J. M. Stil

Science with the Australian Square Kilometre Array Pathfinder

The future of cm and m-wave astronomy lies with the Square Kilometre Array (SKA), a telescope under development by a consortium of 17 countries that will be 50 times more sensitive than any existing radio facility. Most of the key science for the SKA will be addressed through large-area imaging of the Universe at frequencies from a few hundred MHz to a few GHz. The Australian SKA Pathfinder (ASKAP) is a technology demonstrator aimed in the mid-frequency range, and achieves instantaneous wide-area imaging through the development and deployment of phased-array feed systems on parabolic reflectors. The large field-of-view makes ASKAP an unprecedented synoptic telescope that will make substantial advances in SKA key science. ASKAP will be located at the Murchison Radio Observatory in inland Western Australia, one of the most radio-quiet locations on the Earth and one of two sites selected by the international community as a potential location for the SKA. In this paper, we outline the ASKAP project and summarise its headline science goals as defined by the community at large.

An improved map of the galactic Faraday sky

We aim to summarize the current state of knowledge regarding Galactic Faraday rotation in an all-sky map of the Galactic Faraday depth. For this we have assembled the most extensive catalog of Faraday rotation data of compact extragalactic polarized radio sources to date. In the map-making procedure we used a recently developed algorithm that reconstructs the map and the power spectrum of a statistically isotropic and homogeneous field while taking into account uncertainties in the noise statistics. This procedure is able to identify some rotation angles that are offset by an integer multiple of π. The resulting map can be seen as an improved version of earlier such maps and is made publicly available, along with a map of its uncertainty. For the angular power spectrum we find a power law behavior C� ∝ � −2.17 for a Faraday sky where an overall variance profile as a function of Galactic latitude has been removed, in agreement with earlier work. We show that this is in accordance with a 3D Fourier power spectrum P(k) ∝ k −2.17 of the underlying

MODELING THE MAGNETIC FIELD IN THE GALACTIC DISK USING NEW ROTATION MEASURE OBSERVATIONS FROM THE VERY LARGE ARRAY

We have determined 194 Faraday rotation measures (RMs) of polarized extragalactic radio sources using new, multi-channel polarization observations at frequencies around 1.4 GHz from the Very Large Array in the Galactic plane at 17° ≤ l ≤ 63° and 205° ≤ l ≤ 253°. This catalog fills in gaps in the RM coverage of the Galactic plane between the Canadian Galactic Plane Survey and Southern Galactic Plane Survey. Using this catalog we have tested the validity of recently proposed axisymmetric and bisymmetric models of the large-scale (or regular) Galactic magnetic field, and found that of the existing models we tested, an axisymmetric spiral model with reversals occurring in rings (as opposed to along spiral arms) best matched our observations. Building on this, we have performed our own modeling, using RMs from both extragalactic sources and pulsars. By developing independent models for the magnetic field in the outer and inner Galaxy, we conclude that in the inner Galaxy, the magnetic field closely follows the spiral arms, while in the outer Galaxy, the field is consistent with being purely azimuthal. Furthermore, the models contain no reversals in the outer Galaxy, and together seem to suggest the existence of a single reversed region that spirals out from the Galactic center.

RADIO POLARIMETRY OF THE ELAIS N1 FIELD: POLARIZED COMPACT SOURCES

We present deep polarimetric observations at 1420 MHz of the European Large Area ISO Survey North 1 region (ELAIS N1) as part of the Dominion Radio Astrophysical Observatory Planck Deep Fields project. By combining closely spaced aperture synthesis fields, we image a region of 7.43 deg^2 to a maximum sensitivity in Stokes Q and U of 78 μJy beam^(-1), and detect 786 compact sources in Stokes I. Of these, 83 exhibit polarized emission. We find that the differential source counts (log N-log p) for polarized sources are nearly constant down to p > 500 μJy, and that these faint polarized radio sources are more highly polarized than the strong source population. The median fractional polarization is 4.8% ± 0.7% for polarized sources with Stokes I flux density between 10 and 30 mJy, approximately 3 times larger than sources with I > 100 mJy. The majority of the polarized sources have been identified with galaxies in the Spitzer Wide Area Infrared Extragalactic Survey (SWIRE) image of ELAIS N1. Most of the galaxies occupy regions in the IRAC 5.8 μm/3.6 μm versus 8.0 μm/4.5 μm color-color diagram associated with dusty AGNs, or with ellipticals with an aging stellar population. A few host galaxies have colors that suggests significant PAH emission in the near-infrared. A small fraction, 12%, of the polarized sources are not detected in the SWIRE data. None of the polarized sources in our sample appears to be associated with an actively star-forming galaxy.

