Anthony Gray

A large-scale, interstellar Faraday-rotation feature of unknown origin

The disk of the Milky Way contains free electrons and magnetic fields which contribute significantly to the energetics of the interstellar medium. The concentrations of electrons and magnetic fields are too low to be detected by direct methods, but may be investigated using Faraday rotation, a wavelength-dependent shift in linear polarization angle induced by a magneto-ionic medium. Structures in polarization angle arising from Faraday rotation have been detected recently at long radio wavelengths. These structures are disorganized and filamentary, probably arising from interstellar gas in the vicinity of the Sun. Here we report a more distant, highly ordered Faraday-rotation structure of elliptical shape, with its long axis parallel to the plane of the Galaxy. The feature appears to be located in an inter-arm region of the Milky Way, between the spiral arm containing the Sun and the next outer (Perseus) spiral arm. Within the elliptical region, small-scale structure which characterizes the turbulence seen in adjacent regions of the interstellar medium is absent. The origin of this magneto-ionic feature is uncertain, but it must arise from an organization of the magnetic-field and electron-density distributions on a scale of the order of 50 parsecs (165 light years).

H I Distribution and Kinematics of UGCA 86

We present 21 cm H I line and 408 MHz and 1.4 GHz continuum observations of the Magellanic dwarf galaxy UGCA 86, made with the Dominion Radio Astrophysical Observatory (DRAO) Synthesis Telescope. UGCA 86 is detected in the continuum at both frequencies, with 408 MHz flux density S408 = 120 ± 30 mJy and 1.4 GHz flux density S1400 = 79 ± 3 mJy. The H I structure of UGCA 86 is complex, with two separate components: a rotating disk and a highly elongated spur that is kinematically disjunct from the disk. The H I disk is centered on the optical galaxy with similar axial ratio and orientation of the major axis. An area of the disk with a peculiar velocity of ~25 km s-1 relative to the regular rotation of the disk is found on the southern side, where most of the star formation is concentrated. The spur is seen along the minor axis of UGCA 86 and overlaps in part with the disk. Toward the optical center of UGCA 86, the velocity difference between the spur and the disk is 40 km s-1, about one-third of the rotation velocity of the H I disk at 6 kpc from the center. This implies a large radial component of the orbital velocity of the spur and therefore a significantly noncircular orbit. The median H I velocity dispersion of the disk is 8.8 km s-1, similar to other (dwarf) galaxies. The H I velocity dispersion of the spur varies from 10 to 30 km s-1. A possible tidal origin of the spur is considered in view of the proximity of the large Scd galaxy IC 342. However, the orientation of the spur along the minor axis and its spatial overlap with the disk suggest that the spur is located far outside the plane of the H I disk. No evidence is found that the outer H I disk is warped, which poses a problem for the interpretation of the spur as a tidal tail induced by IC 342. Detailed modeling of the IC 342/UGCA 86 system will be required before a tidal origin of the spur can be dismissed conclusively. The possibility that the spur is part of the nascent cloud of UGCA 86 or the remains of a small dwarf galaxy is presented as an alternative interpretation.

Activites of the Dominion Radio Astrophysical Observatory

The Dominion Radio Astrophysical Observatory (DRAO) carries out world-class research in radio astronomy science and engineering, with a focus on science and technologies relevant to the planned International Square- Kilometre Array (SKA). DRAO staff operate three on-site telescopes, and participate in international science projects, with recognized expertise in the theory and practice of wide-field imaging and polarimetry. Active engineering programs include digital signal processing (correlators and beam-forming), phased-array feeds, and high performance composite reflector antennas.