851 research outputs found
Dall'a(ltro) all'altro. Attorno al Seminario Ancora di Jacques Lacan
L'articolo ricostruisce - a partire dal Seminario XX, Ancora, di Jacques Lacan - la posizione dell'oggetto a, come obiezione al rapporto con l'altro. Nella ricostruzione della problematica, riassunta dall'espressione lacaniana "non c'\ue8 rapporto sessuale", un posto importante viene riconosciuto alle riflessioni di S. Freud sull'innamoramento e la melanconia. Un momento esplicativo della problematica dell'oggetto a, viene individuato nel ciclo di opere dedicato alle Ninfee, di Claude Monet, soggetto costantemente ritratto negli ultimi vent'anni di vita del pittore francese. La direzione intravista per il passaggio dall' (a)-ltro all'altro \ue8 affidata ad un lavoro (di imputazione) utile a far cadere l'oggetto: cedere l'oggetto come riabilitazione del desiderio. Cedere l'oggetto a (castrazione) equivale a "non cedere sul proprio desiderio" (Seminario VII sull'etica di J. Lacan) ovvero al pensiero dell'imputabilit\ue0, come riapertura del rapporto con l'altro
Mapping the Cosmic Web with Ly-alpha Emission
We use a high-resolution cosmological simulation to predict the distribution
of HI Ly-alpha emission from the low-redshift (z<0.5) intergalactic medium
(IGM). Our simulation can be used to reliably compute the emission from
optically thin regions of the IGM but not that of self-shielded gas. We
therefore consider several models that bracket the expected emission from
self-shielded regions. Most galaxies are surrounded by extended (>10^2 kpc)
``coronae'' of optically thin gas with Ly-alpha surface brightness close to the
expected background. Most of these regions contain smaller cores of dense, cool
gas. Unless self-shielded gas is able to cool to T<10^4.1 K, these cores are
much brighter than the background. The Ly-alpha coronae represent ``cooling
flows'' of IGM gas accreting onto galaxies. We also estimate the number of
Ly-alpha photons produced through the reprocessing of stellar ionizing
radiation in the interstellar medium of galaxies; while this mechanism is
responsible for the brightest Ly-alpha emission, it occurs on small physical
scales and can be separated using high-resolution observations. In all cases,
we find that Ly-alpha emitters are numerous (with a space density ~0.1 h^3
Mpc^-3) and closely trace the filamentary structure of the IGM, providing a new
way to map gas inside the cosmic web.Comment: 4 pages, 3 figures, accepted by ApJ
A Flexible Analytic Model of Cosmic Variance in the First Billion Years
Cosmic variance is the intrinsic scatter in the number density of galaxies
due to fluctuations in the large-scale dark matter density field. In this work,
we present a simple analytic model of cosmic variance in the high redshift
Universe (). We assume that galaxies grow according to the evolution
of the halo mass function, which we allow to vary with large-scale environment.
Our model produces a reasonable match to the observed ultraviolet luminosity
functions in this era by regulating star formation through stellar feedback and
assuming that the UV luminosity function is dominated by recent star formation.
We find that cosmic variance in the UVLF is dominated by the variance in the
underlying dark matter halo population, and not by differences in halo
accretion or the specifics of our stellar feedback model. We also find that
cosmic variance dominates over Poisson noise for future high- surveys except
for the brightest sources or at very high redshifts (). We
provide a linear approximation of cosmic variance for a variety of redshifts,
magnitudes, and survey areas through the public Python package galcv. Finally,
we introduce a new method for incorporating priors on cosmic variance into
estimates of the galaxy luminosity function and demonstrate that it
significantly improves constraints on that important observable
Hubble Diagram of Gamma-Rays Bursts calibrated with Gurzadyan-Xue Cosmology
Gamma-ray bursts (GRBs) being the most luminous among known cosmic objects
carry an essential potential for cosmological studies if properly used as
standard candles. In this paper we test with GRBs the cosmological predictions
of the Gurzadyan-Xue (GX) model of dark energy, a novel theory that predicts,
without any free parameters, the current vacuum fluctuation energy density
close to the value inferred from the SNIa observations. We also compare the GX
results with those predicted by the concordance scenario -CDM.
