33 research outputs found
Impact of axisymmetric mass models for dwarf spheroidal galaxies on indirect dark matter searches
Dwarf spheroidals are low-luminosity satellite galaxies of the Milky Way
highly dominated by dark matter (DM). Therefore, they are prime targets to
search for signals from dark matter annihilation using gamma-ray observations.
While the typical assumption is that the dark matter density profile of these
satellite galaxies can be described by a spherical symmetric
Navarro-Frenk-White (NFW) profile, recent observational data of stellar
kinematics suggest that the DM halos around these galaxies are better described
by axisymmetric profiles. Motivated by such evidence, we analyse about seven
years of PASS8 Fermi data for seven classical dwarf galaxies, including Draco,
adopting both the widely used NFW profile and observationally-motivated
axisymmetric density profiles. For four of the selected dwarfs (Sextans,
Carina, Sculptor and Fornax) axisymmetric mass models suggest a cored density
profile rather than the commonly adopted cusped profile. We found that upper
limits on the annihilation cross section for some of these dwarfs are
significantly higher than the ones achieved using an NFW profile. Therefore,
upper limits in the literature obtained using spherical symmetric cusped
profiles, such as the NFW, might be overestimated. Our results show that it is
extremely important to use observationally motivated density profiles going
beyond the usually adopted NFW in order to obtain accurate constraints on the
dark matter annihilation cross section.Comment: 9 pages, 5 figure
Cosmological prior for the -factor estimation of dwarf spheroidal galaxies
Dark matter halos of dwarf spheroidal galaxies (dSphs) play important roles
in dark matter detection. Generally we estimate the halo profile using a
kinematical equation of dSphs but the halo profile has a large uncertainty
because we have only a limited number of kinematical dataset. In this paper, we
utilize cosmological models of dark matter subhalos to obtain better
constraints on halo profile of dSphs. The constraints are realized as two
cosmological priors: satellite prior, based on a semi-analytic model of the
accretion history of subhalos and their tidal stripping effect, and
stellar-to-halo mass relation prior, which estimates halo mass of a galaxy from
its stellar mass using empirical correlations. In addition, we adopt a radial
dependent likelihood function by considering velocity dispersion profile, which
allows us to mitigate the parameter degeneracy in the previous analysis using a
radial independent likelihood function with averaged dispersion. Using these
priors, we estimate the squared dark matter density integrated over the
region-of-interest (so-called -factor) of 8 classical and 27 ultra-faint
dSphs. Our method significantly decreases the uncertainty of -factors (upto
about ) compared to the previous radial independent analysis. We confirm
the model dependence of -factor estimates by evaluating Bayes factors of
different model setups and find that the estimates are still stable even when
assuming different cosmological models.Comment: 31 pages, 9 figure
Diel vertical migration of Daphnia in Lake Kizaki : Difference in its pattern depending on the daphnid's body size
The pattern of diel vertical migration (DVM) of Daphnia galeata was analyzed in summer in Lake Kizaki (a small mesotrophic lake) with maximum depth of 29.5 m. Juveniles, small adults and medium-sized adults stayed the whole day at 4-8, 8 and 14 m depths, respectively, and did not perform DVM, while large adults showed DVM between 14 m and near the bottom layer (26-27.5 m). Smaller adults had larger clutch sizes relative to their body size and allocated more energy to reproduction than larger adults. They might have compensated for loss due to predation by producing abundant offspring in the epilimnion where the food level was higher. On the other hand, the larger adults performed DVM to reduce mortality due to predation at the cost of reduced reproductivity induced by environmental factors in the hypolimnion (low concentrations of food and oxygen). The study thus demonstrated that even a single population of Daphnia can show different DVM patterns depending on body size (life stage).Article信州大学理学部紀要 32(2): 77-88(1998)departmental bulletin pape
Constraining dark matter annihilation with HSC Low Surface Brightness Galaxies
Searches for dark matter annihilation signals have been carried out in a
number of target regions such as the Galactic Center and Milky Way dwarf
spheroidal galaxies (dSphs), among a few others. Here we propose low surface
brightness galaxies (LSBGs) asnovel targets for the indirect detection of dark
matter emission. In particular, LSBGs are known to have very large dark matter
contents and be less contaminated by extragalactic gamma-ray sources (e.g.,
blazars) compared to star forming galaxies. We report on an analysis that uses
eight LSBGs (detected by Subaru Hyper Suprime-Cam survey data) with known
redshifts to conduct a search for gamma-ray emission at the positions of these
new objects in Fermi Large Area Telescope data. We found no excesses of
gamma-ray emission and set constraints on the dark matter annihilation
cross-section. We exclude (at the 95% C.L.) dark matter scenarios predicting a
cross-section higher than 10^-23[cm^3/s] for dark matter particles of mass 10
GeV self-annihilating in the b_b channel. Although this constraint is weaker
than the ones reported in recent studies using other targets, we note that in
the near future, the number of detections of new LSBGs will increase by a few
orders of magnitude. We forecast that with the use of the full catalog of
soon-to-be-detected LSBGs the constraint will reach cross-section sensitivities
of ~ 3*10^-25 [cm^3/s] for dark matter particles with masses less than 10 GeV.Comment: 19 pages, 6 figures, accepted to JCA
The Quiescent Intracluster Medium in the Core of the Perseus Cluster
Clusters of galaxies are the most massive gravitationally-bound objects in
the Universe and are still forming. They are thus important probes of
cosmological parameters and a host of astrophysical processes. Knowledge of the
dynamics of the pervasive hot gas, which dominates in mass over stars in a
cluster, is a crucial missing ingredient. It can enable new insights into
mechanical energy injection by the central supermassive black hole and the use
of hydrostatic equilibrium for the determination of cluster masses. X-rays from
the core of the Perseus cluster are emitted by the 50 million K diffuse hot
plasma filling its gravitational potential well. The Active Galactic Nucleus of
the central galaxy NGC1275 is pumping jetted energy into the surrounding
intracluster medium, creating buoyant bubbles filled with relativistic plasma.
These likely induce motions in the intracluster medium and heat the inner gas
preventing runaway radiative cooling; a process known as Active Galactic
Nucleus Feedback. Here we report on Hitomi X-ray observations of the Perseus
cluster core, which reveal a remarkably quiescent atmosphere where the gas has
a line-of-sight velocity dispersion of 164+/-10 km/s in a region 30-60 kpc from
the central nucleus. A gradient in the line-of-sight velocity of 150+/-70 km/s
is found across the 60 kpc image of the cluster core. Turbulent pressure
support in the gas is 4% or less of the thermodynamic pressure, with large
scale shear at most doubling that estimate. We infer that total cluster masses
determined from hydrostatic equilibrium in the central regions need little
correction for turbulent pressure.Comment: 31 pages, 11 Figs, published in Nature July