A Comparison of Multi-Phase Magnetic Field Tracers in a High-Galactic Latitude Region of the Filamentary Interstellar Medium

Abstract

Understanding how the Galactic magnetic field threads the multi-phase interstellar medium (ISM) remains a considerable challenge, as different magnetic field tracers probe dissimilar phases and field components. We search for evidence of a common magnetic field shared between the ionized and neutral ISM by comparing 1.4 GHz radio continuum polarization and HI line emission from the Galactic Arecibo L-Band Feed Array Continuum Transit Survey (GALFACTS) and Galactic Arecibo L-Band Feed Array HI (GALFA-HI) survey, respectively. We compute the polarization gradient of the continuum emission and search for associations with diffuse/translucent HI structures. The polarization gradient is sensitive to changes in the integrated product of the thermal electron density and line-of-sight field strength ($B_{\parallel }$) in warm ionized gas, while narrow HI structures highlight the plane-of-sky field orientation in cold neutral gas. We identified one region in the high-Galactic latitude Arecibo sky, G216+26 centered on $(\ell ,b)\sim (216°,+26°)$, containing filaments in the polarization gradient that are aligned with narrow HI structures roughly parallel to the Galactic plane. We present a comparison of multi-phase observations and magnetic field tracers of this region, demonstrating that the warm ionized and cold neutral media are connected likely via a common magnetic field. We quantify the physical properties of a polarization gradient filament associated with H$\alpha$ emission, measuring a line-of-sight field strength $B_{\parallel }=6\pm 4\mu$G and a plasma beta $\beta =2.1_{-2.1}^{+3.1}$. We discuss the lack of widespread multi-phase magnetic field alignments and consider whether this region is associated with a short-timescale or physically rare phenomenon. This work highlights the utility of multi-tracer analyses for understanding the magnetized ISM.

30 pages, 17 figures, minor edits to reflect published version

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