Kavli Affiliate: Susan E. Clark
| First 5 Authors: Marta Nowotka, Marta Nowotka, , ,
| Summary:
We present a joint analysis of HI absorption Zeeman measurements and the
morphology of filamentary HI emission to investigate the three-dimensional
structure of the magnetic field in the diffuse neutral interstellar medium
(ISM). Our analysis is based on the Arecibo Millennium Survey and new data from
the Five-hundred-meter Aperture Spherical radio Telescope (FAST) toward radio
sources 3C 75, 3C 207, and 3C 409. Toward 3C 409, we make a 4$sigma$ Zeeman
detection and infer $B_LOS$ = 9.1 +/- 1.9$mu$G, in agreement with Arecibo
results. We quantify the dispersion of HI filaments at the locations and
velocities of Zeeman components using GALFA-HI narrow-channel emission maps.
Focusing on a subsample of 42 spectrally distinct components, we find a weak
but statistically significant positive correlation (Spearman r = 0.3, $p =
0.01$) between $|B_LOS|$ and the circular variance of HI filament orientation
angles. To examine its origin, we characterize the environments probed by HI
absorption using dust emission, 3D dust maps, OH absorption, and CO emission.
We find evidence that existing HI absorption Zeeman measurements trace magnetic
fields that are coherent on parsec scales, probe primarily local gas
($100$-$500$ pc, often at distances consistent with the Local Bubble wall), and
exhibit systematic differences in the magnitude of $B_LOS$. We attribute the
correlation between Zeeman measurements and filamentary HI morphology to
large-scale variations in magnetic field strength and/or inclination angle
across different Galactic environments, which could arise due to the Local
Bubble geometry or enhanced total field strength in denser regions.
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