MAGMO: Coherent magnetic fields in the star forming regions of the Carina-Sagittarius spiral arm tangent

We present the pilot results of the `MAGMO' project, targeted observations of ground-state hydroxyl masers towards sites of 6.7-GHz methanol maser emission in the Carina-Sagittarius spiral arm tangent, Galactic longitudes 280 degrees to 295 degrees. The `MAGMO' project aims to determine if Galactic magnetic fields can be traced with Zeeman splitting of masers associated with star formation. Pilot observations of 23 sites of methanol maser emission were made, with the detection of ground-state hydroxyl masers towards 11 of these and six additional offset sites. Of these 17 sites, nine are new detections of sites of 1665-MHz maser emission, three of them accompanied by 1667-MHz emission. More than 70% of the maser features have significant circular polarization, whilst only ~10% have significant linear polarization (although some features with up to 100% linear polarization are found). We find 11 Zeeman pairs across six sites of high-mass star formation with implied magnetic field strengths between -1.5 mG and +3.8 mG and a median field strength of +1.6 mG. Our measurements of Zeeman splitting imply that a coherent field orientation is experienced by the maser sites across a distance of 5.3+/-2.0 kpc within the Carina-Sagittarius spiral arm tangent.Comment: 19 pages, 13 figures, accepted for publication in MNRA

A quantum mechanical approach to establishing the magnetic field orientation from a maser Zeeman profile

Recent comparisons of magnetic field directions derived from maser Zeeman splitting with those derived from continuum source rotation measures have prompted new analysis of the propagation of the Zeeman split components, and the inferred field orientation. In order to do this, we first review differing electric field polarization conventions used in past studies. With these clearly and consistently defined, we then show that for a given Zeeman splitting spectrum, the magnetic field direction is fully determined and predictable on theoretical grounds: when a magnetic field is oriented away from the observer, the left-hand circular polarization is observed at higher frequency and the right-hand polarization at lower frequency. This is consistent with classical Lorentzian derivations. The consequent interpretation of recent measurements then raises the possibility of a reversal between the large-scale field (traced by rotation measures) and the small-scale field (traced by maser Zeeman splitting).Comment: 10 pages, 5 Figures, accepted for publication in MNRA

A quantum mechanical approach to establishing the magnetic field orientation from a maser zeeman profile

Recent comparisons of magnetic field directions derived from maser Zeeman splitting with those derived from continuum source rotation measures have prompted new analysis of the propagation of the Zeeman split components, and the inferred field orientation. In order to do this, we first review differing electric field polarization conventions used in past studies. With these clearly and consistently defined, we then show that for a given Zeeman splitting spectrum, the magnetic field direction is fully determined and predictable on theoretical grounds: when a magnetic field is oriented away from the observer, the left-hand circular polarization is observed at higher frequency and the right-hand polarization at lower frequency. This is consistent with classical Lorentzian derivations. The consequent interpretation of recent measurements then raises the possibility of a reversal between the large-scale field (traced by rotation measures)and the small-scale field (traced by maser Zeeman splitting)

Complex Faraday depth structure of active galactic nuclei as revealed by broad-band radio polarimetry

We present a detailed study of the Faraday depth structure of four bright (>1 Jy), strongly polarized, unresolved radio-loud quasars. The Australia Telescope Compact Array (ATCA) was used to observe these sources with 2 GHz of instantaneous bandwidth from 1.1 to 3.1 GHz. This allowed us to spectrally resolve the polarization structure of spatially unresolved radio sources, and by fitting various Faraday rotation models to the data, we conclusively demonstrate that two of the sources cannot be described by a simple rotation measure (RM) component modified by depolarization from a foreground Faraday screen. Our results have important implications for using background extragalactic radio sources as probes of the Galactic and intergalactic magneto-ionic media as we show how RM estimations from narrow-bandwidth observations can give erroneous results in the presence of multiple interfering Faraday components. We postulate that the additional RM components arise from polarized structure in the compact inner regions of the radio source itself and not from polarized emission from galactic or intergalactic foreground regions. We further suggest that this may contribute significantly to any RM time variability seen in RM studies on these angular scales. Follow-up, high-sensitivity very long baseline interferometry (VLBI) observations of these sources will directly test our predictions

Magnetically Dominated Strands of Cold Hydrogen in the Riegel-Crutcher Cloud

We present new high resolution (100 arcsec) neutral hydrogen (H I) self-absorption images of the Riegel-Crutcher cloud obtained with the Australia Telescope Compact Array and the Parkes Radio Telescope. The Riegel-Crutcher cloud lies in the direction of the Galactic center at a distance of 125 +/- 25 pc. Our observations resolve the very large, nearby sheet of cold hydrogen into a spectacular network of dozens of hair-like filaments. Individual filaments are remarkably elongated, being up to 17 pc long with widths of less than ~0.1 pc. The strands are reasonably cold, with spin temperatures of 40 K and in many places appearing to have optical depths larger than one. Comparing the H I images with observations of stellar polarization we show that the filaments are very well aligned with the ambient magnetic field. We argue that the structure of the cloud has been determined by its magnetic field. In order for the cloud to be magnetically dominated the magnetic field strength must be > 30 microGauss.Comment: To appear in the Astrophysical Journal. 26 pages, 6 figures. Full resolution version available at ftp://ftp.atnf.csiro.au/pub/people/nmcclure/papers/rc_cloud.pd

Do the Unidentified EGRET Sources Trace Annihilating Dark Matter in the Local Group?

