Global Magneto-Ionic Medium Survey Research Articles

Structure in the Magnetic Field of the Milky Way Disk and Halo Traced by Faraday Rotation

Magnetic fields in the ionized medium of the disk and halo of the Milky Way impose Faraday rotation on linearly polarized radio emission. We compare two surveys mapping the Galactic Faraday rotation, one showing the rotation measures of extragalactic sources seen through the Galaxy, and one showing Faraday depth of the diffuse Galactic synchrotron emission from the Global Magneto-Ionic Medium Survey. Comparing the two data sets in 5° × 10° bins shows good agreement at intermediate latitudes, 10° < ∣b∣ < 50°, and little correlation between them at lower and higher latitudes. Where they agree, both tracers show clear patterns as a function of Galactic longitude, ℓ; in the northern hemisphere a strong pattern; and in the southern hemisphere a pattern. Pulsars with height above or below the plane ∣z∣ > 300 pc show similar ℓ dependence in their rotation measures. Nearby nonthermal structures show rotation measure shadows as does the Orion–Eridanus superbubble. We describe families of dynamo models that could explain the observed patterns in the two hemispheres. We suggest that a field reversal, known to cross the plane a few hundred parsecs inside the solar circle, could shift to positive z with increasing Galactic radius to explain the pattern in the northern hemisphere. Correlation shows that rotation measures from extragalactic sources are one to two times the corresponding rotation measure of the diffuse emission, implying Faraday complexity along some lines of sight, especially in the southern hemisphere.

The Global Magneto-ionic Medium Survey: A Faraday Depth Survey of the Northern Sky Covering 1280–1750 MHz

Abstract The Galactic interstellar medium hosts a significant magnetic field, which can be probed through the synchrotron emission produced from its interaction with relativistic electrons. Linearly polarized synchrotron emission is generated throughout the Galaxy and, at longer wavelengths, modified along nearly every path by Faraday rotation in the intervening magneto-ionic medium. Full characterization of the polarized emission requires wideband observations with many frequency channels. We have surveyed polarized radio emission from the Northern sky over the range 1280–1750 MHz, with channel width 236.8 kHz, using the John A. Galt Telescope (diameter 25.6 m) at the Dominion Radio Astrophysical Observatory, as part of the Global Magneto-Ionic Medium Survey. The survey covered 72% of the sky, decl. −30° to +87° at all R.A. The intensity scale was absolutely calibrated, based on the flux density and spectral index of Cygnus A. Polarization angle was calibrated using the extended polarized emission of the Fan Region. Data are presented as brightness temperatures with angular resolution 40′. Sensitivity in Stokes Q and U is 45 mK rms in a 1.18 MHz band. We have applied rotation measure synthesis to the data to obtain a Faraday depth cube of resolution 150 rad m−2 and sensitivity 3 mK rms of polarized intensity. Features in Faraday depth up to a width of 110 rad m−2 are represented. The maximum detectable Faraday depth is ±2 × 104 rad m−2. The survey data are available at the Canadian Astronomy Data Centre.

The Global Magneto-Ionic Medium Survey (GMIMS): the brightest polarized region in the southern sky at 75 cm and its implications for Radio Loop II

ABSTRACT Using the Global Magneto-Ionic Medium Survey (GMIMS) Low-Band South (LBS) southern sky polarization survey, covering 300–480 MHz at 81 arcmin resolution, we reveal the brightest region in the southern polarized sky at these frequencies. The region, G150−50, covers nearly 20 $\deg ^2$, near (l, b) ≈ (150○, −50○). Using GMIMS-LBS and complementary data at higher frequencies (∼0.6–30 GHz), we apply Faraday tomography and Stokes QU-fitting techniques. We find that the magnetic field associated with G150−50 is both coherent and primarily in the plane of the sky, and indicates that the region is associated with Radio Loop II. The Faraday depth spectra across G150−50 are broad and contain a large-scale spatial gradient. We model the magnetic field in the region as an expanding shell, and we can reproduce both the observed Faraday rotation and the synchrotron emission in the GMIMS-LBS band. Using QU fitting, we find that the Faraday spectra are produced by several Faraday dispersive sources along the line of sight. Alternatively, polarization horizon effects that we cannot model are adding complexity to the high-frequency polarized spectra. The magnetic field structure of Loop II dominates a large fraction of the sky, and studies of the large-scale polarized sky will need to account for this object. Studies of G150−50 with high angular resolution could mitigate polarization horizon effects, and clarify the nature of G150−50.

