154 research outputs found
Is an obscured AGN at the centre of the disk galaxy IC 2497 responsible for Hanny's Voorwerp?
We present the results of VLBI and MERLIN observations of the massive disk
galaxy IC 2497. Optical observations of IC 2497 revealed the existence of a
giant emission nebula "Hanny's Voorwerp" in the proximity of the galaxy.
Earlier short-track 18 cm observations with e-VLBI at 18 cm, detected a compact
radio component (C1) at the centre of IC 2497. The brightness temperature of C1
was measured to be greater than 4E5 K. Deeper, long-track e-VLBI observations
presented here, re-confirm the existence of C1 but also reveal the existence of
a second compact component (C2) located about 230 milliarcseconds to the
North-East of C1. The brightness temperature of C2 is measured to be greater
than 1.4E5 K, suggesting that both components may be related to AGN activity
(e.g. a radio core and jet hotspot). Lower resolution 18cm MERLIN observations
show both components. C1 is shown to be compact with a slight elongation along
the direction of Hanny's Voorwerp, and C2 shows a lot of extended emission in
an almost perpendicular direction to the direction of the Voorwerp. Our results
continue to support the hypothesis that IC 2497 contains an Active Galactic
Nucleus (AGN), and that a jet associated with this AGN clears a path that
permits ionising radiation from the AGN to directly illuminate the emission
nebula.Comment: Presented at The 8th International e-VLBI Workshop: the Science and
Technology of Long Baseline Real-Time Interferometry, EXPReS09, June 22-26
2009 Madrid, Spain. 5 pages, 5 article
Controlling the efficiency of spin injection into graphene by carrier drift
Electrical spin injection from ferromagnetic metals into graphene is hindered
by the impedance mismatch between the two materials. This problem can be
reduced by the introduction of a thin tunnel barrier at the interface. We
present room temperature non-local spin valve measurements in
cobalt/aluminum-oxide/graphene structures with an injection efficiency as high
as 25%, where electrical contact is achieved through relatively transparent
pinholes in the oxide. This value is further enhanced to 43% by applying a DC
current bias on the injector electrodes, that causes carrier drift away from
the contact. A reverse bias reduces the AC spin valve signal to zero or
negative values. We introduce a model that quantitatively predicts the behavior
of the spin accumulation in the graphene under such circumstances, showing a
good agreement with our measurements.Comment: 4 pages, 3 color figure
Linear scaling between momentum and spin scattering in graphene
Spin transport in graphene carries the potential of a long spin diffusion
length at room temperature. However, extrinsic relaxation processes limit the
current experimental values to 1-2 um. We present Hanle spin precession
measurements in gated lateral spin valve devices in the low to high (up to
10^13 cm^-2) carrier density range of graphene. A linear scaling between the
spin diffusion length and the diffusion coefficient is observed. We measure
nearly identical spin- and charge diffusion coefficients indicating that
electron-electron interactions are relatively weak and transport is limited by
impurity potential scattering. When extrapolated to the maximum carrier
mobilities of 2x10^5 cm^2/Vs, our results predict that a considerable increase
in the spin diffusion length should be possible
A GBT Survey of the HALOGAS Galaxies and Their Environments I: Revealing the full extent of HI around NGC891, NGC925, NGC4414 & NGC4565
We present initial results from a deep neutral hydrogen (HI) survey of the
HALOGAS galaxy sample, which includes the spiral galaxies NGC891, NGC925,
NGC4414, and NGC4565, performed with the Robert C. Byrd Green Bank Telescope
(GBT). The resulting observations cover at least four deg around these
galaxies with an average 5 detection limit of 1.210
cm over a velocity range of 20 km s and angular scale of 9.1.
In addition to detecting the same total flux as the GBT data, the spatial
distribution of the GBT and original Westerbork Synthesis Radio Telescope
(WSRT) data match well at equal spatial resolutions. The HI mass fraction below
HI column densities of 10 cm is, on average, 2\%. We discuss the
possible origins of low column density HI of nearby spiral galaxies. The
absence of a considerable amount of newly detected HI by the GBT indicates
these galaxies do not have significant extended diffuse HI structures, and
suggests future surveys planned with the SKA and its precursors must go
\textit{at least} as deep as 10 cm in column density to
significantly increase the probability of detecting HI associated with the
cosmic web and/or cold mode accretion.Comment: Accepted for publication in The Astrophysical Journal; 28 pages, 15
figure
Towards a Full Census of the Obscure(d) Vela Supercluster using MeerKAT
Recent spectroscopic observations of a few thousand partially obscured
galaxies in the Vela constellation revealed a massive overdensity on
supercluster scales straddling the Galactic Equator (l 272.5deg) at km/s. It remained unrecognised because it is located just beyond the
boundaries and volumes of systematic whole-sky redshift and peculiar velocity
surveys - and is obscured by the Milky Way. The structure lies close to the
apex where residual bulkflows suggest considerable mass excess. The uncovered
Vela Supercluster (VSCL) conforms of a confluence of merging walls, but its
core remains uncharted. At the thickest foreground dust column densities (|b| <
6 deg) galaxies are not visible and optical spectroscopy is not effective. This
precludes a reliable estimate of the mass of VSCL, hence its effect on the
cosmic flow field and the peculiar velocity of the Local Group. Only systematic
HI-surveys can bridge that gap. We have run simulations and will present
early-science observing scenarios with MeerKAT 32 (M32) to complete the census
of this dynamically and cosmologically relevant supercluster. M32 has been put
forward because this pilot project will also serve as precursor project for HI
MeerKAT Large Survey Projects, like Fornax and Laduma. Our calculations have
shown that a survey area of the fully obscured part of the supercluster, where
the two walls cross and the potential core of the supercluster resides, can be
achieved on reasonable time-scales (200 hrs) with M32.Comment: 10 pages, 3 figures, accepted for publication, Proceedings of
Science, workshop on "MeerKAT Science: On the Pathway to the SKA", held in
Stellenbosch 25-27 May 201
Electronic spin transport in graphene field effect transistors
Spin transport experiments in graphene, a single layer of carbon atoms,
indicate spin relaxation times that are significantly shorter than the
theoretical predictions. We investigate experimentally whether these short spin
relaxation times are due to extrinsic factors, such as spin relaxation caused
by low impedance contacts, enhanced spin flip processes at the device edges or
the presence of an aluminium oxide layer on top of graphene in some samples.
