A Comparison of Radio and X-Ray Morphologies of Four Clusters of Galaxies Containing Radio Halos

Clusters of galaxies may contain cluster-wide, centrally located, diffuse radio sources, called halos. They have been found to show morphologies similar to those of the X-ray emission. To quantify this qualitative statement we performed a point-to-point comparison of the radio and the X-ray emission for four clusters of galaxies containing radio halos: Coma, Abell 2255, Abell 2319, Abell 2744. Our study leads to a linear relation between the radio and the X-ray surface brightness in two clusters, namely Abell 2255 and Abell 2744. In Coma and A2319 the radio and the X-ray brightnesses seem to be related with a sub-linear power law. Implications of these findings within simple radio halo formation models are briefly discussed.Comment: 9 pages, 13 .ps figures, accepted by A&

On the Asymmetries of Extended X-ray Emission from Planetary Nebulae

Chandra X-ray Observatory (CXO) images have revealed that the X-ray emitting regions of the molecule-rich young planetary nebulae (PNs) BD+30 3639 (BD+30) and NGC 7027 are much more asymmetric than their optical nebulosities. To evaluate the potential origins of these X-ray asymmetries, we analyze X-ray images of BD+30, NGC 7027, and another planetary nebula resolved by CXO, NGC 6543, within specific energy bands. Image resolution has been optimized by sub-pixel repositioning of individual X-ray events. The resulting subarcsecond-resolution images reveal that the soft (E < 0.7 keV) X-ray emission from BD+30 is more uniform than the harder emission, which is largely confined to the eastern rim of the optical nebula. In contrast, soft X-rays from NGC 7027 are highly localized and this PN is more axially symmetric in harder emission. The broad-band X-ray morphologies of BD+30 and NGC 7027 are highly anticorrelated with their distributions of visual extinction, as determined from high-resolution, space- and ground-based optical and infrared imaging. Hence, it is likely that the observed X-ray asymmetries of these nebulae are due in large part to the effects of nonuniform intranebular extinction. However, the energy-dependent X-ray structures in both nebulae and in NGC 6543 -- which is by far the least dusty and molecule-rich of the three PNs, and displays very uniform intranebular extinction -- suggests that other mechanisms, such as the action of collimated outflows and heat conduction, are also important in determining the detailed X-ray morphologies of young planetary nebulae.Comment: 33 pages, 12 figures; to appear in the Astrophysical Journa

Grains and grain boundaries in highly crystalline monolayer molybdenum disulfide

Recent progress in large-area synthesis of monolayer molybdenum disulfide, a new two-dimensional direct-bandgap semiconductor, is paving the way for applications in atomically thin electronics. Little is known, however, about the microstructure of this material. Here we have refined chemical vapor deposition synthesis to grow highly crystalline islands of monolayer molybdenum disulfide up to 120 um in size with optical and electrical properties comparable or superior to exfoliated samples. Using transmission electron microscopy, we correlate lattice orientation, edge morphology, and crystallinity with island shape to demonstrate that triangular islands are single crystals. The crystals merge to form faceted tilt and mirror boundaries that are stitched together by lines of 8- and 4- membered rings. Density functional theory reveals localized mid-gap states arising from these 8-4 defects. We find that mirror boundaries cause strong photoluminescence quenching while tilt boundaries cause strong enhancement. In contrast, the boundaries only slightly increase the measured in-plane electrical conductivity

Multi-frequency VLBI observations of faint gigahertz peaked spectrum sources

We present the data and analysis of VLBI observations at 1.6, 5 and 15 GHz of a sample of faint Gigahertz Peaked Spectrum (GPS) sources selected from the Westerbork Northern Sky Survey (WENSS). The 5 GHz observations involved a global array of 16 stations and yielded data on the total sample of 47 sources. A subsample of 26 GPS sources with peak frequencies > 5 GHz and/or peak flux densities > 125 mJy was observed with the VLBA at 15 GHz. A second subsample of 29 sources, with peak frequencies <5 GHz, was observed at 1.6 GHz using a 14 station global VLBI array. In this way, 44 of the 47 sources (94%) in the sample were observed above and at or below their spectral peak. Spectral decomposition allowed us to identify 3, 11, 7, and 2 objects as compact symmetric objects, compact doubles, core-jet and complex sources respectively. However, many of the sources classified as compact double or core-jet sources show only two components making their classification rather tentative. This may explain why the strong morphological dichotomy of GPS quasars and galaxies found for radio-bright GPS sources, is not as clear in this faint sample.Comment: Latex, 18 pages, 8 figures; MNRAS, accepted. The paper, with higher quality figures, may also be obtained from http://www.ast.cam.ac.uk/~snellen . Minor comments of referee incorporate

NICMOS observations of high redshift radio galaxies: witnessing the formation of bright elliptical galaxies?

We present the results of a near infrared imaging program of a sample of 19 radio galaxies with redshift between 1.7 and 3.2, using the NICMOS Camera 2 on the Hubble Space Telescope. The galaxies were observed in H-band which, for 18 of the 19 targets, samples the rest-frame optical emission longwards of the 4000 A break. The images show a wide range of morphologies, from unresolved to compact systems, to systems with substructures such as multiple emission peaks, to systems comprised of several components spread over large areas. The morphologies of 5 of the lowest redshift targets are well represented by a de Vaucouleurs profile, consistent with them being elliptical galaxies. Their average effective radius is a factor of 2 smaller than that of z=1 3CR radio galaxies at similar restframe wavelengths. The near infrared continuum light is generally well aligned with the radio axis and the aligned light has typical V-H colors of around 3.5-4. For several galaxies where WFPC2 images were available, we computed a high resolution map of the optical-to-infrared spectral indices: all multi-component systems present net color gradients between the various clumps. We argue that in these galaxies the continuum emission has probably a stellar origin. Finally in many cases we observe nearby faint objects close to the radio sources. The number density of these faint objects is slightly higher than what is found in the deep NICMOS parallel observations of random fields. These objects also tend to be aligned with the radio sources axis, suggesting that they may be related to the presence of the AGN. (shortened astro-ph version)Comment: To appear in ApJ

Feeding AGN: new results from the NUGA survey

The NUGA project is a high-resolution (0.5''-1'') CO survey of low luminosity AGN including the full sequence of activity types (Seyferts, LINERs and transition objects). NUGA aims to systematically study the different mechanisms for gas fueling of AGNs in the Local Universe. In this paper we discuss the latest results of this recently completed survey, which now includes newly acquired subarcsec resolution observations for all targets of the sample. The large variety of circumnuclear disk morphologies found in NUGA galaxies (m=1, m=2 and stochastic instabilities) is a challenging result that urges the refinement of current dynamical models. In this paper we report on new results obtained in 4 study cases for NUGA: NGC4826, NGC7217, NGC4579 and NGC6951Comment: 4 pages, 2 figures. Contributed talk to appear in "The Interplay among Black Holes, Stars and ISM in Galactic Nuclei," Proc. IAU 222 (Gramado, Brazil), eds. Th. Storchi Bergmann, L.C. Ho, H.R. Schmit