3,350 research outputs found
Sterile neutrinos as subdominant warm dark matter
In light of recent findings which seem to disfavor a scenario with (warm)
dark matter entirely constituted of sterile neutrinos produced via the
Dodelson-Widrow (DW) mechanism, we investigate the constraints attainable for
this mechanism by relaxing the usual hypothesis that the relic neutrino
abundance must necessarily account for all of the dark matter. We first study
how to reinterpret the limits attainable from X-ray non-detection and
Lyman-alpha forest measurements in the case that sterile neutrinos constitute
only a fraction fs of the total amount of dark matter. Then, assuming that
sterile neutrinos are generated in the early universe solely through the DW
mechanism, we show how the X-ray and Lyman-alpha results jointly constrain the
mass-mixing parameters governing their production. Furthermore, we show how the
same data allow us to set a robust upper limit fs < 0.7 at the 2 sigma level,
rejecting the case of dominant dark matter (fs = 1) at the ~ 3 sigma level.Comment: Minor changes; added references; version accepted for publication in
Phys. Rev.
Linking dwarf galaxies to halo building blocks with the most metal-poor star in Sculptor
Current cosmological models indicate that the Milky Way's stellar halo was
assembled from many smaller systems. Based on the apparent absence of the most
metal-poor stars in present-day dwarf galaxies, recent studies claimed that the
true Galactic building blocks must have been vastly different from the
surviving dwarfs. The discovery of an extremely iron-poor star (S1020549) in
the Sculptor dwarf galaxy based on a medium-resolution spectrum cast some doubt
on this conclusion. However, verification of the iron-deficiency and
measurements of additional elements, such as the alpha-element Mg, are
mandatory for demonstrating that the same type of stars produced the metals
found in dwarf galaxies and the Galactic halo. Only then can dwarf galaxy stars
be conclusively linked to early stellar halo assembly. Here we report
high-resolution spectroscopic abundances for 11 elements in S1020549,
confirming the iron abundance of less than 1/4000th that of the Sun, and
showing that the overall abundance pattern mirrors that seen in low-metallicity
halo stars, including the alpha-elements. Such chemical similarity indicates
that the systems destroyed to form the halo billions of years ago were not
fundamentally different from the progenitors of present-day dwarfs, and
suggests that the early chemical enrichment of all galaxies may be nearly
identical.Comment: 16 pages, including 2 figures. Accepted for publication in Nature. It
is embargoed for discussion in the press until formal publication in Natur
Please, don't do it! Fifteen years of progress of non-invasive brain stimulation in action inhibition
open3noThe ability to inhibit prepotent responses is critical for survival. Action inhibition can be investigated using a stop-signal task (SST), designed to provide a reliable measure of the time taken by the brain to suppress motor responses. Here we review the major research advances using the combination of this paradigm with the use of non-invasive brain stimulation techniques in the last fifteen years. We highlight new methodological approaches to understanding and exploiting several processes underlying action control, which is critically impaired in several psychiatric disorders. In this review we present and discuss existing literature demonstrating i) the importance of the use of non-invasive brain stimulation in studying human action inhibition, unveiling the neural network involved ii) the critical role of prefrontal areas, including the pre-supplementary motor area (pre-SMA) and the inferior frontal gyrus (IFG), in inhibitory control iii) the neural and behavioral evidence of proactive and reactive action inhibition. As the main result of this review, the specific literature demonstrated the crucial role of pre-SMA and IFG as evidenced from the field of noninvasive brain stimulation studies. Finally, we discuss the critical questions that remain unanswered about how such non-invasive brain stimulation protocols can be translated to therapeutic treatments.embargoed_20210915Borgomaneri S.; Serio G.; Battaglia S.Borgomaneri S.; Serio G.; Battaglia S
The stellar halo of the Galaxy
Stellar halos may hold some of the best preserved fossils of the formation
history of galaxies. They are a natural product of the merging processes that
probably take place during the assembly of a galaxy, and hence may well be the
most ubiquitous component of galaxies, independently of their Hubble type. This
review focuses on our current understanding of the spatial structure, the
kinematics and chemistry of halo stars in the Milky Way. In recent years, we
have experienced a change in paradigm thanks to the discovery of large amounts
of substructure, especially in the outer halo. I discuss the implications of
the currently available observational constraints and fold them into several
possible formation scenarios. Unraveling the formation of the Galactic halo
will be possible in the near future through a combination of large wide field
photometric and spectroscopic surveys, and especially in the era of Gaia.Comment: 46 pages, 16 figures. References updated and some minor changes.
