
nuclear region, grow fast and resist external perturbations. In this scenario,
however, SN feedback controls the gas supply and the maximum SMBH mass now
depends on the balance between AGN heating and gravity. We conclude that
SMBH/NSC co-evolution is crucial for the growth of SMBH in high-z galaxies, the
progenitors of massive elliptical today.Comment: accepted for publication in MNRA
Proving termination of evaluation for System F with control operators
We present new proofs of termination of evaluation in reduction semantics
(i.e., a small-step operational semantics with explicit representation of
evaluation contexts) for System F with control operators. We introduce a
modified version of Girard's proof method based on reducibility candidates,
where the reducibility predicates are defined on values and on evaluation
contexts as prescribed by the reduction semantics format. We address both
abortive control operators (callcc) and delimited-control operators (shift and
reset) for which we introduce novel polymorphic type systems, and we consider
both the call-by-value and call-by-name evaluation strategies.Comment: In Proceedings COS 2013, arXiv:1309.092
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