9,207 research outputs found
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
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 Magnetic Field of L1544: I. Near-Infrared Polarimetry and the Non-Uniform Envelope
The magnetic field (B-field) of the starless dark cloud L1544 has been
studied using near-infrared (NIR) background starlight polarimetry (BSP) and
archival data in order to characterize the properties of the plane-of-sky
B-field. NIR linear polarization measurements of over 1,700 stars were obtained
in the H-band and 201 of these were also measured in the K-band. The NIR BSP
properties are correlated with reddening, as traced using the RJCE (H-M)
method, and with thermal dust emission from the L1544 cloud and envelope seen
in Herschel maps. The NIR polarization position angles change at the location
of the cloud and exhibit their lowest dispersion of position angles there,
offering strong evidence that NIR polarization traces the plane-of-sky B-field
of L1544. In this paper, the uniformity of the plane-of-sky B-field in the
envelope region of L1544 is quantitatively assessed. This allowed evaluating
the approach of assuming uniform field geometry when measuring relative
mass-to-flux ratios in the cloud envelope and core based on averaging of the
envelope radio Zeeman observations, as in Crutcher et al. (2009). In L1544, the
NIR BSP shows the envelope B-field to be significantly non-uniform and likely
not suitable for averaging Zeeman properties without treating intrinsic
variations. Deeper analyses of the NIR BSP and related data sets, including
estimates of the B-field strength and testing how it varies with position and
gas density, are the subjects of later papers in this series.Comment: 16 pages, 9 figures; accepted for publication in The Astrophysical
Journa
Mid-infrared colour gradients and the colour-magnitude relation in Virgo early-type galaxies
We make use of Spitzer imaging between 4 and 16 micron and near-infrared data
at 2.2 micron to investigate the nature and distribution of the mid-infrared
emission in a sample of early-type galaxies in the Virgo cluster. These data
allow us to conclude, with some confidence, that the emission at 16 micron in
passive ETGs is stellar in origin, consistent with previous work concluding
that the excess mid-infrared emission comes from the dusty envelopes around
evolved AGB stars. There is little evidence for the mid-infrared emission of an
unresolved central component, as might arise in the presence of a dusty torus
associated with a low-luminosity AGN. We nonetheless find that the 16 micron
emission is more centrally peaked than the near-infrared emission, implying a
radial stellar population gradient. By comparing with independent evidence from
studies at optical wavelengths, we conclude that a metallicity that falls with
increasing radius is the principal driver of the observed gradient. We also
plot the mid-infrared colour-magnitude diagram and combine with similar work on
the Coma cluster to define the colour-magnitude relation for absolute K-band
magnitudes from -26 to -19. Because a correlation between mass and age would
produce a relation with a gradient in the opposite sense to that observed, we
conclude that the relation reflects the fact that passive ETGs of lower mass
also have a lower average metallicity. The colour-magnitude relation is thus
driven by metallicity effects. In contrast to what is found in Coma, we do not
find any objects with anomalously bright 16 micron emission relative to the
colour-magnitude relation. Although there is little overlap in the mass ranges
probed in the two clusters, this may suggest that observable ``rejuvenation''
episodes are limited to intermediate mass objects.Comment: 8 pages, 4 figure
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