1,269 research outputs found
Resolving the Spin Crisis: Mergers and Feedback
We model in simple terms the angular momentum (J) problem of galaxy formation
in CDM, and identify the key elements of a scenario that can solve it. The
buildup of J is modeled via dynamical friction and tidal stripping in mergers.
This reveals how over-cooling in incoming halos leads to transfer of J from
baryons to dark matter (DM), in conflict with observations. By incorporating a
simple recipe of supernova feedback, we match the observed J distribution in
disks. Gas removal from small incoming halos, which make the low-J component of
the product, eliminates the low-J baryons. Partial heating and puffing-up of
the gas in larger incoming halos, combined with tidal stripping, reduces the J
loss of baryons. This implies a higher baryonic spin for lower mass halos. The
observed low baryonic fraction in dwarf galaxies is used to calibrate the
characteristic velocity associated with supernova feedback, yielding v_fb sim
100 km/s, within the range of theoretical expectations. The model then
reproduces the observed distribution of spin parameter among dwarf and bright
galaxies, as well as the J distribution inside these galaxies. This suggests
that the model captures the main features of a full scenario for resolving the
spin crisis.Comment: 8 pages, Latex, svmult.cls, subeqnar.sty, sprmindx.sty, physprbb.sty,
cropmark.sty, in The Mass of Galaxies at Low and High Redshift, eds. R.
Bender & A. Renzini (Springer-Verlag, ESO Astrophysics Symposia
PANSATZ: Pulse-based Ansatz for Variational Quantum Algorithms
We develop and implement a novel pulse-based ansatz, which we call PANSATZ,
for more efficient and accurate implementations of variational quantum
algorithms (VQAs) on today's noisy intermediate-scale quantum (NISQ) computers.
Our approach is applied to quantum chemistry. Specifically, finding the
ground-state energy associated with the electron configuration problem, using
the variational quantum eigensolver (VQE) algorithm for several molecules. We
manage to achieve chemical accuracy both in simulation for several molecules
and on one of IBM's NISQ devices for the molecule in the STO-3G basis.
Our results are compared to a gate-based ansatz and show significant latency
reduction - up to shorter ansatz schedules. We also show that this
ansatz has structured adaptivity to the entanglement level required by the
problem
Anisotropic Hubble expansion of large scale structures
We investigate the dynamics of an homogenous distribution of galaxies moving
under the cosmological expansion through Euler-Poisson equations system. The
solutions are interpreted with the aim of understanding the cosmic velocity
fields in the Local Super Cluster, and in particular the presence of a bulk
flow. Among several solutions, we shows a planar kinematics with constant
(eternal) and rotational distortion, the velocity field is not potential
Dependence of the Inner DM Profile on the Halo Mass
I compare the density profile of dark matter (DM) halos in cold dark matter
(CDM) N-body simulations with 1 Mpc, 32 Mpc, 256 Mpc and 1024 Mpc box sizes. In
dimensionless units the simulations differ only for the initial power spectrum
of density perturbations. I compare the profiles when the most massive halos
are composed of about 10^5 DM particles. The DM density profiles of the halos
in the 1 Mpc box show systematically shallower cores with respect to the
corresponding halos in the 32 Mpc simulation that have masses, M_{dm}, typical
of the Milky Way and are fitted by a NFW profile. The DM density profiles of
the halos in the 256 Mpc box are consistent with having steeper cores than the
corresponding halos in the 32 Mpc simulation, but higher mass resolution
simulations are needed to strengthen this result. Combined, these results
indicate that the density profile of DM halos is not universal, presenting
shallower cores in dwarf galaxies and steeper cores in clusters. Physically the
result sustains the hypothesis that the mass function of the accreting
satellites determines the inner slope of the DM profile. In comoving
coordinates, r, the profile \rho_{dm} \propto 1/(X^\alpha(1+X)^{3-\alpha}),
with X=c_\Delta r/r_\Delta, r_\Delta is the virial radius and \alpha
=\alpha(M_{dm}), provides a good fit to all the DM halos from dwarf galaxies to
clusters at any redshift with the same concentration parameter c_\Delta ~ 7.
The slope, \gamma, of the outer parts of the halo appears to depend on the
acceleration of the universe: when the scale parameter is a=(1+z)^{-1} < 1, the
slope is \gamma ~ 3 as in the NFW profile, but \gamma ~ 4 at a > 1 when
\Omega_\Lambda ~ 1 and the universe is inflating.[abridged]Comment: Accepted for publication in MNRAS. 13 pages, including 11 figures and
2 tables. The revised version has an additional discussion section and work
on the velocity dispersion anisotrop
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