In a cold dark matter (CDM) framework of structure formation, the dark matter haloes around galaxies assemble through successive mergers with smaller haloes. This merging process is not completely efficient, and hundreds of surviving halo cores, or {\it subhaloes}, are expected to remain in orbit within the halo of a galaxy like the Milky Way. While the dozen visible satellites of the Milky Way may trace some of these subhaloes, the majority are currently undetected. A large number of high-velocity clouds (HVCs) of neutral hydrogen {\it are} observed around the Milky Way, and it is plausible that some of the HVCs may trace subhaloes undetected in the optical. Confirming the existence of concentrations of dark matter associated with even a few of the HVCs would represent a dramatic step forward in our attempts to understand the nature of dark matter. Supersymmetric (SUSY) extensions of the Standard Model of particle physics currently suggest neutralinos as a natural well-motivated candidate for the non-baryonic dark matter of the universe. If this is indeed the case, then it may be possible to detect dark matter indirectly as it annihilates into neutrinos, photons or positrons. In particular, the centres of subhaloes might show up as point sources in gamma-ray observations. In this work we consider the possibility that some of the unidentified EGRET $\gamma$-ray sources trace annihilating neutralino dark matter in the dark substructure of the Local Group. We compare the observed positions and fluxes of both the unidentified EGRET sources and the HVCs with the positions and fluxes predicted by a model of halo substructure, to determine to what extent any of these three populations could be associated.Comment: 12 Pages, 4 figures, to appear in a special issue of ApSS. Presented at "The Multiwavelength Approach to Unidentified Gamma-Ray Sources" (Hong Kong, June 1 - 4, 2004; Conference organizers: K.S. Cheng and G.E. Romero

From 10 Kelvin to 10 TeraKelvin: Insights on the Interaction Between Cosmic Rays and Gas in Starbursts

Recent work has both illuminated and mystified our attempts to understand cosmic rays (CRs) in starburst galaxies. I discuss my new research exploring how CRs interact with the ISM in starbursts. Molecular clouds provide targets for CR protons to produce pionic gamma rays and ionization, but those same losses may shield the cloud interiors. In the densest molecular clouds, gamma rays and Al-26 decay can provide ionization, at rates up to those in Milky Way molecular clouds. I then consider the free-free absorption of low frequency radio emission from starbursts, which I argue arises from many small, discrete H II regions rather than from a "uniform slab" of ionized gas, whereas synchrotron emission arises outside them. Finally, noting that the hot superwind gas phase fills most of the volume of starbursts, I suggest that it has turbulent-driven magnetic fields powered by supernovae, and that this phase is where most synchrotron emission arises. I show how such a scenario could explain the far-infrared radio correlation, in context of my previous work. A big issue is that radio and gamma-ray observations imply CRs also must interact with dense gas. Understanding how this happens requires a more advanced understanding of turbulence and CR propagation.Comment: Conference proceedings for "Cosmic-ray induced phenomenology in star-forming environments: Proceedings of the 2nd Session of the Sant Cugat Forum of Astrophysics" (April 16-19, 2012). 16 pages, 5 figure

MAGMO: polarimetry of 1720-MHz OH masers towards southern star-forming regions

From targeted observations of ground-state hydroxyl (OH) masers towards 702 Methanol Multibeam survey 6.7-GHz methanol masers, in the Galactic longitude range from 186 degrees C through the Galactic Centre to 20 degrees C, made as part of the 'MAGMO' (Mapping the Galactic Magnetic field through OH masers) project, we present the physical and polarization properties of the 1720-MHz OH maser transition, including the identification of Zeeman pairs. We present 10 new and 23 previously catalogued 1720-MHz OH maser sources detected towards star-forming regions (SFRs). In addition, we also detected 16 1720-MHz OH masers associated with supernova remnants and two sites of diffuse OH emission. Towards the 33 star formation masers, we identify 44 Zeeman pairs, implying magnetic field strengths ranging from −11.4 to +13.2 mG, and a median magnetic field strength of |BLOS| ∼ 6 mG. With limited statistics, we present the in situ magnetic field orientation of the masers and the Galactic magnetic field distribution revealed by the 1720-MHz transition. We also examine the association statistics of 1720-MHz OH SFR masers with other ground-state OH masers, excited-state OH masers, class I and class II methanol masers, and water masers, and compare maser positions with mid-infrared images of the parent SFRs. Of the 33 1720-MHz star formation masers, 10 are offset from their central exciting sources, and appear to be associated with outflow activity.This work was presented as part of CSO’s doctoral program funded by the International Macquarie Research Excellence Scholarship (iMQRES) program. CSO was also a recipient of the Commonwealth Scientific and Industrial Research Organisation (CSIRO) Astronomy and Space Science Student Program grant for which CSO is grateful. JRD acknowledges the support of an Australian Research Council (ARC) DECRA Fellowship (project number DE170101086). This paper includes archived data obtained through the Australia Telescope Online Archive (http://atoa.atnf.csiro.au). The Australia Telescope Compact Array (ATCA) is part of the Australia Telescope National Facility and funded by the CSIRO. This research made use of APLPY, an open-source plotting package for PYTHON (Robitaille & Bressert 2012)

A Limit on the Polarized Anomalous Microwave Emission of Lynds 1622

The dark cloud Lynds 1622 is one of a few specific sites in the Galaxy where, relative to observed free-free and vibrational dust emission, there is a clear excess of microwave emission. In order to constrain models for this microwave emission, and to better establish the contribution which it might make to ongoing and near-future microwave background polarization experiments, we have used the Green Bank Telescope to search for linear polarization at 9.65 Ghz towards Lynds 1622. We place a 95.4% upper limit of 88 micro-Kelvin (123 micro-Kelvin at 99.7 confidence) on the total linear polarization of this source averaged over a 1'.3 FWHM beam. Relative to the observed level of anomalous emission in Stokes I these limits correspond to fractional linear polarizations of 2.7% and 3.5%.Comment: replaced with version accepted by Ap