10,860 research outputs found
Manifestations of the Galactic Center Magnetic Field
Several independent lines of evidence reveal that a relatively strong and
highly ordered magnetic field is present throughout the Galaxy's central
molecular zone (CMZ). The field within dense clouds of the central molecular
zone is predominantly parallel to the Galactic plane, probably as a result of
the strong tidal shear in that region. A second magnetic field system is
present outside of clouds, manifested primarily by a population of vertical,
synchrotron-emitting filamentary features aligned with the field. Whether or
not the strong vertical field is uniform throughout the CMZ remains
undetermined, but is a key central issue for the overall energetics and the
impact of the field on the Galactic center arena. The interactions between the
two field systems are considered, as they are likely to drive some of the
activity within the CMZ. As a proxy for other gas-rich galaxies in the local
group and beyond, the Galactic center region reveals that magnetic fields are
likely to be an important diagnostic, if not also a collimator, of the flow of
winds and energetic particles out of the nucleus.Comment: To appear in "LESSONS FROM THE LOCAL GROUP" - A Conference in Honour
of David Block and Bruce Elmegreen, eds: Freeman, K.C., Elmegreen, B.G.,
Block, D.L. & Woolway, M. (SPRINGER: NEW YORK
On "Sexual contacts and epidemic thresholds," models and inference for Sexual partnership distributions
Recent work has focused attention on statistical inference for the population
distribution of the number of sexual partners based on survey data.
The characteristics of these distributions are of interest as components of
mathematical models for the transmission dynamics of sexually-transmitted
diseases (STDs). Such information can be used both to calibrate theoretical
models, to make predictions for real populations, and as a tool for guiding
public health policy.
Our previous work on this subject has developed likelihood-based statistical
methods for inference that allow for low-dimensional, semi-parametric models.
Inference has been based on several proposed stochastic process models for the
formation of sexual partnership networks. We have also developed model
selection criteria to choose between competing models, and assessed the fit of
different models to three populations: Uganda, Sweden, and the USA. Throughout
this work, we have emphasized the correct assessment of the uncertainty of the
estimates based on the data analyzed. We have also widened the question of
interest to the limitations of inferences from such data, and the utility of
degree-based epidemiological models more generally.
In this paper we address further statistical issues that are important in
this area, and a number of confusions that have arisen in interpreting our
work. In particular, we consider the use of cumulative lifetime partner
distributions, heaping and other issues raised by Liljeros et al. in a recent
working paper.Comment: 22 pages, 5 figures in linked working pape
Discovery of hot supergiant stars near the Galactic center
We report new results of a campaign to find Wolf-Rayet and O (WR/O) stars and
high-mass X-ray binaries (HMXBs) in the Galactic center. We searched for
candidates by cross-correlating the 2MASS catalog with a deep Chandra catalog
of X-ray point sources in the Radio Arches region. Following up with K-band
spectroscopy, we found two massive stellar counterparts to CXOGC
J174555.3-285126 and CXOGC J174617.0-285131, which we classify as a broad-lined
WR star of sub-type WN6b and an O Ia supergiant, respectively. Their X-ray
properties are most consistent with those of known colliding-wind binaries in
the Galaxy and the Large Magellanic Cloud, although a scenario involving
low-rate accretion onto a compact object is also possible. The O Ia star lies
4.4 pc in projection from the Quintuplet cluster, and has a radial velocity
consistent with that of the Quintuplet, suggesting that this star might have
escaped from the cluster. We also present the discovery of a B2 Ia supergiant,
which we identified as a candidate massive star using 8 micron Spitzer maps of
the Galactic center in a region near the known massive X-ray-emitting star
CXOGC J174516.1-290315. We discuss the origin of these stars in the context of
