10,005 research outputs found
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
The Herschel Virgo Cluster Survey. III. A constraint on dust grain lifetime in early-type galaxies
Passive early-type galaxies (ETGs) provide an ideal laboratory for studying the interplay between dust formation around evolved stars and its subsequent destruction in a hot gas. Using Spitzer-IRS and Herschel data we compare the dust production rate in the envelopes
of evolved AGB stars with a constraint on the total dust mass. Early-type galaxies which appear to be truly passively evolving are not detected by Herschel. We thus derive a distance independent upper limit to the dust grain survival time in the hostile environment of ETGs of <46 ± 25 Myr for amorphous silicate grains. This implies that ETGs which are detected at far-infrared wavelengths have
acquired a cool dusty medium via interaction. Given likely time-scales for ram-pressure stripping, this also implies that only galaxies with dust in a cool (atomic) medium can release dust into the intra-cluster medium
Essential countability of treeable equivalence relations
We establish a dichotomy theorem characterizing the circumstances under which
a treeable Borel equivalence relation E is essentially countable. Under
additional topological assumptions on the treeing, we in fact show that E is
essentially countable if and only if there is no continuous embedding of E1
into E. Our techniques also yield the first classical proof of the analogous
result for hypersmooth equivalence relations, and allow us to show that up to
continuous Kakutani embeddability, there is a minimum Borel function which is
not essentially countable-to-one
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
Inextendibility of spacetimes and Lorentzian length spaces
We study the low-regularity (in-)extendibility of spacetimes within the
synthetic-geometric framework of Lorentzian length spaces developed in [KS:17].
To this end, we introduce appropriate notions of geodesics and timelike
geodesic completeness and prove a general inextendibility result. Our results
shed new light on recent analytic work in this direction and, for the first
time, relate low-regularity inextendibility to (synthetic) curvature blow-up.Comment: 21 pages, 1 figure, made assumptions in 5.4 and 5.6 more explicit
(strong causality and local TL geodesic connectedness of extension
A Quantitative Non-radial Oscillation Model for the Subpulses in PSR B0943+10
In this paper, we analyze time series measurements of PSR B0943+10 and fit
them with a non-radial oscillation model. The model we apply was first
developed for total intensity measurements in an earlier paper, and expanded to
encompass linear polarization in a companion paper to this one. We use PSR
B0943+10 for the initial tests of our model because it has a simple geometry,
it has been exhaustively studied in the literature, and its behavior is
well-documented. As prelude to quantitative fitting, we have reanalyzed
previously published archival data of PSR B0943+10 and uncovered subtle but
significant behavior that is difficult to explain in the framework of the
drifting spark model. Our fits of a non-radial oscillation model are able to
successfully reproduce the observed behavior in this pulsar.Comment: 45 pages, 16 figures, accepted Ap
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