16,230 research outputs found
Use of neural networks for the identification of new z>=3.6 QSOs from FIRST-SDSS DR5
We aim to obtain a complete sample of redshift > 3.6 radio QSOs from FIRST
sources having star-like counterparts in the SDSS DR5 photometric survey
(r<=20.2). We found that simple supervised neural networks, trained on sources
with SDSS spectra, and using optical photometry and radio data, are very
effective for identifying high-z QSOs without spectra. The technique yields a
completeness of 96 per cent and an efficiency of 62 per cent. Applying the
trained networks to 4415 sources without DR5 spectra we found 58 z>=3.6 QSO
candidates. We obtained spectra of 27 of them, and 17 are confirmed as high-z
QSOs. Spectra of 13 additional candidates from the literature and from SDSS DR6
revealed 7 more z>=3.6 QSOs, giving and overall efficiency of 60 per cent. None
of the non-candidates with spectra from NED or DR6 is a z>=3.6 QSO,
consistently with a high completeness. The initial sample of z>=3.6 QSOs is
increased from 52 to 76, i.e. by a factor 1.46. From the new identifications
and candidates we estimate an incompleteness of SDSS for the spectroscopic
classification of FIRST 3.6<=z<=4.6 QSOs of 15 percent for r<=20.2.Comment: 16 pages, 9 figures accepted for publication in MNRA
Ammonia observations in the LBV nebula G79.29+0.46. Discovery of a cold ring and some warm spots
The surroundings of Luminous Blue Variable (LBV) stars are excellent
laboratories to study the effects of their high UV radiation, powerful winds,
and strong ejection events onto the surrounding gas and dust. The LBV
G79.29+0.46 powered two concentric infrared rings which may interact with the
infrared dark cloud (IRDC) G79.3+0.3. The Effelsberg 100m telescope was used to
observe the NH_3 (1,1), (2,2) emission surrounding G79.29+0.46 and the IRDC. In
addition, we observed particular positions in the (3,3) transition toward the
strongest region of the IRDC. We report here the first coherent shell-like
structure of dense NH_3 gas associated with an evolved massive star. The shell,
two or three orders of magnitude more tenuous than the IRDC, is well traced in
both ammonia lines, and surrounds the ionized nebula. The NH_3 emission in the
IRDC is characterized by a low and uniform rotational temperature (T_rot ~ 10
K) and moderately high opacities in the (1,1) line. The rest of the observed
field is spotted by warm or hot zones (T_rot > 30 K) and characterized by
optically thin emission of the (1,1) line. The NH_3 abundances are about
10^{-8} in the IRDC, and 10^{-10}-10^{-9} elsewhere. The warm temperatures and
low abundances of NH_3 in the shell suggest that the gas is being heated and
photo-dissociated by the intense UV field of the LBV star. An outstanding
region is found to the south-west (SW) of the LBV star within the IRDC. The
NH_3 (3,3) emission at the centre of the SW region reveals two velocity
components tracing gas at temperatures > 30K. The northern edge of the SW
region agrees with the border of the ring nebula and a region of continuum
enhancement; here, the opacity of the (1,1) line and the NH_3 abundance do not
decrease as expected in a typical clump of an isolated cold dark cloud. This
strongly suggests some kind of interaction between the ring nebula and the
IRDC.Comment: 15 pages, 13 figures, accepted by A&A. Note the change of title with
respect to previous versio
Reducing sample variance: halo biasing, non-linearity and stochasticity
Comparing clustering of differently biased tracers of the dark matter
distribution offers the opportunity to reduce the cosmic variance error in the
measurement of certain cosmological parameters. We develop a formalism that
includes bias non-linearities and stochasticity. Our formalism is general
enough that can be used to optimise survey design and tracers selection and
optimally split (or combine) tracers to minimise the error on the
cosmologically interesting quantities. Our approach generalises the one
presented by McDonald & Seljak (2009) of circumventing sample variance in the
measurement of . We analyse how the bias, the noise,
the non-linearity and stochasticity affect the measurements of and explore
in which signal-to-noise regime it is significantly advantageous to split a
galaxy sample in two differently-biased tracers. We use N-body simulations to
find realistic values for the parameters describing the bias properties of dark
matter haloes of different masses and their number density.
We find that, even if dark matter haloes could be used as tracers and
selected in an idealised way, for realistic haloes, the sample variance limit
can be reduced only by up to a factor .
This would still correspond to the gain from a three times larger survey volume
if the two tracers were not to be split. Before any practical application one
should bear in mind that these findings apply to dark matter haloes as tracers,
while realistic surveys would select galaxies: the galaxy-host halo relation is
likely to introduce extra stochasticity, which may reduce the gain further.Comment: 21 pages, 13 figures. Published version in MNRA
The effective Lagrangian of dark energy from observations
Using observational data on the expansion rate of the universe (H(z)) we
constrain the effective Lagrangian of the current accelerated expansion. Our
results show that the effective potential is consistent with being flat i.e., a
cosmological constant; it is also consistent with the field moving along an
almost flat potential like a pseudo-Goldstone boson. We show that the potential
of dark energy does not deviate from a constant at more than 6% over the
redshift range 0 < z < 1. The data can be described by just a constant term in
the Lagrangian and do not require any extra parameters; therefore there is no
evidence for augmenting the number of parameters of the LCDM paradigm. We also
find that the data justify the effective theory approach to describe
accelerated expansion and that the allowed parameters range satisfy the
expected hierarchy. Future data, both from cosmic chronometers and baryonic
acoustic oscillations, that can measure H(z) at the % level, could greatly
improve constraints on the flatness of the potential or shed some light on
possible mechanisms driving the accelerated expansion. Besides the above
result, it is shown that the effective Lagrangian of accelerated expansion can
be constrained from cosmological observations in a model-independent way and
that direct measurements of the expansion rate H(z) are most useful to do so.Comment: 9 pages, 3 figures, JCAP submitted. This paper presents a
reconstruction of the dark energy potential. It is a companion to Moresco et
al. 2012a, which presents new H(z) results and Moresco et al. 2012b, which
provides cosmological parameter constraint
Perturbations in electromagnetic dark energy
It has been recently proposed that the presence of a temporal electromagnetic
field on cosmological scales could explain the phase of accelerated expansion
that the universe is currently undergoing. The field contributes as a
cosmological constant and therefore, the homogeneous cosmology produced by such
a model is exactly the same as that of CDM. However, unlike a
cosmological constant term, electromagnetic fields can acquire perturbations
which in principle could affect CMB anisotropies and structure formation. In
this work, we study the evolution of inhomogeneous scalar perturbations in this
model. We show that provided the initial electromagnetic fluctuations generated
during inflation are small, the model is perfectly compatible with both CMB and
large scale structure observations at the same level of accuracy as
CDM.Comment: 12 pages, 3 figures. Added new comments to match the published
versio
An effective theory of accelerated expansion
We work out an effective theory of accelerated expansion to describe general
phenomena of inflation and acceleration (dark energy) in the Universe. Our aim
is to determine from theoretical grounds, in a physically-motivated and model
independent way, which and how many (free) parameters are needed to broadly
capture the physics of a theory describing cosmic acceleration. Our goal is to
make as much as possible transparent the physical interpretation of the
parameters describing the expansion. We show that, at leading order, there are
five independent parameters, of which one can be constrained via general
relativity tests. The other four parameters need to be determined by observing
and measuring the cosmic expansion rate only, H(z). Therefore we suggest that
future cosmology surveys focus on obtaining an accurate as possible measurement
of to constrain the nature of accelerated expansion (dark energy and/or
inflation).Comment: In press; minor changes, results unchange
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