A Search for Nitrogen Enriched Quasars in the Sloan Digital Sky Survey Early Data Release

A search for nitrogen-rich quasars in the Sloan Digital Sky Survey Early Data Release (SDSS EDR) catalog has yielded 16 candidates, including five with very prominent emission, but no cases with nitrogen emission as strong as in Q0353-383. The quasar Q0353-383 has long been known to have extremely strong nitrogen intercombination lines at lambda 1486 and lambda 1750 Angstroms, implying an anomalously high nitrogen abundance of about 15 times solar. It is still the only one of its kind known. A preliminary search through the EDR using the observed property of the weak C IV emission seen in Q0353-383 resulted in a sample of 23 objects with unusual emission or absorption-line properties, including one very luminous redshift 2.5 star-forming galaxy. We present descriptions, preliminary emission-line measurements, and spectra for all the objects discussed here.Comment: 20 pages, 5 figures, submitted to AJ; final refereed versio

Measurement of the Integrated Faraday Rotations of BL Lac Objects

We present the results of multi-frequency polarization VLA observations of radio sources from the complete sample of northern, radio-bright BL Lac objects compiled by H. Kuhr and G. Schmidt. These were used to determine the integrated rotation measures of 18 objects, 15 of which had never been measured previously, which hindered analysis of the intrinsic polarization properties of objects in the complete sample. These measurements make it possible to correct the observed orientations of the linear polarizations of these sources for the effect of Faraday rotation. The most probable origin for Faraday rotation in these objects is the Galactic interstellar medium. The results presented complete measurements of the integrated rotation measures for all 34 sources in the complete sample of BL Lac objects.Comment: 9 pages, 7 figure

Determination of complex dielectric functions of ion implanted and implanted‐annealed amorphous silicon by spectroscopic ellipsometry

Measuring with a spectroscopic ellipsometer (SE) in the 1.8–4.5 eV photon energy region we determined the complex dielectric function (ϵ = ϵ1 + iϵ2) of different kinds of amorphous silicon prepared by self‐implantation and thermal relaxation (500 °C, 3 h). These measurements show that the complex dielectric function (and thus the complex refractive index) of implanted a‐Si (i‐a‐Si) differs from that of relaxed (annealed) a‐Si (r‐a‐Si). Moreover, its ϵ differs from the ϵ of evaporated a‐Si (e‐a‐Si) found in the handbooks as ϵ for a‐Si. If we use this ϵ to evaluate SE measurements of ion implanted silicon then the fit is very poor. We deduced the optical band gap of these materials using the Davis–Mott plot based on the relation: (ϵ2E2)1/3 ∼ (E− Eg). The results are: 0.85 eV (i‐a‐Si), 1.12 eV (e‐a‐Si), 1.30 eV (r‐a‐Si). We attribute the optical change to annihilation of point defects

Multi-frequency polarimetry of the Galactic radio background around 350 MHz: I. A region in Auriga around l = 161, b = 16

With the Westerbork Synthesis Radio Telescope (WSRT), multi-frequency polarimetric images were taken of the diffuse radio synchrotron background in a region centered on (l,b) = (161,16). The observations were done simultaneously in 5 frequency bands from 341 MHz to 375 MHz, with 5 arcmin resolution. Ubiquitous structure on arcminute and degree scales in the polarized intensity and polarization angle, combined with no observed structure in total intensity, indicates that the structure in the polarized radiation must be due to Faraday rotation and depolarization mostly in the warm nearby Galactic interstellar medium (ISM). Beam depolarization most likely creates "depolarization canals" of one beam wide, while depth depolarization is responsible for creating most of the structure on scales larger than a beam width. Rotation measures RM are in the range -17 < RM < 10 rad/m2 with a non-zero average of about -3.4 rad/m2. The gradient and average RM are consistent with a regular magnetic field of about 1 uG which has a pitch angle of p = -14 degrees. 13 Extragalactic sources in the field have |RM| < 13 rad/m2, with an estimated intrinsic source contribution of 3.6 rad/m2. The RMs of the extragalactic sources show a gradient (with a sign reversal) that is about 3 times larger than the gradient in the RMs of the diffuse emission, and that is approximately in Galactic latitude. This difference is ascribed to a vastly different effective length of the line of sight. The observations are interpreted in terms of a single-cell-size model of the warm ISM which contains gas and magnetic fields, with a polarized background.Comment: 15 pages, 14 figures, to be published in A&