Full-resolution version available at
http://www.astro.rug.nl/~ahelmi/stellar-halo-review.pd
Reduction in Phencyclidine Induced Sensorimotor Gating Deficits in the Rat Following Increased System Xc â Activity in the Medial Prefrontal Cortex
Rationale: Aspects of schizophrenia, including deficits in sensorimotor gating, have been linked to glutamate dysfunction and/or oxidative stress in the prefrontal cortex. System xc â, a cystineâglutamate antiporter, is a poorly understood mechanism that contributes to both cellular antioxidant capacity and glutamate homeostasis.
Objectives: Our goal was to determine whether increased system xc â activity within the prefrontal cortex would normalize a rodent measure of sensorimotor gating.
Methods: In situ hybridization was used to map messenger RNA (mRNA) expression of xCT, the active subunit of system xc â, in the prefrontal cortex. Prepulse inhibition was used to measure sensorimotor gating; deficits in prepulse inhibition were produced using phencyclidine (0.3â3 mg/kg, sc). N-Acetylcysteine (10â100 ÎŒM) and the system xc â inhibitor (S)-4-carboxyphenylglycine (CPG, 0.5 ÎŒM) were used to increase and decrease system xc â activity, respectively. The uptake of 14C-cystine into tissue punches obtained from the prefrontal cortex was used to assay system xc â activity.
Results: The expression of xCT mRNA in the prefrontal cortex was most prominent in a lateral band spanning primarily the prelimbic cortex. Although phencyclidine did not alter the uptake of 14C-cystine in prefrontal cortical tissue punches, intraprefrontal cortical infusion of N-acetylcysteine (10â100 ÎŒM) significantly reduced phencyclidine- (1.5 mg/kg, sc) induced deficits in prepulse inhibition. N-Acetylcysteine was without effect when coinfused with CPG (0.5 ÎŒM), indicating an involvement of system xc â.
Conclusions: These results indicate that phencyclidine disrupts sensorimotor gating through system xc â independent mechanisms, but that increasing cystineâglutamate exchange in the prefrontal cortex is sufficient to reduce behavioral deficits produced by phencyclidine
Cold gas accretion in galaxies
Evidence for the accretion of cold gas in galaxies has been rapidly
accumulating in the past years. HI observations of galaxies and their
environment have brought to light new facts and phenomena which are evidence of
ongoing or recent accretion:
1) A large number of galaxies are accompanied by gas-rich dwarfs or are
surrounded by HI cloud complexes, tails and filaments. It may be regarded as
direct evidence of cold gas accretion in the local universe. It is probably the
same kind of phenomenon of material infall as the stellar streams observed in
the halos of our galaxy and M31. 2) Considerable amounts of extra-planar HI
have been found in nearby spiral galaxies. While a large fraction of this gas
is produced by galactic fountains, it is likely that a part of it is of
extragalactic origin. 3) Spirals are known to have extended and warped outer
layers of HI. It is not clear how these have formed, and how and for how long
the warps can be sustained. Gas infall has been proposed as the origin. 4) The
majority of galactic disks are lopsided in their morphology as well as in their
kinematics. Also here recent accretion has been advocated as a possible cause.
In our view, accretion takes place both through the arrival and merging of
gas-rich satellites and through gas infall from the intergalactic medium (IGM).
The infall may have observable effects on the disk such as bursts of star
formation and lopsidedness. We infer a mean ``visible'' accretion rate of cold
gas in galaxies of at least 0.2 Msol/yr. In order to reach the accretion rates
needed to sustain the observed star formation (~1 Msol/yr), additional infall
of large amounts of gas from the IGM seems to be required.Comment: To appear in Astronomy & Astrophysics Reviews. 34 pages.