According to the statistical approach by Schaefer (2007), the use of several
empirical relations obtained from GRBs observables, after a consistent
calibration for a specific model, enables one to probe current cosmological
models. Based on this recently introduced method, we use the 69 GRBs sample
collected by Schaefer (2007); and the most recently released SWIFT satellite
data (Sakamoto et al. 2007) together with the 41 GRBs sample collected by
Rizzuto et al. (2007), which has the more firmly determined redshifts. Both
data samples span a distance scale up to redshift about 7. We show that the GX
models are compatible with the Hubble diagram of the Schaefer (2007) 69 GRBs
sample. Such adjustment is almost identical to the one for the concordance
-CDM.Comment: 9 pages, 17 figures, 11 tables; Astr. & Astrophys. (in press
Reionization History from Coupled CMB/21cm Line Data
We study CMB secondary anisotropies produced by inhomogeneous reionization by
means of cosmological simulations coupled with the radiative transfer code
CRASH. The reionization history is consistent with the WMAP Thomson optical
depth determination. We find that the signal arising from this process
dominates over the primary CMB component for l > 4000 and reaches a maximum
amplitude of l(l+1)C_l/2\pi ~ 1.6 x 10^{-13} on arcmin scale, i.e. l as large
as several thousands. We then cross-correlate secondary CMB anisotropy maps
with neutral hydrogen 21cm line emission fluctuations obtained from the same
simulations. The two signals are highly anti-correlated on angular scales
corresponding to the typical size of HII regions (including overlapping) at the
21cm map redshift. We show how the CMB/21cm cross-correlation can be used to:
(a) study the nature of the reionization sources, (b) reconstruct the cosmic
reionization history, (c) infer the mean cosmic ionization level at any
redshift. We discuss the feasibility of the proposed experiment with
forthcoming facilities.Comment: 7 pages, 5 figures, MNRAS in pres
The extent of metal enrichment from galactic winds during the Cosmic Dawn
One of the key processes driving galaxy evolution during the Cosmic Dawn is
supernova feedback. This likely helps regulate star formation inside of
galaxies, but it can also drive winds that influence the large-scale
intergalactic medium. Here, we present a simple semi-analytic model of
supernova-driven galactic winds and explore the contributions of different
phases of galaxy evolution to metal enrichment in the high-redshift (z > 6)
Universe. We show that models calibrated to the observed galaxy luminosity
function at z~6-8 have filling factors ~1% at z~6 and ~0.1% at z~12, with
different star formation prescriptions providing about an order of magnitude
uncertainty. Despite the small fraction of space filled by winds, these
scenarios still provide more than enough enriched volume to explain the
observed abundance of metal-line absorbers in quasar spectra at z > 5. We also
consider enrichment through winds driven by Pop III star formation in
minihaloes. We find that these can dominate the total filling factor at z > 10
and even compete with winds from normal galaxies at z~6, at least in terms of
the total enriched volume. But these regions have much lower overall
metallicities, because each one is generated by a small burst of star
formation. Finally, we show that Compton cooling of these supernova-driven
winds at z > 6 has only a small effect on the cosmic microwave background.Comment: 12 pages, 8 figures, submitted to MNRA
Radio Recombination Lines at Decametre Wavelengths: Prospects for the Future
This paper considers the suitability of a number of emerging and future
instruments for the study of radio recombination lines (RRLs) at frequencies
below 200 MHz. These lines arise only in low-density regions of the ionized
interstellar medium, and they may represent a frequency-dependent foreground
for next-generation experiments trying to detect H I signals from the Epoch of
Reionization and Dark Ages ("21-cm cosmology"). We summarize existing
decametre-wavelength observations of RRLs, which have detected only carbon
RRLs. We then show that, for an interferometric array, the primary instrumental
factor limiting detection and study of the RRLs is the areal filling factor of
the array. We consider the Long Wavelength Array (LWA-1), the LOw Frequency
ARray (LOFAR), the low-frequency component of the Square Kilometre Array
(SKA-lo), and a future Lunar Radio Array (LRA), all of which will operate at
decametre wavelengths. These arrays offer digital signal processing, which
should produce more stable and better defined spectral bandpasses; larger
frequency tuning ranges; and better angular resolution than that of the
previous generation of instruments that have been used in the past for RRL
observations. Detecting Galactic carbon RRLs, with optical depths at the level
of 10^-3, appears feasible for all of these arrays, with integration times of
no more than 100 hr. The SKA-lo and LRA, and the LWA-1 and LOFAR at the lowest
frequencies, should have a high enough filling factor to detect lines with much
lower optical depths, of order 10^-4 in a few hundred hours. The amount of
RRL-hosting gas present in the Galaxy at the high Galactic latitudes likely to
be targeted in 21-cm cosmology studies is currently unknown. If present,
however, the spectral fluctuations from RRLs could be comparable to or exceed
the anticipated H I signals.Comment: 9 pages; Astron. & Astrophys., in pres
Constraining the Collisional Nature of the Dark Matter Through Observations of Gravitational Wakes
We propose to use gravitational wakes as a direct observational probe of the
collisional nature of the dark matter. We calculate analytically the structure
of a wake generated by the motion of a galaxy in the core of an X-ray cluster
for dark matter in the highly-collisional and collisionless limits. We show
that the difference between these limits can be recovered from detailed X-ray
or weak lensing observations. We also discuss the sizes of sub-halos in these
limits. Preliminary X-ray data on the motion of NGC 1404 through the Fornax
group disfavors fluid-like dark matter but does not exclude scenarios in which
the dark matter is weakly collisional.Comment: 29 pages, 3 figures, submitted to Ap
Cosmological Parameter Estimation Using 21 cm Radiation from the Epoch of Reionization
A number of radio interferometers are currently being planned or constructed
to observe 21 cm emission from reionization. Not only will such measurements
provide a detailed view of that epoch, but, since the 21 cm emission also
traces the distribution of matter in the Universe, this signal can be used to
constrain cosmological parameters at 6 < z < 20. The sensitivity of an
interferometer to the cosmological information in the signal may depend on how
precisely the angular dependence of the 21 cm 3-D power spectrum can be
measured. Utilizing an analytic model for reionization, we quantify all the
effects that break the spherical symmetry of the 3-D 21 cm power spectrum and
produce physically motivated predictions for this power spectrum. We find that
upcoming observatories will be sensitive to the 21 cm signal over a wide range
of scales, from larger than 100 to as small as 1 comoving Mpc. We consider
three methods to measure cosmological parameters from the signal: (1) direct
fitting of the density power spectrum to the signal, (2) using only the
velocity field fluctuations in the signal, (3) looking at the signal at large
enough scales such that all fluctuations trace the density field. With the
foremost method, the first generation of 21 cm observations should moderately
improve existing constraints on cosmological parameters for certain
low-redshift reionization scenarios, and a two year observation with the second
generation interferometer MWA5000 can improve constraints on Omega_w, Omega_m
h^2, Omega_b h^2, Omega_nu, n_s, and alpha_s. If the Universe is substantially
ionized by z = 12 or if spin temperature fluctuations are important, we show
that it will be difficult to place competitive constraints on cosmological
parameters with any of the considered methods.Comment: 20 pages, 12 figures, accepted by Ap
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