61,712 research outputs found
The Dark Disk of the Milky Way
Massive satellite accretions onto early galactic disks can lead to the
deposition of dark matter in disk-like configurations that co-rotate with the
galaxy. This phenomenon has potentially dramatic consequences for dark matter
detection experiments. We utilize focused, high-resolution simulations of
accretion events onto disks designed to be Galaxy analogues, and compare the
resultant disks to the morphological and kinematic properties of the Milky
Way's thick disk in order to bracket the range of co-rotating accreted dark
matter. We find that the Milky Way's merger history must have been unusually
quiescent compared to median LCDM expectations and therefore its dark disk must
be relatively small: the fraction of accreted dark disk material near the Sun
is about 20% of the host halo density or smaller and the co-rotating dark
matter fraction near the Sun, defined as particles moving with a rotational
velocity lag less than 50 km/s, is enhanced by about 30% or less compared to a
standard halo model. Such a dark disk could contribute dominantly to the low
energy (of order keV for a dark matter particle with mass 100 GeV) nuclear
recoil event rate of direct dectection experiments, but it will not change the
likelihood of detection significantly. These dark disks provide testable
predictions of weakly-interacting massive particle dark matter models and
should be considered in detailed comparisons to experimental data. Our findings
suggest that the dark disk of the Milky Way may provide a detectable signal for
indirect detection experiments, contributing up to about 25% of the dark matter
self-annihilation signal in the direction of the center of the Galaxy, lending
the signal a noticeably oblate morphology.Comment: 11 pages, 6 figures, 1 table; submitted to Ap
Low mass X-ray binaries as a stellar mass indicator for the host galaxy
Using results of Chandra observations of old stellar systems in eleven nearby
galaxies of various morphological types and the census of LMXBs in the Milky
Way, we study the population of low mass X-ray binaries and their relation to
the mass of the host galaxy. We show that the azimuthally averaged spatial
distribution of the number of LMXBs and, in the majority of cases, of their
collective luminosity closely follows that of the near-infrared light.
Considering galaxies as a whole, we find that in a broad mass range,
log(M)~9-11.5, the total number of LMXBs and their combined luminosity are
proportional to the stellar mass of the host galaxy. Within the accuracy of the
light-to-mass conversion, we cannot rule out the possibility of a weak
dependence of the X/M ratio on morphological type. However, the effect of such
a dependence, if any, does not exceed a factor of ~1.5-2.
The luminosity distributions of LMXBs observed in different galaxies are
similar to each other and, with the possible exception of NGC1553, are
consistent with the average luminosity function derived from all data. The
average XLF of LMXBs in nearby galaxies has a complex shape and is
significantly different from that of HMXBs. It follows a power law with a
differential slope of ~1 at low luminosities, gradually steepens at
log(Lx)>37.0-37.5 and has a rather abrupt cut-off at log(Lx)~39.0-39.5. This
value of the cut-off luminosity is significantly, by an order of magnitude,
lower than found for high mass X-ray binaries.Comment: Accepted for publication in MNRA
Herschel Far-IR counterparts of SDSS galaxies: Analysis of commonly used Star Formation Rate estimates
We study a hundred of galaxies from the spectroscopic Sloan Digital Sky
Survey with individual detections in the Far-Infrared Herschel PACS bands (100
or 160 m) and in the GALEX Far-UltraViolet band up to z0.4 in the
COSMOS and Lockman Hole fields. The galaxies are divided into 4 spectral and 4
morphological types. For the star forming and unclassifiable galaxies we
calculate dust extinctions from the UV slope, the H/H ratio and
the ratio. There is a tight correlation between the
dust extinction and both and metallicity. We calculate
SFR and compare it with other SFR estimates (H, UV, SDSS)
finding a very good agreement between them with smaller dispersions than
typical SFR uncertainties. We study the effect of mass and metallicity, finding
that it is only significant at high masses for SFR. For the AGN and
composite galaxies we find a tight correlation between SFR and L
(0.29), while the dispersion in the SFR - L relation is
larger (0.57). The galaxies follow the prescriptions of the
Fundamental Plane in the M-Z-SFR space.Comment: 24 pages, 23 figures, accepted for publication in MNRA
Automatic active acoustic target detection in turbulent aquatic environments
This work is funded by the Environment and Food Security theme Ph.D. studentship from the University of Aberdeen, the Natural Environment Research Council (NERC) and Department for Environment, Food, and Rural Affairs (Defra grant NE/J004308/1), and the Marine Collaboration Research Forum (MarCRF). We would like to gratefully acknowledge the support from colleagues at Marine Scotland Science.Peer reviewedPublisher PD
Exploration of a 100 TeV gamma-ray northern sky using the Tibet air-shower array combined with an underground water-Cherenkov muon-detector array
Aiming to observe cosmic gamma rays in the 10 - 1000 TeV energy region, we
propose a 10000 m^2 underground water-Cherenkov muon-detector (MD) array that
operates in conjunction with the Tibet air-shower (AS) array. Significant
improvement is expected in the sensitivity of the Tibet AS array towards
celestial gamma-ray signals above 10 TeV by utilizing the fact that
gamma-ray-induced air showers contain far fewer muons compared with
cosmic-ray-induced ones. We carried out detailed Monte Carlo simulations to
assess the attainable sensitivity of the Tibet AS+MD array towards celestial
TeV gamma-ray signals. Based on the simulation results, the Tibet AS+MD array
will be able to reject 99.99% of background events at 100 TeV, with 83% of
gamma-ray events remaining. The sensitivity of the Tibet AS+MD array will be
~20 times better than that of the present Tibet AS array around 20 - 100 TeV.
