1,153 research outputs found
Dark Matter Search Using Chandra Observations of Willman 1, and a Spectral Feature Consistent with a Decay Line of a 5 keV Sterile Neutrino
We report the results of a search for an emission line from radiatively
decaying dark matter in the Chandra X-ray Observatory spectrum of the
ultra-faint dwarf spheroidal galaxy Willman 1. 99% confidence line flux upper
limits over the 0.4-7 keV Chandra bandpass are derived and mapped to an allowed
region in the sterile neutrino mass-mixing angle plane that is consistent with
recent constraints from Suzaku X-ray Observatory and Chandra observations of
the Ursa Minor and Draco dwarf spheroidals. A significant excess to the
continuum, detected by fitting the particle-background-subtracted source
spectrum, indicates the presence of a narrow emission feature with energy 2.51
+/- 0.07 (0.11) keV and flux [3.53 +/- 1.95 (2.77)] X 10^(-6) photons/cm^2/s at
68% (90%) confidence. Interpreting this as an emission line from sterile
neutrino radiative decay, we derive the corresponding allowed range of sterile
neutrino mass and mixing angle using two approaches. The first assumes that
dark matter is solely composed of sterile neutrinos, and the second relaxes
that requirement. The feature is consistent with the sterile neutrino mass of
5.0 +/- 0.2 keV and a mixing angle in a narrow range for which neutrino
oscillations can produce all of the dark matter and for which sterile neutrino
emission from the cooling neutron stars can explain pulsar kicks, thus
bolstering both the statistical and physical significance of our measurement.Comment: 34 pages, including 20 figures; accepted for publication in ApJ;
substantially expanded discussion session, results unchange
An abundance study of red-giant-branch stars in the Hercules dwarf spheroidal galaxy
Using high-resolution spectroscopy, we provide a determination of [Fe/H] and
[Ca/H] for confirmed red-giant branch member stars of the Hercules dwarf
spheroidal galaxy. Based on this we explore the ages of the prevailing stellar
populations in Hercules, and the enrichment history from supernovae.
Additionally, we provide a new simple metallicity calibration for Stromgren
photometry for metal-poor, red giant branch stars. We find that the red-giant
branch stars of the Hercules dSph galaxy are more metal-poor than estimated in
our previous study that was based on photometry alone. Additionally, we find an
abundance trend such that [Ca/Fe] is higher for more metal-poor stars, and
lower for more metal-rich stars, with a spread of about 0.8 dex. The [Ca/Fe]
trend suggests an early rapid chemical enrichment through supernovae of type
II, followed by a phase of slow star formation dominated by enrichment through
supernovae of type Ia. A comparison with isochrones indicates that the red
giants in Hercules are older than 10 Gyr.Comment: 12 pages, 11 figures. Accepted for publication in A&
A Dark Matter Hurricane: Measuring the S1 Stream with Dark Matter Detectors
The recently discovered S1 stream passes through the Solar neighbourhood on a
low inclination, counter-rotating orbit. The progenitor of S1 is a dwarf galaxy
with a total mass comparable to the present-day Fornax dwarf spheroidal, so the
stream is expected to have a significant DM component. We compute the effects
of the S1 stream on WIMP and axion detectors as a function of the density of
its unmeasured dark component. In WIMP detectors the S1 stream supplies more
high energy nuclear recoils so will marginally improve DM detection prospects.
We find that even if S1 comprises less than 10% of the local density, multi-ton
xenon WIMP detectors can distinguish the S1 stream from the bulk halo in the
relatively narrow mass range between 5 and 25 GeV. In directional WIMP
detectors such as CYGNUS, S1 increases DM detection prospects more
substantially since it enhances the anisotropy of the WIMP signal. Finally, we
show that axion haloscopes possess by far the greatest potential sensitivity to
the S1 stream. Once the axion mass has been discovered, the distinctive
velocity distribution of S1 can easily be extracted from the axion power
spectrum.Comment: 21 pages, 11 figure
WIMP searches with gamma rays in the Fermi era: challenges, methods and results
The launch of the gamma-ray telescope Fermi Large Area Telescope (Fermi-LAT)
started a pivotal period in indirect detection of dark matter. By outperforming
expectations, for the first time a robust and stringent test of the paradigm of
weakly interacting massive particles (WIMPs) is within reach. In this paper, we
discuss astrophysical targets for WIMP detection and the challenges they
present, review the analysis tools which have been employed to tackle these
challenges, and summarize the status of constraints on and the claimed
detections in the WIMP parameter space. Methods and results will be discussed
in comparison to Imaging Air Cherenkov Telescopes. We also provide an outlook
on short term and longer term developments.Comment: 72 pages, 7 figures, Invited review for Journal of Experimental and
Theoretical Physics,v3: added a few references, addressed referee comment
Next decade of sterile neutrino studies
We review the status of sterile neutrino dark matter and discuss
astrophysical and cosmological bounds on its properties as well as future
prospects for its experimental searches. We argue that if sterile neutrinos are
the dominant fraction of dark matter, detecting an astrophysical signal from
their decay (the so-called 'indirect detection') may be the only way to
identify these particles experimentally. However, it may be possible to check
the dark matter origin of the observed signal unambiguously using its
characteristic properties and/or using synergy with accelerator experiments,
searching for other sterile neutrinos, responsible for neutrino flavor
oscillations. We argue that to fully explore this possibility a dedicated
cosmic mission - an X-ray spectrometer - is needed.Comment: 23 pages, 6 figure
First Stellar Abundances in the Dwarf Irregular Galaxy Sextans A
We present the abundance analyses of three isolated A-type supergiant stars
in the dwarf irregular galaxy Sextans A from high-resolution spectra the UVES
spectrograph at the VLT. Detailed model atmosphere analyses have been used to
determine the stellar atmospheric parameters and the elemental abundances of
the stars. The mean iron group abundance was determined from these three stars
to be [(FeII,CrII)/H]=-0.99+/-0.04+/-0.06. This is the first determination of
the present-day iron group abundances in Sextans A. These three stars now
represent the most metal-poor massive stars for which detailed abundance
analyses have been carried out. The mean stellar alpha element abundance was
determined from the alpha element magnesium as
[alpha(MgI)/H]=-1.09+/-0.02+/-0.19. This is in excellent agreement with the
nebular alpha element abundances as determined from oxygen in the H II regions.
