1,734 research outputs found
The evolution of the star forming sequence in hierarchical galaxy formation models
It has been argued that the specific star formation rates of star forming
galaxies inferred from observational data decline more rapidly below z = 2 than
is predicted by hierarchical galaxy formation models. We present a detailed
analysis of this problem by comparing predictions from the GALFORM
semi-analytic model with an extensive compilation of data on the average star
formation rates of star-forming galaxies. We also use this data to infer the
form of the stellar mass assembly histories of star forming galaxies. Our
analysis reveals that the currently available data favour a scenario where the
stellar mass assembly histories of star forming galaxies rise at early times
and then fall towards the present day. In contrast, our model predicts stellar
mass assembly histories that are almost flat below z = 2 for star forming
galaxies, such that the predicted star formation rates can be offset with
respect to the observational data by factors of up to 2-3. This disagreement
can be explained by the level of coevolution between stellar and halo mass
assembly that exists in contemporary galaxy formation models. In turn, this
arises because the standard implementations of star formation and supernova
feedback used in the models result in the efficiencies of these process
remaining approximately constant over the lifetime of a given star forming
galaxy. We demonstrate how a modification to the timescale for gas ejected by
feedback to be reincorporated into galaxy haloes can help to reconcile the
model predictions with the data.Comment: 30 Pages, 16 Figures, MNRAS accepte
Assumptions of the primordial spectrum and cosmological parameter estimation
The observables of the perturbed universe, CMB anisotropy and large
structures, depend on a set of cosmological parameters, as well as, the assumed
nature of primordial perturbations. In particular, the shape of the primordial
power spectrum (PPS) is, at best, a well motivated assumption. It is known that
the assumed functional form of the PPS in cosmological parameter estimation can
affect the best fit parameters and their relative confidence limits. In this
paper, we demonstrate that a specific assumed form actually drives the best fit
parameters into distinct basins of likelihood in the space of cosmological
parameters where the likelihood resists improvement via modifications to the
PPS. The regions where considerably better likelihoods are obtained allowing
free form PPS lie outside these basins. In the absence of a preferred model of
inflation, this raises a concern that current cosmological parameters estimates
are strongly prejudiced by the assumed form of PPS. Our results strongly
motivate approaches toward simultaneous estimation of the cosmological
parameters and the shape of the primordial spectrum from upcoming cosmological
data. It is equally important for theorists to keep an open mind towards early
universe scenarios that produce features in the PPS.Comment: 11 pages, 2 figures, discussions extended, main results unchanged,
matches published versio
Multi-spin dynamics of the solid-state NMR Free Induction Decay
We present a new experimental investigation of the NMR free induction decay
(FID) in a lattice of spin-1/2 nuclei in a strong Zeeman field. Following a
pi/2 pulse, evolution under the secular dipolar Hamiltonian preserves coherence
number in the Zeeman eigenbasis, but changes the number of correlated spins in
the state. The observed signal is seen to decay as single-spin, single-quantum
coherences evolve into multiple-spin coherences under the action of the dipolar
Hamiltonian. In order to probe the multiple-spin dynamics during the FID, we
measured the growth of coherence orders in a basis other than the usual Zeeman
eigenbasis. This measurement provides the first direct experimental observation
of the growth of coherent multiple-spin correlations during the FID.
Experiments were performed with a cubic lattice of spins (19F in calcium
fluoride) and a linear spin chain (19F in fluorapatite). It is seen that the
geometrical arrangement of the spins plays a significant role in the
development of higher order correlations. The results are discussed in light of
existing theoretical models.Comment: 7 pages, 6 figure
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