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Register a new userRadio polarization from the inner Galaxy at arcminute resolution
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Bryan Gaensler
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The Southern Galactic Plane Survey (SGPS) is an HI and 1.4-GHz continuum survey of the 4th quadrant of the Galaxy at arcmin resolution. We present here results on linearly polarized continuum emission from an initial 28-square-degree Test Region for the SGPS, consisting of 190 mosaiced pointings of the Australia Telescope Compact Array, and covering the range 325.5 < l < 332.5, -0.5 < b < +3.5. Complicated extended structure is seen in linear polarization throughout the Test Region, almost all of which has no correlation with total intensity. We interpret the brightest regions of polarized emission as representing intrinsic structure in extended polarization, most likely originating in the Crux spiral arm at a distance of 3.5 kpc; fainter polarized structure is imposed by Faraday rotation in foreground material. Two large areas in the field are devoid of polarization. We argue that these voids are produced by foreground HII regions in which the magnetic field is disordered on scales of ~0.1-0.2 pc. We also identify a depolarized halo around the HII region RCW 94, which we suggest results from the interaction of the HII region with a surrounding molecular cloud.
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We present our results from the Southern Galactic Plane Survey, an effort to map the fourth quadrant of the Milky Way in linear polarization at a frequency of 1.4 GHz and at a resolution of 1-2 arcmin. These data are a powerful probe of both the turbulence and large-scale structure of magneto-ionic gas, and have revealed a variety of new features in the interstellar medium.
Recently, Saxena et al. (2018) reported the discovery of a possible radio galaxy, J1530$+$1049 at a redshift of z=5.72. We observed the source with the European Very Long Baseline Interferometry Network at $1.7$ GHz. We detected two faint radio features with a separation of $sim 400$ mas. The radio power calculated from the VLA flux density by Saxena et al. (2018), and the projected source size derived from our EVN data place J1530$+$1049 among the medium-sized symmetric objects (MSOs) which are thought to be young counterparts of radio galaxies (An and Baan 2012). Thus, our finding is consistent with a radio galaxy in an early phase of its evolution as proposed by Saxena et al. (2018).
We present high sensitivity and absolutely calibrated images of diffuse radio polarisation at a resolution of about 10 arcmin covering the range 10 degr <l<34 degr and |b|<5 degr at 2.3 GHz from the S-band Parkes All Sky Survey and at 4.8 GHz from the Sino-German 6 cm polarisation survey of the Galactic plane. Strong depolarisation near the Galactic plane is seen at 2.3 GHz, which correlates with strong Halpha emission. We ascribe the depolarisation to spatial Faraday rotation measure fluctuations of about 65 rad/m2 on scales smaller than 6-9 pc. We argue that most (about 90%) of the polarised emission seen at 4.8 GHz originates from a distance of 3-4 kpc in the Scutum arm and that the random magnetic field dominates the regular field there. A branch extending from the North Polar Spur towards lower latitudes can be identified from the polarisation image at 4.8 GHz but only partly from the polarised image at 2.3 GHz, implying the branch is at a distance larger than 2-3 kpc. We show that comparison of structure functions of complex polarised intensity with those of polarised intensity can indicate whether the observed polarised structures are intrinsic or caused by Faraday screens. The probability distribution function of gradients from the polarisation images at 2.3 GHz indicates the turbulence in the warm ionised medium is transonic.
I review our current state of knowledge about non-thermal radiation from the Galactic Centre (GC) and Inner Galaxy. Definitionally, the Galactic nucleus is at the bottom of the Galaxys gravitational well, rendering it a promising region to seek the signatures of dark matter decay or annihilation. It also hosts, however, the Milky Ways resident supermassive black hole and up to 10% of current massive star formation in the Galaxy. Thus the Galactic nucleus is a dynamic and highly-energized environment implying that extreme caution must be exercised in interpreting any unusual or unexpected signal from (or emerging from) the region as evidence for dark matter-related processes. One spectacular example of an `unexpected signal is the discovery within the last few years of the `Fermi Bubbles and, subsequently, their polarised radio counterparts. These giant lobes extend ~7 kpc from the nucleus into both north and south Galactic hemispheres. Hard-spectrum, microwave emission coincident with the lower reaches of the Bubbles has also been detected, first in WMAP, and more recently in Planck data. Debate continues as to the origin of the Bubbles and their multi-wavelength emissions: are they the signatures of relatively recent (in the last ~Myr) activity of the supermassive black hole or, alternatively, nuclear star formation? I will briefly review evidence that points to the latter interpretation.
We have carried out spectroscopic observations of the high redshift ($zapprox 3.4$) radio galaxy 0902+34 at intermediate resolution with the William Herschel Telescope. The dynamical spectral ranges covered are 4600-5480 AA and 5920-7680 AA with resolutions of 5.4 AA and 9.5 AA, respectively. We detect a continuum that is almost flat and resolve three emission lines: Ly$alpha$, C IV $lambda$1549 and He II $lambda$1640, the last one previously undetected. The line ratios are similar to the typical values found for narrow-line high redshift radio galaxies. Line ratios observed in different regions of the galaxy seem to indicate the presence of strong ionization and/or dust density gradients. We have not detected any Ly$alpha$ absorption at z=3.3968 (red wing of the Ly$alpha$ emission line) as might be expected from the absorption found at 21 cm by other authors using the VLA and Arecibo antennas. We discuss possible models for the H I absorbing cloud.
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