29 research outputs found
Gas and Stellar Motions and Observational Signatures of Co-Rotating Spiral Arms
We have observed a snapshot of our N-body/Smoothed Particle Hydrodynamics
simulation of a Milky Way-sized barred spiral galaxy in a similar way to how we
can observe the Milky Way. The simulated galaxy shows a co-rotating spiral arm,
i.e. the spiral arm rotates with the same speed as the circular speed. We
observed the rotation and radial velocities of the gas and stars as a function
of the distance from our assumed location of the observer at the three lines of
sight on the disc plane, (l, b) = (90, 0), (120, 0) and (150,0) deg. We find
that the stars tend to rotate slower (faster) behind (at the front of) the
spiral arm and move outward (inward), because of the radial migration. However,
because of their epicycle motion, we see a variation of rotation and radial
velocities around the spiral arm. On the other hand, the cold gas component
shows a clearer trend of rotating slower (faster) and moving outward (inward)
behind (at the front of) the spiral arm, because of the radial migration. We
have compared the results with the velocity of the maser sources from Reid et
al. (2014), and find that the observational data show a similar trend in the
rotation velocity around the expected position of the spiral arm at l = 120
deg. We also compared the distribution of the radial velocity from the local
standard of the rest, V_LSR, with the APOGEE data at l = 90 deg as an example.Comment: 10 pages, 7 figures, accepted for publication in MNRA
The stellar kinematics of co-rotating spiral arms in Gaia mock observations
We have observed an N-body/Smoothed Particle Hydrodynamics simulation of a
Milky Way like barred spiral galaxy. We present a simple method that samples
N-body model particles into mock Gaia stellar observations and takes into
account stellar populations, dust extinction and Gaia's science performance
estimates. We examine the kinematics around a nearby spiral arm at a similar
position to the Perseus arm at three lines of sight in the disc plane;
(l,b)=(90,0), (120,0) and (150,0) degrees. We find that the structure of the
peculiar kinematics around the co-rotating spiral arm, which is found in Kawata
et al. (2014b), is still visible in the observational data expected to be
produced by Gaia despite the dust extinction and expected observational errors
of Gaia. These observable kinematic signatures will enable testing whether the
Perseus arm of the Milky Way is similar to the co-rotating spiral arms commonly
seen in N-body simulations.Comment: 9 pages 4 Figures, submitted to MNRAS 22nd Dec 201
Radial Distribution of Stellar Motions in Gaia DR2
By taking advantage of the superb measurements of position and velocity for
an unprecedented large number of stars provided in Gaia DR2, we have generated
the first maps of the rotation velocity, , and vertical velocity,
, distributions as a function of the Galactocentric radius, , across a radial range of ~kpc. In the
map, we have identified many diagonal ridge features, which are compared with
the location of the spiral arms and the expected outer Lindblad resonance of
the Galactic bar. We have detected also radial wave-like oscillations of the
peak of the vertical velocity distribution.Comment: 5 pages, 3 figures, accepted for publication in MNRAS Lette
Spiral and bar driven peculiar velocities in Milky Way sized galaxy simulations
We investigate the kinematic signatures induced by spiral and bar structure
in a set of simulations of Milky Way-sized spiral disc galaxies. The set
includes test particle simulations that follow a quasi-stationary density
wave-like scenario with rigidly rotating spiral arms, and -body simulations
that host a bar and transient, co-rotating spiral arms. From a location similar
to that of the Sun, we calculate the radial, tangential and line-of-sight
peculiar velocity fields of a patch of the disc and quantify the fluctuations
by computing the power spectrum from a two-dimensional Fourier transform. We
find that the peculiar velocity power spectrum of the simulation with a bar and
transient, co-rotating spiral arms fits very well to that of APOGEE red clump
star data, while the quasi-stationary density wave spiral model without a bar
does not. We determine that the power spectrum is sensitive to the number of
spiral arms, spiral arm pitch angle and position with respect to the spiral
arm. However, it is necessary to go beyond the line of sight velocity field in
order to distinguish fully between the various spiral models with this method.
