Zero patterns and unitary similarity

A subspace of the space, L(n), of traceless complex $n\times n$ matrices can be specified by requiring that the entries at some positions $(i,j)$ be zero. The set, $I$, of these positions is a (zero) pattern and the corresponding subspace of L(n) is denoted by $L_I(n)$. A pattern $I$ is universal if every matrix in L(n) is unitarily similar to some matrix in $L_I(n)$. The problem of describing the universal patterns is raised, solved in full for $n\le3$, and partial results obtained for $n=4$. Two infinite families of universal patterns are constructed. They give two analogues of Schur's triangularization theorem.Comment: 39 page

Four hot DOGs eaten up with the EVN

Hot dust-obscured galaxies (hot DOGs) are a rare class of hyperluminous infrared galaxies recently identified with the Wide-field Infrared Survey Explorer (WISE) satellite. The majority of the ~1000-member all-sky population should be at high redshifts (z~2-3), at the peak of star formation in the history of the Universe. This class most likely represents a short phase during galaxy merging and evolution, a transition from starburst- to AGN-dominated phases. For the first time, we observed four hot DOGs with known mJy-level radio emission using the European VLBI Network (EVN) at 1.7 GHz, in a hope to find compact radio features characteristic to AGN activity. All four target sources are detected at ~15-30 mas angular resolution, confirming the presence of an active nucleus. The sources are spatially resolved, i.e. the flux density of the VLBI-detected components is smaller than the total flux density, suggesting that a fraction of the radio emission originates from larger-scale (partly starburst-related) activity. Here we show the preliminary results of our e-EVN observations made in 2014 February, and discuss WISE J1814+3412, an object with kpc-scale symmetric radio structure, in more detail.Comment: 6 pages, 1 figure; appears in the proceedings of the 12th European VLBI Network Symposium and Users Meeting (7-10 October 2014, Cagliari, Italy), eds. A. Tarchi, M. Giroletti & L. Feretti. JREF Proceedings of Science, PoS(EVN 2014)003, http://pos.sissa.it/archive/conferences/230/003/EVN%202014_003.pd

Four hot DOGs in the microwave

Hot dust-obscured galaxies (hot DOGs) are a rare class of hyperluminous infrared galaxies identified with the Wide-field Infrared Survey Explorer (WISE) satellite. The majority of them is at high redshifts (z~2-3), at the peak epoch of star formation in the Universe. Infrared, optical, radio, and X-ray data suggest that hot DOGs contain heavily obscured, extremely luminous active galactic nuclei (AGN). This class may represent a short phase in the life of the galaxies, signifying the transition from starburst- to AGN-dominated phases. Hot DOGs are typically radio-quiet, but some of them show mJy-level emission in the radio (microwave) band. We observed four hot DOGs using the technique of very long baseline interferometry (VLBI). The 1.7-GHz observations with the European VLBI Network (EVN) revealed weak radio features in all sources. The radio is free from dust obscuration and, at such high redshifts, VLBI is sensitive only to compact structures that are characteristic of AGN activity. In two cases (WISE J0757+5113, WISE J1603+2745), the flux density of the VLBI-detected components is much smaller than the total flux density, suggesting that ~70-90 per cent of the radio emission, while still dominated by AGN, originates from angular scales larger than probed by the EVN. The source WISE J1146+4129 appears a candidate compact symmetric object, and WISE J1814+3412 shows a 5.1-kpc double structure, reminiscent of hot spots in a medium-sized symmetric object. Our observations support that AGN residing in hot DOGs may be genuine young radio sources where starburst and AGN activities coexist.Comment: 8 pages, 4 tables, 1 figure; accepted for publication in the Monthly Notices of the Royal Astronomical Societ

Two in one? A possible dual radio-emitting nucleus in the quasar SDSS J1425+3231

The radio-emitting quasar SDSS J1425+3231 (z=0.478) was recently found to have double-peaked narrow [O III] optical emission lines. Based on the analysis of the optical spectrum, Peng et al. (2011) suggested that this object harbours a dual active galactic nucleus (AGN) system, with two supermassive black holes (SMBHs) separated on the kpc scale. SMBH pairs should be ubiquitous according to hierarchical galaxy formation scenarios in which the host galaxies and their central black holes grow together via interactions and eventual mergers. Yet the number of presently-confirmed dual SMBHs on kpc or smaller scales remains small. A possible way to obtain direct observational evidence for duality is to conduct high-resolution radio interferometric measurements, provided that both AGN are in an evolutionary phase when some activity is going on in the radio. We used the technique of Very Long Baseline Interferometry (VLBI) to image SDSS J1425+3231. Observations made with the European VLBI Network (EVN) at 1.7 GHz and 5 GHz frequencies in 2011 revealed compact radio emission at sub-mJy flux density levels from two components with a projected linear separation of \sim2.6 kpc. These two components support the possibility of a dual AGN system. The weaker component remained undetected at 5 GHz, due to its steep radio spectrum. Further study will be necessary to securely rule out a jet--shock interpretation of the less dominant compact radio source. Assuming the dual AGN interpretation, we discuss black hole masses, luminosities, and accretion rates of the two components, using available X-ray, optical, and radio data. While high-resolution radio interferometric imaging is not an efficient technique to search blindly for dual AGN, it is an invaluable tool to confirm the existence of selected candidates.Comment: 7 pages, 2 figures. Accepted for publication in Monthly Notices of the Royal Astronomical Societ

