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 $\mu$m) polarization across the galaxy. These observations place an upper limit of $P_H<0.05$ on the $H$-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 $H$-band polarization remains undetected. The polarization upper limit at $H$-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 ($P_H/P{VRI} \lesssim 0.05$) across the face of M51.Comment: 4 pages, 2 figures, Accepted for publication in ApJ

The Magnetic Field of Cloud 3 in L204

The L204 dark cloud complex is a nearby filamentary structure in Ophiuchus North that has no signs of active star formation. Past studies show that L204 is interacting with the nearby runaway O star, $\zeta$ Oph, and hosts a magnetic field that is coherent across parsec-length scales. Near-infrared $H$-band (1.6$\mu$m) linear polarization measurements were obtained for 3,896 background stars across a $1\deg \times 1.5\deg$ region centered on the dense Cloud 3 in L204, using the Mimir near-infrared instrument on the 1.8m Perkins Telescope. Analysis of these observations reveals both large-scale properties and small-scale changes in the magnetic field direction in Cloud 3. In the northern and western $\zeta$ Oph facing regions of the cloud, the magnetic field appears to be pushed up against the face of the cloud. This may indicate that the UV flux from $\zeta$ Oph has compressed the magnetic field on the western edge of L204. The plane-of-sky magnetic field strength is estimated to be $\sim 11 - 26$ $\mu$G using the Chandrasekhar-Fermi method. The polarimetry data also reveal that the polarization efficiency (PE $\equiv P_{\rm H}/A_{\rm V}$) steadily decreases with distance from $\zeta$ Oph ($-0.09 \pm 0.03 \% \, {\rm mag}^{-1} \, {\rm pc}^{-1}$). Additionally, power-law fits of PE versus $A_{\rm V}$ for localized samples of probe stars show steeper negative indices with distance from $\zeta$ Oph. Both findings highlight the importance of external illumination, here from $\zeta$ Oph, in aligning dust grains to embedded magnetic fields.Comment: 13 pages, 14 figures, 1 tabl

ON THE MULTI-PREFERENCE APPROACH TO EVALUATING OPPORTUNITIES

The purpose of the paper is to provide a general framework for analyzing ""preference for opportunities."" Based on two simple axioms a fundamental result due to Kreps is used in order to represent rankings of opportunity sets in terms of multiple preferences. The paper provides several refinements of the basic representation theorem. In particular, a condition of ""closedness under compromise"" is suggested in order to distinguish the flexibility interpretation of the model from normative interpretations which play a crucial role in justifying the intrinsic value of opportunities. Moreover, the paper clarifies the link between the multiple preference approach and the ""choice function"" approach to evaluating opportunities. In particular, it is shown how the well-known Aizerman/Malishevski result on rationalizability of choice functions can be obtained as a corollary from the more general multiple preference representation of a ranking of opportunity sets.

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

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

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