These results are consistent from star-to-star with no significant spatial
variations over a length of 0.8 kpc in Sextans A. This supports the nebular
abundance studies of dwarf irregular galaxies, where homogeneous oxygen
abundances are found throughout, and argues against in situ enrichment. The
alpha/Fe abundance ratio is [alpha(MgI)/FeII,CrII]=-0.11+/-0.02+/-0.10, which
is consistent with the solar ratio. This is consistent with the results from
A-supergiant analyses in other Local Group dwarf irregular galaxies but in
stark contrast with the high [alpha/Fe] results from metal-poor stars in the
Galaxy, and is most clearly seen from these three stars in Sextans A because of
their lower metallicities. The low [alpha/Fe] ratios are consistent with the
slow chemical evolution expected for dwarf galaxies from analyses of their
stellar populations.Comment: 40 pages, 8 figures, accepted for publication in A
Star formation histories of dwarf galaxies from the Colour-Magnitude diagrams of their resolved stellar populations
In this tutorial paper we summarize how the star formation (SF) history of a
galactic region can be derived from the colour-magnitude diagram (CMD) of its
resolved stars. The procedures to build synthetic CMDs and to exploit them to
derive the SF histories (SFHs) are described, as well as the corresponding
uncertainties. The SFHs of resolved dwarf galaxies of all morphological types,
obtained from the application of the synthetic CMD method, are reviewed and
discussed. In short: 1) Only early-type galaxies show evidence of long
interruptions in the SF activity; late-type dwarfs present rather continuous,
or gasping, SF regimes; 2) A few early-type dwarfs have experienced only one
episode of SF activity concentrated at the earliest epochs, whilst many others
show extended or recurrent SF activity; 3) No galaxy experiencing now its first
SF episode has been found yet; 4) No frequent evidence of strong SF bursts is
found; 5) There is no significant difference in the SFH of dwarf irregulars and
blue compact dwarfs, except for the current SF rates. Implications of these
results on the galaxy formation scenarios are briefly discussed.Comment: 29 pages. Tutorial Review to appear in the special issue
"Dwarf-Galaxy Cosmology" in Advances in Astronom
Sensitivity Projections for Dark Matter Searches with the Fermi Large Area Telescope
The nature of dark matter is a longstanding enigma of physics; it may consist
of particles beyond the Standard Model that are still elusive to experiments.
Among indirect search techniques, which look for stable products from the
annihilation or decay of dark matter particles, or from axions coupling to
high-energy photons, observations of the -ray sky have come to
prominence over the last few years, because of the excellent sensitivity of the
Large Area Telescope (LAT) on the Fermi Gamma-ray Space Telescope mission. The
LAT energy range from 20 MeV to above 300 GeV is particularly well suited for
searching for products of the interactions of dark matter particles. In this
report we describe methods used to search for evidence of dark matter with the
LAT, and review the status of searches performed with up to six years of LAT
data. We also discuss the factors that determine the sensitivities of these
searches, including the magnitudes of the signals and the relevant backgrounds,
considering both statistical and systematic uncertainties. We project the
expected sensitivities of each search method for 10 and 15 years of LAT data
taking. In particular, we find that the sensitivity of searches targeting dwarf
galaxies, which provide the best limits currently, will improve faster than the
square root of observing time. Current LAT limits for dwarf galaxies using six
years of data reach the thermal relic level for masses up to 120 GeV for the
annihilation channel for reasonable dark matter density profiles.
With projected discoveries of additional dwarfs, these limits could extend to
about 250 GeV. With as much as 15 years of LAT data these searches would be
sensitive to dark matter annihilations at the thermal relic cross section for
masses to greater than 400 GeV (200 GeV) in the ()
annihilation channels.Comment: Updated with a few additional and corrected references; otherwise,
text is identical to previous version. Submitted on behalf of the Fermi-LAT
collaboration. Accepted for publication in Physics Reports, 59 pages, 34
figures; corresponding author: Eric Charles ([email protected]
- …