STRUCTURE IN THE ROTATION MEASURE SKY

An analysis of structure in rotation measure (RM) across the sky based on the RM catalog of Taylor et al. is presented. Several resolved RM structures are identified with structure in the local interstellar medium, including radio loops I, II, and III, the Gum nebula, and the Orion-Eridanus superbubble. Structure functions (SFs) of RM are presented for selected areas, and maps of SF amplitude and slope across the sky are compared with Hα intensity and diffuse polarized intensity. RM variance on an angular scale of 1° is correlated with length of the line of sight through the Galaxy, with a contribution from local structures. The slope of the SFs is less concentrated to the Galactic plane and less correlated with length of the line of sight through the Galaxy, suggesting a more local origin for RM structure on angular scales ~10°. The RM variance is a factor of ~2 higher toward the South Galactic Pole than toward the North Galactic Pole, reflecting a more wide-spread asymmetry between the northern and southern Galactic hemispheres. Depolarization of diffuse Galactic synchrotron emission at latitudes <30° can be explained largely by Faraday dispersion related to small-scale variance in RM, but the errors allow a significant contribution from differential Faraday rotation along the line of sight.

THE INTEGRATED POLARIZATION OF SPIRAL GALAXY DISKS

We present integrated polarization properties of nearby spiral galaxies at 4.8 GHz, and models for the integrated polarization of spiral galaxy disks as a function of inclination. Spiral galaxies in our sample have observed integrated fractional polarization in the range 1%-17.6%. At inclinations less than 50°, the fractional polarization depends mostly on the ratio of random to regular magnetic field strength. At higher inclinations, Faraday depolarization associated with the regular magnetic field becomes more important. The observed degree of polarization is lower ( 2 × 1021 W Hz–1. The polarization angle of the integrated emission is aligned with the apparent minor axis of the disk for galaxies without a bar. In our axially symmetric models, the polarization angle of the integrated emission is independent of wavelength. Simulated distributions of fractional polarization for randomly oriented spiral galaxies at 4.8 GHz and 1.4 GHz are presented. We conclude that polarization measurements, e.g., with the Square Kilometre Array, of unresolved spiral galaxies allow statistical studies of the magnetic field in disk galaxies using large samples in the local universe and at high redshift. As these galaxies behave as idealized background sources without internal Faraday rotation, they can be used to detect large-scale magnetic fields in the intergalactic medium.

NEW CONSTRAINTS ON THE GALACTIC HALO MAGNETIC FIELD USING ROTATION MEASURES OF EXTRAGALACTIC SOURCES TOWARDS THE OUTER GALAXY

We present a study of the Milky Way disk and halo magnetic field, determined from observations of Faraday rotation measure (RM) towards 641 polarized extragalactic radio sources in the Galactic longitude range 100 117 , within 30 of the Galactic plane. Forjbj < 15 , we observe a symmetric RM distribution about the Galactic plane. This is consistent with a disk field in the Perseus arm of even parity across the Galactic midplane. In the range 15 <jbj < 30 , we find median rotation measures of -15 4 rad m -2 and -62 5 rad m -2 in the northern and southern Galactic hemispheres, respectively. If the RM distribution is a signature of the large-scale field parallel to the Galactic plane, this suggests that the halo magnetic field toward the outer Galaxy does not reverse direction across the mid-plane. The variation of RM as a function of Galactic latitude in this longitude range is such that RMs become more negative at largerjbj. This is consistent with an azimuthal magnetic field of strength 2 G (7 G) at a height 0.8-2 kpc above (below) the Galactic plane between the local and the Perseus spiral arm. We propose that the Milky Way could possess spiral-like halo magnetic fields similar to those observed in M51. Subject headings: magnetic fields —Faraday rotation—polarization—Galaxy: halo

