The large-scale magnetic field in the fourth Galactic quadrant

Abstract

Aims. We have re-examined the published rotation measures (RMs) of extragalactic point sources and pulsars with |b| < 3 • to study the magnetic field in the fourth Galactic quadrant. Methods. We reduced the influence of structure in electron density as much as possible by excluding objects for which Hα-data indicate large fluctuations in n e somewhere along the line of sight. We also excluded objects for which the RM may have been significantly "corrupted" by an intervening supernova remnant. We modeled RM(l), the longitude dependence of RM of the unaffected extragalactic sources and pulsars. We assumed several geometries for the large-scale field. All but one of those are based on logarithmic spiral arms (with various pitch angles and widths), while one has circular symmetry. We also made different assumptions about the large-scale n e -distribution. Results. The data suggest the following generic behaviour of the large-scale field in the 4th Galactic quadrant. The field is most likely organized along logarithmic spiral arms and shows two significant reversals: from the Norma arm (CCW field) to the Norma-Crux interarm region (CW field), and from the Norma-Crux interarm region to the Crux arm (CCW field). The present data do not constrain the field in and beyond the Crux-Carina interarm region. Although the models give a good description of the global character of RM(l), individual RM-estimates deviate by typically 15 times their measurement errors. We argue that these large deviations are most likely due to the "small-scale" field that dominates on scales of up to several hundred pc. Conclusions. The picture that emerges is thus of a field that has significant structure on smaller scales, but for which the average values in arms and interarm regions are nevertheless well-defined. In addition, this smaller-amplitude large-scale field appears to reverse at each arm-interarm boundary that we can study with the present data. We briefly discuss the link between these results and theoretical predictions.

Modeling the Magnetic Field in the Galactic Disk Using New Rotation Measure Observations From the Very Large Array

“…Weisberg et al 2004;Han et al 2006;Vallée 2008) or have looked at only specific regions of the disk (e.g. Brown et al 2007;Nota & Katgert 2010), we chose to take a 'hybrid' approach. Our modeling work examines the entire Galactic disk, but in 3 separate sectors to see if we can determine any common features or structure.…”

“…While it is possible to construct magnetic field models using a constant electron density (e.g. Nota & Katgert 2010), such models put all of the structure of the RMs into the magnetic field. Instead, we were interested in attempting to explore the relationship between the spiral arms and the magnetic field.…”

“…To that end, we merged all separate regions beyond the Sagittarius-Carina arm into one magnetic field region, and redefined the field in this new outer-Galaxy region to be purely azimuthal, and still retained the R −1 dependence in this region, consistent with Brown et al (2003b). However, for the remaining regions in the inner Galaxy, we reverted to a constant field strength, as suggested by Nota & Katgert (2010).…”

“…The number of reversals depends on the interpretation of the existing RM data and is presently a very controversial subject (e.g. Brown & Taylor 2001;Weisberg et al 2004;Vallée 2005;Han et al 2006;Brown et al 2007;Sun et al 2008;Vallée 2008;Men et al 2008;Jansson et al 2009;Kronberg & Newton-McGee 2009;Nota & Katgert 2010). Models of the Galactic magnetic field are generally classified as axisymmetric spiral (ASS), which requires symmetry under the same rotation of the disk by π around the Galactic center, bisymmetric spiral (BSS), which requires anti-symmetry under rotation by π, or mixed spiral structure (MSS), which contain both ASS and BSS components (Beck et al 1996).…”

Radio Continuum Surveys with Square Kilometre Array Pathfinders

“…Beck 2011;Noutsos et al 2008;Van Eck et al 2011;Sun et al 2008;Jansson et al 2009;Nota & Katgert 2010;Mao et al 2010b), all of which are key to understanding the evolution of galactic magnetic fields. However, controversy still exists about the existence and location of reversals, and the overall 3D configuration of the magnetic field in the Milky Way's disc and halo.…”