We compute the power spectrum for different regions of the spiral discs, and
discuss the application of this analysis technique to external galaxies.Comment: 14 pages, 11 figures. Improved and MNRAS Accepte
Compton Scattering of Fe K\alpha Lines from Accreting White Dwarfs
Compton scattering in the bulk accretion flow of the accretion column in
magnetic cataclysmic variables (mCVs) can significantly shift photon energies
in the X-ray emission lines resulting from accretion shocks. In particular,
Compton recoil can potentially broaden the 6.7 and 6.97 keV Fe K
emission lines produced in the post-shock region, and contaminate the
fluorescent 6.4 keV neutral Fe K line reflected off the white dwarf
surface. We present nonlinear Monte Carlo simulations demonstrating these
effects, and we discuss the interpretation of observed Fe K linewidths
in mCVs in light of these new results. The implications for other accreting
compact objects are also discussed.Comment: 7 pages, 3 figs, LaTeX, accepted by PAS
On the shear estimation bias induced by the spatial variation of colour across galaxy profiles
The spatial variation of the colour of a galaxy may introduce a bias in the
measurement of its shape if the PSF profile depends on wavelength. We study how
this bias depends on the properties of the PSF and the galaxies themselves. The
bias depends on the scales used to estimate the shape, which may be used to
optimise methods to reduce the bias. Here we develop a general approach to
quantify the bias. Although applicable to any weak lensing survey, we focus on
the implications for the ESA Euclid mission.
Based on our study of synthetic galaxies we find that the bias is a few times
10^-3 for a typical galaxy observed by Euclid. Consequently, it cannot be
neglected and needs to be accounted for. We demonstrate how one can do so using
spatially resolved observations of galaxies in two filters. We show that HST
observations in the F606W and F814W filters allow us to model and reduce the
bias by an order of magnitude, sufficient to meet Euclid's scientific
requirements. The precision of the correction is ultimately determined by the
number of galaxies for which spatially-resolved observations in at least two
filters are available. We use results from the Millennium Simulation to
demonstrate that archival HST data will be sufficient for the tomographic
cosmic shear analysis with the Euclid dataset.Comment: MNRAS submitted, 18 pages, 13 Figure
Demarcation of Sepsis-Induced Peripheral and Central Acidosis with pH (Low) Insertion Cycle Peptide
Acidosis is a key driver for many diseases, including cancer, sepsis, and stroke. The spatiotemporal dynamics of dysregulated pH across disease remain elusive, and current diagnostic strategies do not provide localization of pH alterations. We sought to explore if PET imaging using hydrophobic cyclic peptides that partition into the cellular membrane at low extracellular pH (denoted as pH [low] insertion cycles, or pHLIC) can permit accurate in vivo visualization of acidosis. Methods: Acid-sensitive cyclic peptide c[E4W5C] pHLIC was conjugated to bifunctional maleimide-NO2A and radiolabeled with 64Cu (half-life, 12.7 h). C57BL/6J mice were administered lipopolysaccharide (15 mg/kg) or saline (vehicle) and serially imaged with [64Cu]Cu-c[E4W5C] over 24 h. Ex vivo autoradiography was performed on resected brain slices and subsequently stained with cresyl violet to enable high-resolution spatial analysis of tracer accumulation. A nonâpH-sensitive cell-penetrating control peptide (c[R4W5C]) was used to confirm specificity of [64Cu]Cu-c[E4W5C]. CD11b (macrophage/microglia) and TMEM119 (microglia) immunostaining was performed to correlate extent of neuroinflammation with [64Cu]Cu-c[E4W5C] PET signal. Results: [64Cu]Cu-c[E4W5C] radiochemical yield and purity were more than 95% and more than 99%, respectively, with molar activity of more than 0.925 MBq/nmol. Significantly increased [64Cu]Cu-c[E4W5C] uptake was observed in lipopolysaccharide-treated mice (vs. vehicle) within peripheral tissues, including blood, lungs, liver, and small intestines (P \u3c 0.001â0.05). Additionally, there was significantly increased [64Cu]Cu-c[E4W5C] uptake in the brains of lipopolysaccharide-treated animals. Autoradiography confirmed increased uptake in the cerebellum, cortex, hippocampus, striatum, and hypothalamus of lipopolysaccharide-treated mice (vs. vehicle). Immunohistochemical analysis revealed microglial or macrophage infiltration, suggesting activation in brain regions containing increased tracer uptake. [64Cu]Cu-c[R4W5C] demonstrated significantly reduced uptake in the brain and periphery of lipopolysaccharide mice compared with the acid-mediated [64Cu]Cu-c[E4W5C] tracer. Conclusion: Here, we demonstrate that a pH-sensitive PET tracer specifically detects acidosis in regions associated with sepsis-driven proinflammatory responses. This study suggests that [64Cu]Cu-pHLIC is a valuable tool to noninvasively assess acidosis associated with both central and peripheral innate immune activation