The Tibet AS+MD array will measure the directions of the celestial TeV
gamma-ray sources and the cutoffs of their energy spectra. Furthermore, the
Tibet AS+MD array, along with imaging atmospheric Cherenkov telescopes as well
as the Fermi Gamma-ray Space Telescope and X-ray satellites such as Suzaku and
MAXI, will make multiwavelength observations and conduct morphological studies
on sources in the quest for evidence of the hadronic nature of the cosmic-ray
acceleration mechanism.Comment: Accepted by Astroparticle Physic
Box- and peanut-shaped bulges. III. A new class of bulges: Thick Boxy Bulges
Inspecting all 1224 edge-on disk galaxies larger than 2' in the RC3 on DSS
images we have found several galaxies with extraordinary bulges meeting two
criteria: They are box shaped and large in respect to the diameters of their
galaxies. These bulges are often disturbed, show frequently prominent
irregularities and asymmetries, and some possess possible merger remnants or
merging satellites. For these bulges we have introduced the term "Thick Boxy
Bulges" (TBBs). About 2% of all disk galaxies (S0-Sd), respectively 4% of all
galaxies with box- and peanut-shaped (b/p) bulges, belong to this class of
galaxies. Using multicolour CCD and NIR data we have enlarged and followed up
our sample of nearly 20 galaxies with a TBB. The disturbed morphology of a
large fraction of these galaxies shows that many of the TBB galaxies are not
dynamically settled. For the TBBs the extent of the box shape seems to be too
large to result from a normal bar potential. Therefore we conclude that two
classes of b/p bulges exist with different origins. While most (~96%) b/p
bulges can be explained by bars alone, the extended boxy structures of TBBs
result most likely from accreted material by infalling satellite companions
(soft merging).Comment: LaTeX, 14 pages, 19 figures, accepted to be published in A&A. Figs.
5, 6, 7, 8, 9, 12 and 14 are available as jpg-file
Studying the properties of galaxy cluster morphology estimators
X-ray observations of galaxy clusters reveal a large range of morphologies
with various degrees of disturbance, showing that the assumptions of
hydrostatic equilibrium and spherical shape which are used to determine the
cluster mass from X-ray data are not always satisfied. It is therefore
important for the understanding of cluster properties as well as for
cosmological applications to detect and quantify substructure in X-ray images
of galaxy clusters. Two promising methods to do so are power ratios and center
shifts. Since these estimators can be heavily affected by Poisson noise and
X-ray background, we performed an extensive analysis of their statistical
properties using a large sample of simulated X-ray observations of clusters
from hydrodynamical simulations. We quantify the measurement bias and error in
detail and give ranges where morphological analysis is feasible. A new,
computationally fast method to correct for the Poisson bias and the X-ray
background contribution in power ratio and center shift measurements is
presented and tested for typical XMM-Newton observational data sets. We studied
the morphology of 121 simulated cluster images and establish structure
boundaries to divide samples into relaxed, mildly disturbed and disturbed
clusters. In addition, we present a new morphology estimator - the peak of the
0.3-1 r500 P3/P0 profile to better identify merging clusters. The analysis
methods were applied to a sample of 80 galaxy clusters observed with
XMM-Newton. We give structure parameters (P3/P0 in r500, w and P3/P0_max) for
all 80 observed clusters. Using our definition of the P3/P0 (w) substructure
boundary, we find 41% (47%) of our observed clusters to be disturbed.Comment: Replaced to match version published in A&A, Eq. 1 correcte
Bimodal morphologies of massive galaxies at the core of a protocluster at z=3.09 and the strong size growth of a brightest cluster galaxy
We present the near-infrared high resolution imaging of an extremely dense
group of galaxies at the core of the protocluster at in the SSA22
field by using the adaptive optics AO188 and the Infrared Camera and
Spectrograph (IRCS) on Subaru Telescope. Wide morphological variety of them
suggests their on-going dramatic evolutions. One of the two quiescent galaxies
(QGs), the most massive one in the group, is a compact elliptical with an
effective radius kpc. It supports the two-phase formation
scenario of giant ellipticals today that a massive compact elliptical is formed
at once and evolves in the size and stellar mass by series of mergers. Since
this object is a plausible progenitor of a brightest cluster galaxy (BCG) of
one of the most massive clusters today, it requires strong size (\ga10) and
stellar mass ( four times by ) growths. Another QG hosts an AGN(s)
and is fitted with a model composed from an nuclear component and S\'ersic
model. It shows spatially extended [O{\footnotesize III}]5007 emission
line compared to the continuum emission, a plausible evidence of outflows.
Massive star forming galaxies (SFGs) in the group are two to three times larger
than the field SFGs at similar redshift. Although we obtained the -band
image deeper than the previous one, we found no candidate new members. This
implies a physical deficiency of low mass galaxies with stellar mass
M_{\star}\la4\times10^{10}~M_{\odot} and/or poor detection completeness of
them owing to their diffuse morphologies.Comment: 18 pages, 14 figures, MNRAS accepte
- âŠ