9,558 research outputs found
Using Red Clump Stars to Decompose the Galactic Magnetic Field with Distance
A new method for measuring the large-scale structure of the Galactic magnetic
field is presented. The Galactic magnetic field has been probed through the
Galactic disk with near-infrared starlight polarimetry, however the distance to
each background star is unknown. Using red clump stars as near-infrared
standard candles, this work presents the first attempt to decompose the line of
sight structure of the sky-projected Galactic magnetic field. Two example
lines-of-sight are decomposed: toward a field with many red clump stars and
toward a field with few red clump stars. A continuous estimate of magnetic
field orientation over several kiloparsecs of distance is possible in the field
with many red clump stars, while only discrete estimates are possible in the
sparse example. toward the Outer Galaxy, there is a continuous field
orientation with distance that shows evidence of perturbation by the Galactic
warp. toward the Inner Galaxy, evidence for a large-scale change in the
magnetic field geometry is consistent with models of magnetic field reversals,
independently derived from Faraday rotation studies. A photo-polarimetric
method for identifying candidate intrinsically polarized stars is also
presented. The future application of this method to large regions of the sky
will begin the process of mapping the Galactic magnetic field in a way never
before possible.Comment: 11 pages, 8 figures, 2 tables, accepted for publication in The
Astronomical Journa
H II Region Driven Galactic Bubbles And Their Relationship To The Galactic Magnetic Field
The relative alignments of mid-infrared traced Galactic bubbles are compared to the orientation of the mean Galactic magnetic field in the disk. The orientations of bubbles in the northern Galactic plane were measured and are consistent with random orientations-no preferential alignment with respect to the Galactic disk was found. A subsample of H II region driven Galactic bubbles was identified, and as a single population they show random orientations. When this subsample was further divided into subthermal and suprathermal H II regions, based on hydrogen radio recombination linewidths, the subthermal H II regions showed a marginal deviation from random orientations, but the suprathermal H II regions showed significant alignment with the Galactic plane. The mean orientation of the Galactic disk magnetic field was characterized using new near-infrared starlight polarimetry and the suprathermal H II regions were found to preferentially align with the disk magnetic field. If suprathermal linewidths are associated with younger H II regions, then the evolution of young H II regions is significantly affected by the Galactic magnetic field. As H II regions age, they cease to be strongly linked to the Galactic magnetic field, as surrounding density variations come to dominate their morphological evolution. From the new observations, the ratios of magnetic-to-ram pressures in the expanding ionization fronts were estimated for younger H II regions.NSF AST 06-07500, 09-07790NASAW. M. Keck FoundationAstronom
On the Question of Effective Sample Size in Network Modeling: An Asymptotic Inquiry
The modeling and analysis of networks and network data has seen an explosion
of interest in recent years and represents an exciting direction for potential
growth in statistics. Despite the already substantial amount of work done in
this area to date by researchers from various disciplines, however, there
remain many questions of a decidedly foundational nature - natural analogues of
standard questions already posed and addressed in more classical areas of
statistics - that have yet to even be posed, much less addressed. Here we raise
and consider one such question in connection with network modeling.
Specifically, we ask, "Given an observed network, what is the sample size?"
Using simple, illustrative examples from the class of exponential random graph
models, we show that the answer to this question can very much depend on basic
properties of the networks expected under the model, as the number of vertices
in the network grows. In particular, adopting the (asymptotic) scaling of
the variance of the maximum likelihood parameter estimates as a notion of
effective sample size (), we show that when modeling the
overall propensity to have ties and the propensity to reciprocate ties, whether
the networks are sparse or not under the model (i.e., having a constant or an
increasing number of ties per vertex, respectively) is sufficient to yield an
order of magnitude difference in , from to
. In addition, we report simulation study results that suggest
similar properties for models for triadic (friend-of-a-friend) effects. We then
explore some practical implications of this result, using both simulation and
data on food-sharing from Lamalera, Indonesia.Comment: Published at http://dx.doi.org/10.1214/14-STS502 in the Statistical
Science (http://www.imstat.org/sts/) by the Institute of Mathematical
Statistics (http://www.imstat.org
A Lack of Resolved Near-Infrared Polarization Across the Face of M51
The galaxy M51 was observed using the Mimir instrument on the Perkins
telescope to constrain the resolved H-band (1.6 m) polarization across the
galaxy. These observations place an upper limit of on the -band
polarization across the face of M51, at 0.6 arcsecond pixel sampling. Even with
smoothing to coarser angular resolutions, to reduce polarization uncertainty,
the -band polarization remains undetected. The polarization upper limit at
-band, when combined with previous resolved optical polarimetry, rules out a
Serkowski-like polarization dependence on wavelength. Other polarization
mechanisms cannot account for the observed polarization ratio () across the face of M51.Comment: 4 pages, 2 figures, Accepted for publication in ApJ
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