Magnetic fields in the absence of spiral density waves - NGC 4414

We present three-frequency VLA observations of the flocculent spiral galaxy NGC 4414 made in order to study the magnetic field structure in absence of strong density wave flows. NGC 4414 shows a regular spiral pattern of observed polarization B-vectors with a radial component comparable in strength to the azimuthal one. The average pitch angle of the magnetic field is about 20\degr, similar to galaxies with a well-defined spiral pattern. This provides support for field generation by a turbulent dynamo without significant ``contamination'' from streaming motions in spiral arms. While the stellar light is very axisymmetric, the magnetic field structure shows a clear asymmetry with a stronger regular field and a smaller magnetic pitch angle in the northern disk. Extremely strong Faraday rotation is measured in the southern part of the disk, becoming Faraday thick at 6cm. The distribution of Faraday rotation suggests a mixture of axisymmetric and higher-mode magnetic fields. The strong Faraday effects in the southern region suggest a much thicker magnetoionic disk and a higher content of diffuse ionized gas than in the northern disk portion. An elongation of the 20cm total power emission is also seen towards the South. Although NGC 4414 is currently an isolated spiral, the asymmetries in the polarized radio emission may be sensitive tracers of previous encounters, including weak interactions which would chiefly affect the diffuse gas component without generating obvious long-term perturbations in the optical structure.Comment: 12 pages, 14 figures, A&A accepte

Properties of the warm magnetized ISM, as inferred from WSRT polarimetric imaging

We describe a first attempt to derive properties of the regular and turbulent Galactic magnetic field from multi-frequency polarimetric observations of the diffuse Galactic synchrotron background. A single-cell-size model of the thin Galactic disk is constructed which includes random and regular magnetic fields and thermal and relativistic electrons. The disk is irradiated from behind with a uniform partially polarized background. Radiation from the background and from the thin disk is Faraday rotated and depolarized while propagating through the medium. The model parameters are estimated from a comparison with 350 MHz observations in two regions at intermediate latitudes done with the Westerbork Synthesis Radio Telescope. We obtain good consistency between the estimates for the random and regular magnetic field strengths and typical scales of structure in the two regions. The regular magnetic field strength found is a few microGauss, and the ratio of random to regular magnetic field strength is 0.7 +/- 0.5, for a typical scale of the random component of 15 +/- 10 pc. Furthermore, the regular magnetic field is directed almost perpendicular to the line of sight. This modeling is a potentially powerful method to estimate the structure of the Galactic magnetic field, especially when more polarimetric observations of the diffuse synchrotron background at intermediate latitudes become available.Comment: 12 pages, 6 figures, accepted by A&

Magnetic field structures of galaxies derived from analysis of Faraday rotation measures, and perspectives for the SKA

The forthcoming new-generation radio telescope SKA (Square Kilometre Array) and its precursors will provide a rapidly growing number of polarized radio sources. Our analysis aims on what can be learned from these sources concerning the structure and evolution of magnetic fields of external galaxies. Recognition of magnetic structures is possible from Faraday rotation measures (${\rm RM}$) towards background sources behind galaxies. We construct models for the ionized gas and magnetic field patterns of different azimuthal symmetry (axisymmetric, bisymmetric and quadrisymmetric spiral, and superpositions) plus a halo magnetic field. \RM fluctuations with a Kolmogorov spectrum due to turbulent fields and/or fluctuations in ionized gas density are superimposed. Assuming extrapolated number density counts of polarized sources, we generate a sample of \RM values within the solid angle of the galaxy. Applying various templates, we derive the minimum number of background sources and the minimum quality of the observations. For a large number of sources, reconstruction of the field structure without precognition becomes possible. Any large-scale regular component of the magnetic field can be clearly recognized from \RM data with help of the $\chi^2$ criterium. Under favourite conditions, about a few dozens of polarized sources are sufficient for a reliable result.Comment: 16 pages, 18 figures, accepted for publication in A&

A2255: the First Detection of Filamentary Polarized Emission in a Radio Halo

A deep radio observation of the A2255 cluster of galaxies has been carried out at 1.4 GHz with the Very Large Array synthesis telescope. Thanks to the excellent (u,v) coverage and sensitivity achieved by our observation, the low brightness diffuse extended sources in the cluster (radio halo and relic) have been imaged with unprecedented resolution and dynamic range. We find that the radio halo has filamentary structures that are strongly polarized. The fractional linear polarization reaches levels of $\simeq$ 20$-$40% and the magnetic fields appear ordered on scales of $\sim$400 kpc. This is the first successful attempt to detect polarized emission from a radio halo and provides strong evidence that in this cluster the magnetic field is ordered on large scales.Comment: 4 pages, 3 figures, accepted for publication in Astronomy and Astrophysics Letter

Revealing the magnetic field in a distant galaxy cluster: discovery of the complex radio emission from MACS J0717.5 +3745

Aims. To study at multiple frequencies the radio emission arising from the massive galaxy cluster MACS J0717.5+3745 (z=0.55). Known to be an extremely complex cluster merger, the system is uniquely suited for an investigation of the phenomena at work in the intra-cluster medium (ICM) during cluster collisions. Methods. We use multi-frequency and multi-resolution data obtained with the Very Large Array radio telescope, and X-ray features revealed by Chandra, to probe the non-thermal and thermal components of the ICM, their relations and interactions. Results. The cluster shows highly complex radio emission. A bright, giant radio halo is detected at frequencies as high as 4.8 GHz. MACS J0717.5+3745 is the most distant cluster currently known to host a radio halo. This radio halo is also the most powerful ever observed, and the second case for which polarized radio emission has been detected, indicating that the magnetic field is ordered on large scales.Comment: 14 pages, 13 figures, Astronomy and Astrophysics, accepte