Comparing Submillimeter Polarized Emission with Near-infrared Polarization of Background Stars for the Vela C Molecular Cloud

Abstract

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” (Reff ), a quantity that is expected to depend only on grainintrinsic
physical properties. It is defined as p500 (pI tV ), where p500 and pI are polarization fractions at 500 mm and
the I band, respectively, and tV 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 Reff 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 Reff appears to be relatively flat as a function of
the cloud depth for the range of visual extinctions probed.