Far-infrared Polarization Properties of Nearby Star-forming Regions: A New Compendium of SOFIA/HAWC+ Observations

Abstract

We present a comprehensive polarimetric study of 26 nearby molecular clouds in four far-infrared bands (53 $μ$m to 214 $μ$m) using 52 archival SOFIA/HAWC+ datasets. Far-infrared dust polarization observations probe the plane-of-sky magnetic field. To investigate scale-dependent trends, we group the molecular clouds by distance and analyze the data at common angular ($25''$) and common physical (0.052 pc and 0.32 pc) resolutions. The two shorter wavelengths are more impacted by smoothing, exhibiting a larger decrease in percent polarization. We analyze the polarization spectrum -- the polarization fraction as a function of wavelength -- and find that it depends more strongly on column density than dust temperature. We find a "falling" spectrum at the 0.052 pc resolution, but find a "flat" spectrum at the 0.32 pc resolution, suggesting that resolution plays an important role in the observed polarization spectra. We propose that warm dust grain emission in small-scale structures ($\lesssim$ 0.1 pc) traces different magnetic field geometries only resolved in our close regime data. There is no preferred magnetic field orientation across our data, which suggests that the magnetic field in our $\sim$ parsec scale regions is decoupled from the large-scale field that is primarily parallel to the Galactic plane. The relationship between percent polarization and column density varies between clouds, but the correlation between percent polarization and angular dispersion is consistent across regions. This compendium of dust polarization maps highlights the value of observing at multiple far-infrared wavelengths and will enable additional population-level studies of magnetic fields and dust across star-forming environments.

Figures (14)

• Figure 1: Sky distribution of the 26 regions used in this paper. Each region is marked with a pink dot on a galactic projection full-sky map of the Planck 857 GHz data with histogram equalized color mapping. The vectors represent the average magnetic field orientation for each region as a function of wavelength (colors shown in legend).
• Figure 2: A representative HAWC+ dataset from each of the 26 regions. Each region is labeled with its name and a letter denoting the HAWC+ band shown. The purple circle shows the beam size. The vectors are the magnetic field pseudovectors and their length corresponds to the percent polarization. The pseudovectors are sampled so there is one vector per beam. The scale bar denotes the physical size in parsecs calculated from the distances in \ref{['tab:regions']}.
• Figure 3: Same as \ref{['fig:fam_port']} for the regions that we have coverage in all four HAWC+ bands.
• Figure 4: Example maps of the column density with magnetic field pseudovectors overplotted. The vectors are color coded by the angular dispersion and have constant length. The blue circle shows the size of the disk that the dispersion is calculated over and the purple circle shows the $25"$ beam.
• Figure 5: Histograms of the angular dispersion calculated over three different areas with radii of 1, 3/2, and 2 beams. Top: All of the data smoothed to $25"$. Middle: The close regime data smoothed to $25"$ (solid lines) and smoothed to 0.052 pc (shaded regions). Bottom: The far regime data smoothed to $25"$ (solid lines) and smoothed to 0.32 pc (shaded regions).