19,519 research outputs found
Radio Observations of AGN in Low Surface Brightness Galaxies
We present preliminary results of a study of the low frequency radio
continuum emission from the nuclei of Giant Low Surface Brightness (LSB)
galaxies. We have mapped the emission and searched for extended features such
as radio lobes/jets associated with AGN activity. LSB galaxies are poor in star
formation and generally less evolved compared to nearby bright spirals. This
paper presents low frequency observations of 3 galaxies; PGC 045080 at 1.4 GHz,
610 MHz, 325MHz, UGC 1922 at 610 MHz and UGC 6614 at 610 MHz. The observations
were done with the GMRT. Radio cores as well as extended structures were
detected and mapped in all three galaxies; the extended emission may be
assocated with jets/lobes associated with AGN activity. Our results indicate
that although these galaxies are optically dim, their nuclei can host AGN that
are bright in the radio domain.Comment: To appear in proceedings IAU Symp 244, 'Dark Galaxies and Lost
Baryons', June 2007, 2 pages including 1 figur
Radio Observations of the AGN and Gas in Low Surface Brightness Galaxies
LSB galaxies have low metallicities, diffuse stellar disks, and massive HI
disks. We have detected molecular gas in two giant LSB galaxies, UGC 6614 and
F568-6. A millimeter continuum source has been detected in UGC 6614 as well. At
centimeter wavelengths we have detected and mapped the continuum emission from
the giant LSB galaxy 1300+0144. The emission is extended about the nucleus and
is most likely originating from the AGN in the galaxy. The HI gas distribution
and velocity field in 1300+0144 was also mapped. The HI disk extends well
beyond the optical disk and appears lopsided in the intensity maps.Comment: one page; submitted to proceedings of IAU Symposium 235: Galaxy
Evolution across the Hubble Tim
The AGN and Gas Disk in the Low Surface Brightness Galaxy PGC045080
We present radio observations and optical spectroscopy of the giant low
surface brightness (LSB) galaxy PGC 045080 (or 1300+0144). PGC 045080 is a
moderately distant galaxy having a highly inclined optical disk and massive HI
gas content. Radio continuum observations of the galaxy were carried out at 320
MHz, 610 MHz and 1.4 GHz. Continuum emission was detected and mapped in the
galaxy. The emission appears extended over the inner disk at all three
frequencies. At 1.4 GHz and 610 MHz it appears to have two distinct lobes. We
also did optical spectroscopy of the galaxy nucleus; the spectrum did not show
any strong emission lines associated with AGN activity but the presence of a
weak AGN cannot be ruled out. Furthermore, comparison of the H flux and
radio continuum at 1.4 GHz suggests that a significant fraction of the emission
is non-thermal in nature. Hence we conclude that a weak or hidden AGN may be
present in PGC 045080. The extended radio emission represents lobes/jets from
the AGN. These observations show that although LSB galaxies are metal poor and
have very little star formation, their centers can host significant AGN
activity. We also mapped the HI gas disk and velocity field in PGC 045080. The
HI disk extends well beyond the optical disk and appears warped. In the HI
intensity maps, the disk appears distinctly lopsided. The velocity field is
disturbed on the lopsided side of the disk but is fairly uniform in the other
half. We derived the HI rotation curve for the galaxy from the velocity field.
The rotation curve has a flat rotation speed of ~ 190 km/s.Comment: Paper contains 14 figures and 4 tables. Figures 8, 10 (color) and 13
supplied separately. Accepted for publication in MNRA
Third type of domain wall in soft magnetic nanostrips
Magnetic domain walls (DWs) in nanostructures are low-dimensional objects
that separate regions with uniform magnetisation. Since they can have different
shapes and widths, DWs are an exciting playground for fundamental research, and
became in the past years the subject of intense works, mainly focused on
controlling, manipulating, and moving their internal magnetic configuration. In
nanostrips with in-plane magnetisation, two DWs have been identified: in thin
and narrow strips, transverse walls are energetically favored, while in thicker
and wider strips vortex walls have lower energy. The associated phase diagram
is now well established and often used to predict the low-energy magnetic
configuration in a given magnetic nanostructure. However, besides the
transverse and vortex walls, we find numerically that another type of wall
exists in permalloy nanostrips. This third type of DW is characterised by a
three-dimensional, flux closure micromagnetic structure with an unusual length
and three internal degrees of freedom. Magnetic imaging on
lithographically-patterned permalloy nanostrips confirms these predictions and
shows that these DWs can be moved with an external magnetic field of about 1mT.
An extended phase diagram describing the regions of stability of all known
types of DWs in permalloy nanostrips is provided.Comment: 19 pages, 7 figure
Phase diagram of magnetic domain walls in spin valve nano-stripes
We investigate numerically the transverse versus vortex phase diagram of
head-to-head domain walls in Co/Cu/Py spin valve nano-stripes (Py: Permalloy),
in which the Co layer is mostly single domain while the Py layer hosts the
domain wall. The range of stability of the transverse wall is shifted towards
larger thickness compared to single Py layers, due to a magnetostatic screening
effect between the two layers. An approached analytical scaling law is derived,
which reproduces faithfully the phase diagram.Comment: 4 page
Tailoring and enhancing spontaneous two-photon emission processes using resonant plasmonic nanostructures
The rate of spontaneous emission is known to depend on the environment of a
light source, and the enhancement of one-photon emission in a resonant cavity
is known as the Purcell effect. Here we develop a theory of spontaneous
two-photon emission for a general electromagnetic environment including
inhomogeneous dispersive and absorptive media. This theory is used to evaluate
the two-photon Purcell enhancement in the vicinity of metallic nanoparticles
and it is demonstrated that the surface plasmon resonances supported by these
particles can enhance the emission rate by more than two orders of magnitude.
The control over two-photon Purcell enhancement given by tailored
nanostructured environments could provide an emitter with any desired spectral
response and may serve as an ultimate route for designing light sources with
novel properties
Tomographic reconstruction of quantum correlations in excited Bose-Einstein condensates
We propose to use quantum tomography to characterize the state of a perturbed
Bose-Einstein condensate. We assume knowledge of the number of particles in the
zero-wave number mode and of density distributions in space at different times,
and we treat the condensate in the Bogoliubov approximation. For states that
can be treated with the Gross-Pitaevskii equation, we find that the
reconstructed density operator gives excellent predictions of the second
moments of the atomic creation- and annihilation operators, including the
one-body density matrix. Additional inclusion of the momentum distribution at
one point of time enables somewhat reliable predictions to be made for the
second moments for mixed states, making it possible to distinguish between
coherent and thermal perturbations of the condensate. Finally, we find that
with observation of the zero-wave number mode's anomalous second moment the
reconstructed density operator gives reliable predictions of the second moments
of locally amplitude squeezed states.Comment: 12 pages, 7 figure
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