The Global Magneto-Ionic Medium Survey: Polarimetry of the Southern Sky from 300 to 480 MHz

Abstract Much data on the Galactic polarized radio emission has been gathered in the last five decades. All-sky surveys have been made, but only in narrow, widely spaced frequency bands—and the data are inadequate for the characterization of Faraday rotation, the main determinant of the appearance of the polarized radio sky at decimeter wavelengths. We describe a survey of polarized radio emission from the southern sky, aiming to characterize the magneto-ionic medium, particularly the strength and configuration of the magnetic field. This work is part of the Global Magneto-Ionic Medium Survey (GMIMS). We have designed and built a feed and receiver covering the band 300–900 MHz for the CSIRO Parkes 64 m Telescope. We have surveyed the entire sky between decl. −90° and +20°. We present data covering 300–480 MHz with angular resolution 81′–45′. The survey intensity scale is absolutely calibrated, based on measurements of resistors at known temperatures and on an assumed flux density and spectral index for Taurus A. Data are presented as brightness temperatures. We have applied Rotation Measure Synthesis to the data to obtain a Faraday depth cube of resolution 5.9 rad m−2, sensitivity of 60 mK of polarized intensity, and angular resolution 1.°35. The data presented in this paper are available at the Canadian Astronomy Data Centre.

Through thick or thin: multiple components of the magneto-ionic medium towards the nearby H ii region Sharpless 2–27 revealed by Faraday tomography

Sharpless 2-27 (Sh2-27) is a nearby ${\rm H\,{\small II}}$ region excited by $\zeta$Oph. We present observations of polarized radio emission from 300 to 480$\,$MHz towards Sh2-27, made with the Parkes 64$\,$m Radio Telescope as part of the Global Magneto-Ionic Medium Survey. These observations have an angular resolution of $1.35^{\circ}$, and the data are uniquely sensitive to magneto-ionic structure on large angular scales. We demonstrate that background polarized emission towards Sh2-27 is totally depolarized in our observations, allowing us to investigate the foreground. We analyse the results of Faraday tomography, mapping the magnetised interstellar medium along the 165$\,$pc path to Sh2-27. The Faraday dispersion function in this direction has peaks at three Faraday depths. We consider both Faraday thick and thin models for this observation, finding that the thin model is preferred. We further model this as Faraday rotation of diffuse synchrotron emission in the Local Bubble and in two foreground neutral clouds. The Local Bubble extends for 80$\,$pc in this direction, and we find a Faraday depth of $-0.8 \pm 0.4\,$rad$\,$m$^{-2}$. This indicates a field directed away from the Sun with a strength of $-2.5\pm1.2\,\mu$G. The near and far neutral clouds are each about 30$\,$pc thick, and we find Faraday depths of $-6.6\pm0.6\,$rad$\,$m$^{-2}$ and $+13.7\pm0.8\,$rad$\,$m$^{-2}$, respectively. We estimate that the line-of-sight magnetic strengths in the near and far cloud are $B_{\parallel, \text{near}} \approx -15\,\mu\text{G}$ and $B_{\parallel, \text{far}} \approx +30\,\mu\text{G}$. Our results demonstrate that Faraday tomography can be used to investigate the magneto-ionic properties of foreground features in front of nearby ${\rm H\,{\small II}}$ regions.

The Galactic Magneto-ionic Medium Survey: Moments of the Faraday Spectra

Abstract Faraday rotation occurs along every line of sight in the Galaxy; rotation measure (RM) synthesis allows a 3D representation of the interstellar magnetic field. This study uses data from the Global Magneto-Ionic Medium Survey, a combination of single-antenna spectro-polarimetric studies, including northern sky data from the Dominion Radio Astrophysical Observatory (DRAO) 26 m telescope (1270–1750 MHz) and southern sky data from the Parkes 64 m telescope (300–480 MHz). From the synthesized Faraday spectral cubes we compute the zeroth, first, and second moments to find the total polarized emission, mean RM, and RM width of the polarized emission. From DRAO first moments we find a weak vertical field directed from Galactic North to South, but Parkes data reveal fields directed toward the Sun at high latitudes in both hemispheres: the two surveys clearly sample different volumes. DRAO second moments show feature widths in Faraday spectra increasing with decreasing positive latitudes, implying that longer lines of sight encounter more Faraday rotating medium, but this is not seen at negative latitudes. Parkes data show the opposite: at positive latitudes the second moment decreases with decreasing latitude, but not at negative latitudes. Comparing first moments with RMs of pulsars and extragalactic sources and a study of depolarization together confirm that the DRAO survey samples to larger distances than the Parkes data. Emission regions in the DRAO survey are typically 700–1000 pc away, slightly beyond the scale height of the magneto-ionic medium; emission detected in the Parkes survey is entirely within the magneto-ionic disk, less than 500 pc away.

