825 research outputs found
Diverse metallicities of Fermi bubble clouds indicate dual origins in the disk and halo
The Galactic Center is surrounded by two giant plasma lobes known as the
Fermi Bubbles, extending ~10 kpc both above and below the Galactic plane.
Spectroscopic observations of Fermi Bubble directions at radio, ultraviolet,
and optical wavelengths have detected multi-phase gas clouds thought to be
embedded within the bubbles referred to as Fermi Bubble high-velocity clouds
(FB HVCs). While these clouds have kinematics that can be modeled by a
biconical nuclear wind launched from the Galactic center, their exact origin is
unknown because, until now, there has been little information on their
heavy-metal abundance (metallicity). Here we show that FB HVCs have a wide
range of metallicities from <20% solar to ~320% solar. This result is based on
the first metallicity survey of FB HVCs. These metallicities challenge the
previously accepted tenet that all FB HVCs are launched from the Galactic
center into the Fermi Bubbles with solar or super-solar metallicities. Instead,
we suggest that FB HVCs originate in both the Milky Way's disk and halo. As
such, some of these clouds may characterize circumgalactic medium that the
Fermi Bubbles expand into, rather than material carried outward by the nuclear
wind, changing the canonical picture of FB HVCs. More broadly, these results
reveal that nuclear outflows from spiral galaxies can operate by sweeping up
gas in their halos while simultaneously removing gas from their disks.Comment: This version of the article has been accepted for publication on
Nature Astronomy after peer review. This version is not the Version of Record
(https://doi.org/10.1038/s41550-022-01720-0) and does not reflect
post-acceptance improvements, or any correction
Axions In String Theory
In the context of string theory, axions appear to provide the most plausible
solution of the strong CP problem. However, as has been known for a long time,
in many string-based models, the axion coupling parameter F_a is several orders
of magnitude higher than the standard cosmological bounds. We re-examine this
problem in a variety of models, showing that F_a is close to the GUT scale or
above in many models that have GUT-like phenomenology, as well as some that do
not. On the other hand, in some models with Standard Model gauge fields
supported on vanishing cycles, it is possible for F_a to be well below the GUT
scale.Comment: 62 pages, v2; references, acknowledgements and minor corrections
adde
MAPPING THE NUCLEAR OUTFLOW OF THE MILKY WAY: STUDYING THE KINEMATICS AND SPATIAL EXTENT OF THE NORTHERN FERMI BUBBLE
We report new observations from a systematic, spectroscopic, ultraviolet
absorption-line survey that maps the spatial and kinematic properties of the
high-velocity gas in the Galactic Center region. We examine the hypothesis that
this gas traces the biconical nuclear outflow. We use ultraviolet spectra of 47
background QSOs and halo stars projected inside and outside the northern Fermi
Bubble from the Hubble Space Telescope to study the incidence of high velocity
absorption around it. We use five lines of sight inside the northern Fermi
Bubble to constrain the velocity and column densities of outflowing gas traced
by O I, Al II, C II, C IV, Si II, Si III, Si IV and other species. All five
lines of sight inside the northern Fermi Bubble exhibit blueshifted high
velocity absorption components, whereas only 9 out of the 42 lines of sight
outside the northern Fermi Bubble exhibit blueshifted high velocity absorption
components. The observed outflow velocity profile decreases with Galactic
latitude and radial distance (R) from the Galactic Center. The observed
blueshifted velocities change from =-265 km/s at R~2.3 kpc to
=-91 km/s at R~6.5 kpc. We derive the metallicity of the entrained gas
along the 1H1613-097 sightline, which passes through the center of the northern
Fermi Bubble, finding [O/H] . A simple kinematic model
tuned to match the observed absorption component velocities along the five
lines of sight inside the Bubble, constrains the outflow velocities to
~10001300 km/s, and the age of the outflow to be ~ 69 Myr. We estimate a
minimum mass outflow rate for the nuclear outflow to be 0.2 . Combining the age and mass outflow rates, we determine a
minimum mass of total UV absorbing cool gas entrained in the Fermi Bubbles to
be .Comment: 24 pages, 9 9 figures, Accepted for Publication in Ap
Detectors for the James Webb Space Telescope Near-Infrared Spectrograph I: Readout Mode, Noise Model, and Calibration Considerations
We describe how the James Webb Space Telescope (JWST) Near-Infrared
Spectrograph's (NIRSpec's) detectors will be read out, and present a model of
how noise scales with the number of multiple non-destructive reads
sampling-up-the-ramp. We believe that this noise model, which is validated
using real and simulated test data, is applicable to most astronomical
near-infrared instruments. We describe some non-ideal behaviors that have been
observed in engineering grade NIRSpec detectors, and demonstrate that they are
unlikely to affect NIRSpec sensitivity, operations, or calibration. These
include a HAWAII-2RG reset anomaly and random telegraph noise (RTN). Using real
test data, we show that the reset anomaly is: (1) very nearly noiseless and (2)
can be easily calibrated out. Likewise, we show that large-amplitude RTN
affects only a small and fixed population of pixels. It can therefore be
tracked using standard pixel operability maps.Comment: 55 pages, 10 figure
The Atacama Cosmology Telescope: Two-Season ACTPol Lensing Power Spectrum
We report a measurement of the power spectrum of cosmic microwave background
(CMB) lensing from two seasons of Atacama Cosmology Telescope Polarimeter
(ACTPol) CMB data. The CMB lensing power spectrum is extracted from both
temperature and polarization data using quadratic estimators. We obtain results
that are consistent with the expectation from the best-fit Planck LCDM model
over a range of multipoles L=80-2100, with an amplitude of lensing A_lens =
1.06 +/- 0.15 (stat.) +/- 0.06 (sys.) relative to Planck. Our measurement of
the CMB lensing power spectrum gives sigma_8 Omega_m^0.25 = 0.643 +/- 0.054;
including baryon acoustic oscillation scale data, we constrain the amplitude of
density fluctuations to be sigma_8 = 0.831 +/- 0.053. We also update
constraints on the neutrino mass sum. We verify our lensing measurement with a
number of null tests and systematic checks, finding no evidence of significant
systematic errors. This measurement relies on a small fraction of the ACTPol
data already taken; more precise lensing results can therefore be expected from
the full ACTPol dataset.Comment: 17 pages, 11 figures, to be submitted to Physical Review
New interpretation of variational principles for gauge theories. I. Cyclic coordinate alternative to ADM split
I show how there is an ambiguity in how one treats auxiliary variables in
gauge theories including general relativity cast as 3 + 1 geometrodynamics.
Auxiliary variables may be treated pre-variationally as multiplier coordinates
or as the velocities corresponding to cyclic coordinates. The latter treatment
works through the physical meaninglessness of auxiliary variables' values
applying also to the end points (or end spatial hypersurfaces) of the
variation, so that these are free rather than fixed. [This is also known as
variation with natural boundary conditions.] Further principles of dynamics
workings such as Routhian reduction and the Dirac procedure are shown to have
parallel counterparts for this new formalism. One advantage of the new scheme
is that the corresponding actions are more manifestly relational. While the
electric potential is usually regarded as a multiplier coordinate and Arnowitt,
Deser and Misner have regarded the lapse and shift likewise, this paper's
scheme considers new {\it flux}, {\it instant} and {\it grid} variables whose
corresponding velocities are, respectively, the abovementioned previously used
variables. This paper's way of thinking about gauge theory furthermore admits
interesting generalizations, which shall be provided in a second paper.Comment: 11 page
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