Four dual AGN candidates observed with the VLBA

According to hierarchical structure formation models, merging galaxies are expected to be seen in different stages of their coalescence. However, currently there are no straightforward observational methods neither to select nor to confirm a large number of dual active galactic nuclei (AGN) candidates. Most attempts involve the better understanding of double-peaked narrow emission line sources, to distinguish the objects where the emission lines originate from narrow-line kinematics or jet-driven outflows from those which might harbour dual AGN. We observed four such candidate sources with the Very Long Baseline Array (VLBA) at 1.5 GHz with $\sim$ 10 milli-arcsecond angular resolution where spectral profiles of AGN optical emission suggested the existence of dual AGN. In SDSS J210449.13-000919.1 and SDSS J23044.82-093345.3, the radio structures are aligned with the optical emission features, thus the double-peaked emission lines might be the results of jet-driven outflows. In the third detected source SDSS J115523.74+150756.9, the radio structure is less extended and oriented nearly perpendicular to the position angle derived from optical spectroscopy. The fourth source remained undetected with the VLBA but it has been imaged with the Very Large Array at arcsec resolution a few months before our observations, suggesting the existence of extended radio structure. In none of the four sources did we detect two radio-emitting cores, a convincing signature of duality.Comment: 35 pages, 3 figures, 2 tables, accepted for publication in Ap

Parsec-scale jet properties of the gamma-ray quasar 3C 286

The quasar 3C~286 is one of two compact steep spectrum sources detected by the {\it Fermi}/LAT. Here, we investigate the radio properties of the parsec(pc)-scale jet and its (possible) association with the $\gamma$-ray emission in 3C~286. The Very Long Baseline Interferometry (VLBI) images at various frequencies reveal a one-sided core--jet structure extending to the southwest at a projected distance of $\sim$1 kpc. The component at the jet base showing an inverted spectrum is identified as the core, with a mean brightness temperature of $2.8\times 10^{9}$~K. The jet bends at about 600 pc (in projection) away from the core, from a position angle of $-135^\circ$ to $-115^\circ$. Based on the available VLBI data, we inferred the proper motion speed of the inner jet as $0.013 \pm 0.011$ mas yr$^{-1}$ ($\beta_{\rm app} = 0.6 \pm 0.5$), corresponding to a jet speed of about $0.5\,c$ at an inclination angle of $48^\circ$ between the jet and the line of sight of the observer. The brightness temperature, jet speed and Lorentz factor are much lower than those of $\gamma$-ray-emitting blazars, implying that the pc-scale jet in 3C~286 is mildly relativistic. Unlike blazars in which $\gamma$-ray emission is in general thought to originate from the beamed innermost jet, the location and mechanism of $\gamma$-ray emission in 3C~286 may be different as indicated by the current radio data. Multi-band spectrum fitting may offer a complementary diagnostic clue of the $\gamma$-ray production mechanism in this source.Comment: 9 pages, 4 figures, accept for publication in MNRA

The polymeric conformational effect on capacitive deionization performance of graphene oxide/polypyrrole composite electrode

Exploitation of novel faradic materials is an alternative implementation for solving the problem of poor specific electrosorption capacity that conventional carbon materials are encountered in capacitive deionization. Particularly, composite electrode is just a suitable choice because of its potentially high ion-storage ability. Herein, a cyclic voltammetric treatment method with different low limit of potential window was used to manipulate the polymeric conformation and doping level of graphene oxide/polypyrrole (GO/PPy) composite electrode. Based on it, the effect of polymeric structure on the electrosorption performance was systematically studied. When the low limit of potential window is shifted negatively enough, the irreversible polymeric conformational shrinks of GO/PPy are promoted, which not only hinders the insertion process of ions, but also decreases the doping level of polymer due to the intensive interchain-action produced by more entangled polymeric chain. Thus, the number of intercalated ions should decrease, which is expressed by electrochemical impedance spectroscopy (EIS) results and is proportional to the electrosorption capacity of GO/PPy composite electrode in membrane capacitive deionization (MCDI) process. Our work suggests that the less packing density, higher doping level and more charge delocalization on PPy backbone in electrode are beneficial to enhance its capacitive deionization performance