Full-resolution version available at
http://www.astron.nl/~oosterlo/accretionRevie
Exciting dark matter in the galactic center
We reconsider the proposal of excited dark matter (DM) as an explanation for
excess 511 keV gamma rays from positrons in the galactic center. We
quantitatively compute the cross section for DM annihilation to nearby excited
states, mediated by exchange of a new light gauge boson with off-diagonal
couplings to the DM states. In models where both excited states must be heavy
enough to decay into e^+ e^- and the ground state, the predicted rate of
positron production is never large enough to agree with observations, unless
one makes extreme assumptions about the local circular velocity in the Milky
Way, or alternatively if there exists a metastable population of DM states
which can be excited through a mass gap of less than 650 keV, before decaying
into electrons and positrons.Comment: Dedicated to the memory of Lev Kofman; 16 pages, 9 figures; v3 added
refs, minor changes, accepted to PR
Study of the Sextans dwarf spheroidal galaxy from the DART CaII triplet survey
We use VLT/FLAMES intermediate resolution (R~6500) spectra of individual red
giant branch stars in the near-infrared CaII triplet (CaT) region to
investigate the wide-area metallicity properties and internal kinematics of the
Sextans dwarf spheroidal galaxy (dSph). Our final sample consists of 174
probable members of Sextans with accurate line-of-sight velocities (+- 2 km/s)
and CaT [Fe/H] measurements (+- 0.2 dex). We use the MgI line at 8806.8 \AA\,
as an empirical discriminator for distinguishing between probable members of
the dSph (giant stars) and probable Galactic contaminants (dwarf stars).
Sextans shows a similar chemo-dynamical behaviour to other Milky Way dSphs,
with its central regions being more metal rich than the outer parts and with
the more metal-rich stars displaying colder kinematics than the more metal-poor
stars. Hints of a velocity gradient are found along the projected major axis
and along an axis at P.A.=191 deg, however a larger and more spatially extended
sample may be necessary to pin down the amplitude and direction of this
gradient. We detect a cold kinematic substructure at the centre of Sextans,
consistent with being the remnant of a disrupted very metal poor stellar
cluster. We derive the most extended line-of-sight velocity dispersion profile
for Sextans, out to a projected radius of 1.6 deg. From Jeans modelling of the
observed line-of-sight velocity dispersion profile we find that this is
consistent with both a cored dark matter halo with large core radius and cuspy
halo with low concentration. The mass within the last measured point is in the
range 2-4 x 10^8 M_sun, giving very large mass-to-light ratios, from 460 to 920
(M/L)_(V,sun).Comment: 30 pages, 17 figures; 4 tables. Accepted for publication in MNRAS.
Table 4 will appear as online material in the published version of the
manuscript. Typo correcte
Gamma rays from ultracompact primordial dark matter minihalos
Ultracompact minihalos have recently been proposed as a new class of dark
matter structure. These minihalos would be produced by phase transitions in the
early Universe or features in the inflaton potential, and constitute
non-baryonic massive compact halo objects (MACHOs) today. We examine the
prospect of detecting ultracompact minihalos in gamma-rays if dark matter
consists of self-annihilating particles. We compute present-day fluxes from
minihalos produced in the electron-positron annihilation epoch, and the QCD and
electroweak phase transitions in the early Universe. Even at a distance of 100
pc, minihalos produced during the electron-positron annihilation epoch should
be eminently detectable today, either by the Fermi satellite, current Air
Cherenkov telescopes, or even in archival EGRET data. Within ~1 pc, minihalos
formed in the QCD phase transition would have similar predicted fluxes to the
dwarf spheroidal galaxies targeted by current indirect dark matter searches, so
might also be detectable by present or upcoming experiments.Comment: 5 pages, 3 figures. Minor update to match published version of
erratu
Where to find a dark matter sterile neutrino?
We propose a strategy of how to look for dark matter (DM) particles
possessing a radiative decay channel and derive constraints on their parameters
from observations of X-rays from our own Galaxy and its dwarf satellites. When
applied to the sterile neutrinos in keV mass range, it allows a significant
improvement of restrictions to its parameters, as compared with previous works.Comment: 5 pp, revtex; v3: 1-sigma limits have been replaced by more
conservative 3-sigma limits, a picture illustrating the data analysis methods
has been ade
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