Infrared-Faint Radio Sources: A New Population of High-redshift Radio Galaxies

We present a sample of 1317 Infrared-Faint Radio Sources (IFRSs) that, for the first time, are reliably detected in the infrared, generated by cross-correlating the Wide-Field Infrared Survey Explorer (WISE) all-sky survey with major radio surveys. Our IFRSs are brighter in both radio and infrared than the first generation IFRSs that were undetected in the infrared by the Spitzer Space Telescope. We present the first spectroscopic redshifts of IFRSs, and find that all but one of the IFRSs with spectroscopy has z > 2. We also report the first X-ray counterparts of IFRSs, and present an analysis of radio spectra and polarization, and show that they include Gigahertz-Peaked Spectrum, Compact Steep Spectrum, and Ultra-Steep Spectrum sources. These results, together with their WISE infrared colours and radio morphologies, imply that our sample of IFRSs represents a population of radio-loud Active Galactic Nuclei at z > 2. We conclude that our sample consists of lower-redshift counterparts of the extreme first generation IFRSs, suggesting that the fainter IFRSs are at even higher redshift.Comment: 23 pages, 17 figures. Submitted to MNRA

Estimating extragalactic Faraday rotation

(abridged) Observations of Faraday rotation for extragalactic sources probe magnetic fields both inside and outside the Milky Way. Building on our earlier estimate of the Galactic contribution, we set out to estimate the extragalactic contributions. We discuss the problems involved; in particular, we point out that taking the difference between the observed values and the Galactic foreground reconstruction is not a good estimate for the extragalactic contributions. We point out a degeneracy between the contributions to the observed values due to extragalactic magnetic fields and observational noise and comment on the dangers of over-interpreting an estimate without taking into account its uncertainty information. To overcome these difficulties, we develop an extended reconstruction algorithm based on the assumption that the observational uncertainties are accurately described for a subset of the data, which can overcome the degeneracy with the extragalactic contributions. We present a probabilistic derivation of the algorithm and demonstrate its performance using a simulation, yielding a high quality reconstruction of the Galactic Faraday rotation foreground, a precise estimate of the typical extragalactic contribution, and a well-defined probabilistic description of the extragalactic contribution for each data point. We then apply this reconstruction technique to a catalog of Faraday rotation observations. We vary our assumptions about the data, showing that the dispersion of extragalactic contributions to observed Faraday depths is most likely lower than 7 rad/m^2, in agreement with earlier results, and that the extragalactic contribution to an individual data point is poorly constrained by the data in most cases.Comment: 20 + 6 pages, 19 figures; minor changes after bug-fix; version accepted for publication by A&A; results are available at http://www.mpa-garching.mpg.de/ift/faraday

Electrons in the supernova-driven interstellar medium

Context. Interstellar gas is in a highly turbulent dynamic state driven by successive supernova explosions and stellar winds, while its electron distribution is determined by microscopic processes such as ionization and recombination. In order to understand the properties of the electrons in the interstellar medium (ISM) it is necessary to follow numerically the nonlinear spatial and temporal evolution of the gas, its ionization structure, and its emission properties. Aims. We study the time evolution of the electrons in the ISM and how line of sight observations compare to volume analysis of the simulated medium populated with atoms and ions of the ten most abundant species. In particular, we make quantitative predictions about the occupation fractions and averaged densities of electrons, the dispersion measures, and their vantage point dependence. Methods. We carried out state-of-the-art adaptive mesh refinement simulations of the supernova-driven interstellar gas tracing the evolution of 112 ions and atoms of H, He, C, N, O, Ne, Mg, Si, S, and Fe and their emissivities in a time-dependent fashion. The gas is followed with the magnetohydrodynamical adaptive mesh refinement parallel code coupled with the Collisional + Photo Ionization Plasma Emission Software to trace the ionic structure and radiative emission of the plasma. Results. We show that more than 60% of the electrons are in thermally unstable regimes: about 50% at 200 < T ≤ 103.9 K and 14% at 104.2 < T ≤ 105.5 K. The probability density functions for the electron distribution in different temperature regimes is rather broad, also a result of turbulence in the ISM. Comparing the calculated dispersion measures along different lines of sight to observation, we find a very good agreement. They increase linearly for distances greater than 300 pc from the observer at an average rate of 27 cm−3 pc per kpc. The dispersion regarding the average dispersion measures does not decrease with distance along the line of sight, pointing to a high clumpiness of the electrons and of the turbulent ISM. The mean electron density in the Galactic midplane derived from the volume analysis varies between 0.029 and 0.031 cm−3, while that derived from the dispersion measures, varies between 0.0264 and 0.03 cm−3 depending on the vantage point and on the time averaged period. These variations can be as high as 8.3% between vantage points

Electrons in the supernova-driven interstellar medium

Context.Interstellar gas is in a highly turbulent dynamic state driven by successive supernova explosions and stellar winds, while its electron distribution is determined by microscopic processes such as ionization and recombination. In order to understand the properties of the electrons in the interstellar medium (ISM) it is necessary to follow numerically the nonlinear spatial and temporal evolution of the gas, its ionization structure, and its emission properties.Aims.We study the time evolution of the electrons in the ISM and how line of sight observations compare to volume analysis of the simulated medium populated with atoms and ions of the ten most abundant species. In particular, we make quantitative predictions about the occupation fractions and averaged densities of electrons, the dispersion measures, and their vantage point dependence.Methods.We carried out state-of-the-art adaptive mesh refinement simulations of the supernova-driven interstellar gas tracing the evolution of 112 ions and atoms of H, He, C, N, O, Ne, Mg, Si, S, and Fe and their emissivities in a time-dependent fashion. The gas is followed with the magnetohydrodynamical adaptive mesh refinement parallel code coupled with the Collisional + Photo Ionization Plasma Emission Software to trace the ionic structure and radiative emission of the plasma.Results.We show that more than 60% of the electrons are in thermally unstable regimes: about 50% at 200 &lt;T≤ 103.9K and 14% at 104.2&lt;T≤ 105.5K. The probability density functions for the electron distribution in different temperature regimes is rather broad, also a result of turbulence in the ISM. Comparing the calculated dispersion measures along different lines of sight to observation, we find a very good agreement. They increase linearly for distances greater than 300 pc from the observer at an average rate of 27 cm−3pc per kpc. The dispersion regarding the average dispersion measures does not decrease with distance along the line of sight, pointing to a high clumpiness of the electrons and of the turbulent ISM. The mean electron density in the Galactic midplane derived from the volume analysis varies between 0.029 and 0.031 cm−3, while that derived from the dispersion measures, varies between 0.0264 and 0.03 cm−3depending on the vantage point and on the time averaged period. These variations can be as high as 8.3% between vantage points.</jats:p