evolving stellar clusters in the Galactic center.Comment: 21 pages, 5 figures, accepted for publication in the Astrophysical
Journa
Evidence for A Parsec-scale Jet from The Galactic Center Black Hole: Interaction with Local Gas
Despite strong physical reasons that they should exist and decades of search,
jets from the Galactic Center Black Hole, Sgr A*, have not yet been
convincingly detected. Based on high-resolution Very Large Array images and
ultra-deep imaging-spectroscopic data produced by the Chandra X-ray
Observatory, we report new evidence for the existence of a parsec-scale jet
from Sgr A*, by associating a linear feature G359.944-0.052, previously
identified in X-ray images of the Galactic Center, with a radio shock front on
the Eastern Arm of the Sgr A West HII region. We show that the shock front can
be explained in terms of the impact of a jet having a sharp momentum peak along
the Galaxy's rotation axis, whereas G359.944-0.052, a quasi-steady feature with
a power-law spectrum, can be understood as synchrotron radiation from
shock-induced ultrarelativistic electrons cooling in a finite post-shock region
downstream along the jet path. Several interesting implications of the jet
properties are discussed.Comment: 33 pages, 7 figures; Accepted for publication in The Astrophysical
Journa
Uniform Silicon Isotope Ratios Across the Milky Way Galaxy
We report the relative abundances of the three stable isotopes of silicon,
Si, Si and Si, across the Galaxy using the transition of silicon monoxide. The chosen sources represent a range in
Galactocentric radii () from 0 to 9.8 kpc. The high spectral
resolution and sensitivity afforded by the GBT permit isotope ratios to be
corrected for optical depths. The optical-depth-corrected data indicate that
the secondary-to-primary silicon isotope ratios
and vary much less than predicted on the basis of
other stable isotope ratio gradients across the Galaxy. Indeed, there is no
detectable variation in Si isotope ratios with . This lack of an
isotope ratio gradient stands in stark contrast to the monotonically decreasing
trend with exhibited by published secondary-to-primary oxygen
isotope ratios. These results, when considered in the context of the
expectations for chemical evolution, suggest that the reported oxygen isotope
ratio trends, and perhaps that for carbon as well, require further
investigation. The methods developed in this study for SiO isotopologue ratio
measurements are equally applicable to Galactic oxygen, carbon and nitrogen
isotope ratio measurements, and should prove useful for future observations of
these isotope systems.Comment: 18 pages, 12 figures, 2 tables. Published in The Astrophysical
Journal, Volume 839, Issue
A Nonthermal Radio Filament Connected to the Galactic Black Hole?
Using the Very Large Array, we have investigated a non-thermal radio filament
(NTF) recently found very near the Galactic black hole and its radio
counterpart, SgrA*. While this NTF -- the Sgr A West Filament (SgrAWF) --
shares many characteristics with the population of NTFs occupying the central
few hundred parsecs of the Galaxy, the SgrAWF has the distinction of having an
orientation and sky location that suggest an intimate physical connection to
SgrA*. We present 3.3 and 5.5 cm images constructed using an innovative
methodology that yields a very high dynamic range, providing an unprecedentedly
clear picture of the SgrAWF. While the physical association of the SgrAWF with
SgrA* is not unambiguous, the images decidedly evoke this interesting
possibility. Assuming that the SgrAWF bears a physical relationship to SgrA*,
we examine the potential implications. One is that SgrA* is a source of
relativistic particles constrained to diffuse along ordered local field lines.
The relativistic particles could also be fed into the local field by a
collimated outflow from SgrA*, perhaps driven by the Poynting flux accompanying
the black hole spin in the presence of a magnetic field threading the event
horizon. Second, we consider the possibility that the SgrAWF is the
manifestation of a low-mass-density cosmic string that has become anchored to
the black hole. The simplest form of these hypotheses would predict that the
filament be bi-directional, whereas the SgrAWF is only seen on one side of
SgrA*, perhaps because of the dynamics of the local medium.Comment: 9 pages, 4 figures, accepted for ApJ Letter
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