Lateral spin valve devices using a field effect transistor geometry allowed for
the investigation of the spin relaxation as a function of the charge density,
going continuously from metallic hole to electron conduction (charge densities
of cm) via the Dirac charge neutrality point (). The results are quantitatively described by a one dimensional spin
diffusion model where the spin relaxation via the contacts is taken into
account. Spin valve experiments for various injector/detector separations and
spin precession experiments reveal that the longitudinal (T) and the
transversal (T) relaxation times are similar. The anisotropy of the spin
relaxation times and , when the spins are injected
parallel or perpendicular to the graphene plane, indicates that the effective
spin orbit fields do not lie exclusively in the two dimensional graphene plane.
Furthermore, the proportionality between the spin relaxation time and the
momentum relaxation time indicates that the spin relaxation mechanism is of the
Elliott-Yafet type. For carrier mobilities of 2-5 cm2^/Vs and
for graphene flakes of 0.1-2 m in width, we found spin relaxation times of
the order of 50-200 ps, times which appear not to be determined by the
extrinsic factors mentioned above.Comment: 11 pages, 13 figure
HALOGAS observations of NGC 5023 and UGC 2082: Modeling of non-cylindrically symmetric gas distributions in edge-on galaxies
In recent years it has become clear that the vertical structure of disk
galaxies is a key ingredient for understanding galaxy evolution. In particular,
the presence and structure of extra-planar gas has been a focus of research.
The Hydrogen Accretion in LOcal GAlaxieS (HALOGAS) survey aims to provide a
census on the rate of cold neutral gas accretion in nearby galaxies as well as
a statistically significant set of galaxies that can be investigated for their
extra-planar gas properties.
In order to better understand the the vertical structure of the neutral
hydrogen in the two edge-on HALOGAS galaxies NGC 5023 and UGC 2082 we construct
detailed tilted ring models. The addition of distortions resembling arcs or
spiral arms significantly improves the fit of the models to these galaxies. In
the case of UGC 2082 no vertical gradient in rotational velocity is required in
either symmetric models nor non-symmetric models to match the observations. The
best fitting model features two arcs of large vertical extent that may be due
to accretion. In the case of NGC 5023 a vertical gradient is required in
symmetric models (dV/dz = km s kpc) and its
magnitude is significantly lowered when non-symmetric models are considered
(dV/dz = km s kpc). Additionally it is shown that the
underlying disk of NGC 5023 can be made symmetric, in all parameters except the
warp, in non-symmetric models. In comparison to the "classical" modeling these
models fit the data significantly better with a limited addition of free
parameters.Comment: 27 Pages, 22 Figures. Accepted for publication in MNRA
USE OF GROWTH ANALYSIS TO EVALUATE GENETIC MECHANISMS AFFECTING ACHENE YIELD FORMATION OF SUNFLOWER
The main objective of this study was to investigate the process of dry matter accumulation (DMA) in achenes during the grain-filling period of fifty sunflower genotypes by using the functional method of growth analysis in a field trial at Bicsérd, Hungary. The Hunt-formula of lnY= P0 + P1*X + P2*X2 was fitted to data. Maximum yield (Ymax), the average of the absolute growth rate (AGRavg), maximum growth rate (AGRmax), date of the maximum growth rate (Xagrmax), and the average of the relative growth rate (RGRavg) were calculated from growth curves for hybrids and replications. Significant differences among hybrids and their interaction with sampling dates indicate hybrid differences in the intensity of DMA
accumulation. The strongest correlation was observed between the parameters of Ymax and AGRmax
H I content in Coma cluster substructure
Context. Galaxy clusters are some of largest structures in the universe. These very dense environments tend to be home to higher numbers of evolved galaxies than found in lower-density environments. It is well known that dense environments can influence the evolution of galaxies through the removal of the neutral gas (H I) reservoirs that fuel star formation. It is unclear which environment has a stronger effect: the local environment (i.e., the substructure within the cluster), or the cluster itself. Aims: Using the new H I data from the Westerbork Coma Survey, we explore the average H I content of galaxies across the cluster comparing galaxies that reside in substructure to those that do not. Methods: We applied the Dressler-Shectman test to our newly compiled redshift catalogue of the Coma cluster to search for substructure. With so few of the Coma galaxies directly detected in H I, we used the H I stacking technique to probe the average H I content below what can be directly detected. Results: Using the Dressler-Shectman test, we find 15 substructures within the footprint of the Westerbork Coma Survey. We compare the average H I content for galaxies within substructure to those not in substructure. Using the H I stacking technique, we find that those Coma galaxies not detected in H I are more than 10-50 times more H I deficient than expected, which supports the scenario of an extremely efficient and rapid quenching mechanism. By studying the galaxies that are not directly detected in H I, we also find Coma to be more H I deficient than previously thought. Full Table C.1 is only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/650/A7
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