Tracking the Outer Spiral Arms of the Galaxy in H I Absorption

We have analyzed 793 atomic hydrogen emission and absorption spectrum pairs toward continuum background sources in the northern and southern Galactic plane. In this paper we focus on absorption features from cold gas in the outermost arms of the Galaxy. We find H I absorption associated on a global scale with the outer spiral arm (first and second Galactic quadrants). To a lesser extent we also see absorption associated with the most distant spiral arm in the third and fourth Galactic quadrants. In total, 236 spectra contain clear absorption features associated with the outermost spiral arms. Cool-phase gas therefore exists throughout these spiral arms. The mean distances between absorbing clouds are on the order of 90-220 pc. We identify a number of striking H I structures with masses on the order of 105-106 M⊙ containing cool H I gas with temperatures below 100 K. These clouds are only marginally stable against gravitational collapse.

Compact H I Clouds at High Forbidden Velocities in the Inner Galaxy

The VLA Galactic Plane Survey (VGPS) of the first Galactic quadrant was searched for H I emission with velocities well above the maximum velocity allowed by Galactic rotation. A sample of 17 small fast-moving clouds was identified. The distribution of the ensemble of clouds in longitude and velocity indicates that the clouds are part of the Galactic disk, despite their large forbidden velocity. The median angular diameter of the clouds detected in the VGPS is 34. These clouds would not be noticed in previous low-resolution surveys because of strong beam dilution. Assuming each cloud is located at the tangent point, a median cloud has a diameter of 10 pc, an H I mass of 60 M☉, and a velocity more than 25 km s-1 beyond the local terminal velocity derived from 12CO observations. Three clouds in the sample have a velocity between 50 and 60 km s-1 in excess of the local terminal velocity. The longitude distribution of the sample peaks near l = 30°, while the latitude distribution of the clouds is nearly flat. The observed longitude and latitude distributions are compared with simulated distributions taking into account the selection criteria of the cloud search. It is found that the number of clouds declines with distance from the Galactic center, with an exponential scale length 2.8-8 kpc at the 99% confidence level. We find a lower limit to the scale height of the clouds of 180 pc (HWHM), but the true value is likely significantly higher.

Comparison of algorithms for determination of rotation measure and Faraday structure. I. 1100–1400 MHz

Faraday rotation measures (RMs) and more general Faraday structures are key parameters for studying cosmic magnetism and are also sensitive probes of faint ionized thermal gas. A definition of which derived quantities are required for various scientific studies is needed, as well as addressing the challenges in determining Faraday structures. A wide variety of algorithms has been proposed to reconstruct these structures. In preparation for the Polarization Sky Survey of the Universes Magnetism (POSSUM) to be conducted with the Australian Square Kilometre Array Pathfinder and the ongoing Galactic Arecibo L-band Feeds Array Continuum Transit Survey (GALFACTS), we run a Faraday structure determination data challenge to benchmark the currently available algorithms, including Faraday synthesis (previously called RM synthesis in the literature), wavelet, compressive sampling, and QU-fitting. The input models include sources with one Faraday thin component, two Faraday thin components, and one Faraday thick component. The frequency set is similar to POSSUM/GALFACTS with a 300 MHz bandwidth from 1.1 to 1.4 GHz. We define three figures of merit motivated by the underlying science: (1) an average RM weighted by polarized intensity, , (2) the separation of two Faraday components, and (3) the reduced chi-squared . Based on the current test data with a signal-to-noise ratio of about 32, we find the following. (1) When only one Faraday thin component is present, most methods perform as expected, with occasional failures where two components are incorrectly found. (2) For two Faraday thin components, QU-fitting routines perform the best, with errors close to the theoretical ones for but with significantly higher errors for . All other methods, including standard Faraday synthesis, frequently identify only one component when is below or near the width of the Faraday point-spread function. (3) No methods as currently implemented work well for Faraday thick components due to the narrow bandwidth. (4) There exist combinations of two Faraday components that produce a large range of acceptable fits and hence large uncertainties in the derived single RMs; in these cases, different RMs lead to the same behavior, so no method can recover a unique input model. Further exploration of all these issues is required before upcoming surveys will be able to provide reliable results on Faraday structures.