Is There a Polarization Horizon?

Modern radio spectrometers make measurement of polarized intensity as a function of Faraday depth possible. I investigate the effect of depolarization along a model line of sight. I model sightlines with two components informed by observations: a warm ionized medium with a lognormal electron density distribution and a narrow, denser component simulating a spiral arm or Hii region, all with synchrotron-emitting gas mixed in. I then calculate the polarized intensity from 300–1800 MHz and calculate the resulting Faraday depth spectrum. The idealized synthetic observations show far more Faraday complexity than is observed in Global Magneto-Ionic Medium Survey observations. In a model with a very nearby Hii region observed at low frequencies, most of the effects of a “depolarization wall” are evident: the Hii region depolarizes background emission, and less (but not zero) information from beyond the Hii region reaches the observer. In other cases, the effects are not so clear, as significant amounts of information reach the observer even through significant depolarization, and it is not clear that low-frequency observations sample largely different volumes of the interstellar medium than high-frequency observations. The observed Faraday depth can be randomized such that it does not always have any correlation with the true Faraday depth.

The Fan Region at 1.5 GHz – I. Polarized synchrotron emission extending beyond the Perseus Arm

The Fan Region is one of the dominant features in the polarized radio sky, long thought to be a local (distance < 500 pc) synchrotron feature. We present 1.3-1.8 GHz polarized radio continuum observations of the region from the Global Magneto-Ionic Medium Survey (GMIMS) and compare them to maps of Halpha and polarized radio continuum intensity from 0.408-353 GHz. The high-frequency (> 1 GHz) and low-frequency (< 600 MHz) emission have different morphologies, suggesting a different physical origin. Portions of the 1.5 GHz Fan Region emission are depolarized by about 30% by ionized gas structures in the Perseus Arm, indicating that this fraction of the emission originates >2 kpc away. We argue for the same conclusion based on the high polarization fraction at 1.5 GHz (about 40%). The Fan Region is offset with respect to the Galactic plane, covering -5{\deg} < b < +10{\deg}; we attribute this offset to the warp in the outer Galaxy. We discuss origins of the polarized emission, including the spiral Galactic magnetic field. This idea is a plausible contributing factor although no model to date readily reproduces all of the observations. We conclude that models of the Galactic magnetic field should account for the > 1 GHz emission from the Fan Region as a Galactic-scale, not purely local, feature.

FARADAY TOMOGRAPHY OF THE NORTH POLAR SPUR: CONSTRAINTS ON THE DISTANCE TO THE SPUR AND ON THE MAGNETIC FIELD OF THE GALAXY

We present radio continuum and polarization images of the North Polar Spur (NPS) from the Global Magneto-Ionic Medium Survey (GMIMS) conducted with the Dominion Radio Astrophysical Observatory 26-m Telescope. We fit polarization angle versus wavelength squared over 2048 frequency channels from 1280 to 1750 MHz to obtain a Faraday Rotation Measure (RM) map of the NPS. Combining this RM map with a published Faraday depth map of the entire Galaxy in this direction, we derive the Faraday depth introduced by the NPS and the Galactic interstellar medium (ISM) in front of and behind the NPS. The Faraday depth contributed by the NPS is close to zero, indicating that the NPS is an emitting only feature. The Faraday depth caused by the ISM in front of the NPS is consistent with zero at b>50 degree, implying that this part of the NPS is local at a distance of approximately several hundred parsecs. The Faraday depth contributed by the ISM behind the NPS gradually increases with Galactic latitude up to b=44 degree, and decreases at higher Galactic latitudes. This implies that either the part of the NPS at b<44 degree is distant or the NPS is local but there is a sign change of the large-scale magnetic field. If the NPS is local, there is then no evidence for a large-scale anti-symmetry pattern in the Faraday depth of the Milky Way. The Faraday depth introduced by the ISM behind the NPS at latitudes b>50 degree can be explained by including a coherent vertical magnetic field.

GMIMS: the Global Magneto-Ionic Medium Survey

AbstractThe Global Magneto-Ionic Medium Survey (GMIMS) is a project to map the diffuse polarized emission over the entire sky, Northern and Southern hemispheres, from 300 MHz to 1.8 GHz. With an angular resolution of 30–60 arcmin and a frequency resolution of 1 MHz or better, GMIMS will provide the first spectro-polarimetric data set of the large-scale polarized emission over the entire sky, observed with single-dish telescopes. GMIMS will provide an invaluable resource for studies of the magneto-ionic medium of the Galaxy in the local disk, halo, and its transition.