Interstellar Polarization Survey. II. General Interstellar Medium

Abstract

Magnetic fields permeate the entire Galaxy and are essential to, for example, the regulation of several stages of the star formation process and cosmic-ray transportation. Unraveling their properties, such as intensity and topology, is an observational challenge that requires combining different and complementary techniques. The polarization of starlight due to the absorption by field-aligned nonspherical dust grains provides a unique source of information about the interstellar magnetic field in the optical band. This work introduces a first analysis of a new catalog of optical observations of linearly polarized starlight in the diffuse interstellar medium (ISM), the Interstellar Polarization Survey–General ISM (IPS-GI). We used data from the IPS-GI, focusing on 38 fields sampling lines of sight in the diffuse medium. The fields are about 0.°3 × 0.°3 in size, and each of them contains ∼1000 stars, on average. The IPS-GI catalog has polarimetric measurements of over 40,000 stars, over 18,000 of which have P / σ _P > 5. We added distances and other parameters from auxiliary catalogs to over 36,000 of these stars. We analyzed parameter distributions and correlations between parameters of a high-quality subsample of 10,516 stars (i.e., ∼275 stars per field). As expected, the degree of polarization tends to increase with the extinction, producing higher values of polarization at greater distances or lower absolute Galactic latitudes. Furthermore, we find evidence for a large-scale ordered Galactic magnetic field.

Keywords

• Optical astronomy
• Starlight polarization
• Interstellar medium
• Polarimetry
• Interstellar magnetic fields