| Summary: | We present a large-scale combination of near-infrared (near-IR) interstellar polarization data from background starlight with polarized emission data at submillimeter wavelengths for the Vela C molecular cloud. The near-IR data consist of more than 6700 detections probing a range of visual extinctions between 2 and 20 mag in and around the cloud. The submillimeter data were collected in Antarctica by the Balloon-borne Large Aperture Submillimeter Telescope for Polarimetry. This is the first direct combination of near-IR and submillimeter polarization data for a molecular cloud aimed at measuring the "polarization efficiency ratio" R_(eff), a quantity that is expected to depend only on grain-intrinsic physical properties. It is defined as p_(500)/(pt_TV), where p 500 and p I are polarization fractions at 500 µm and the I band, respectively, and t_V is the optical depth. To ensure that the same column density of material is producing both polarization from emission and from extinction, we conducted a careful selection of near-background stars using 2MASS, Herschel, and Planck data. This selection excludes objects contaminated by the Galactic diffuse background material as well as objects located in the foreground. Accounting for statistical and systematic uncertainties, we estimate an average R_(eff) value of 2.4 ± 0.8, which can be used to test the predictions of dust grain models designed for molecular clouds when such predictions become available. The ratio R_(eff) appears to be relatively flat as a function of the cloud depth for the range of visual extinctions probed. © 2017 American Astronomical Society. Received 2016 May 27. Accepted 2017 February 21. Published 2017 March 15. We are grateful to the anonymous referee for the valuable suggestions and comments. The BLASTPol collaboration acknowledges support from NASA (through grant numbers NAG5-12785, NAG5-13301, NNGO-6GI11G, NNX0-9AB98G, and the Illinois Space Grant Consortium), the Canadian Space Agency (CSA), the Leverhulme Trust through the Research ...
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