107,838 research outputs found
MusE GAs FLOw and Wind (MEGAFLOW) I: First MUSE results on background quasars
The physical properties of galactic winds are one of the keys to understand
galaxy formation and evolution. These properties can be constrained thanks to
background quasar lines of sight (LOS) passing near star-forming galaxies
(SFGs). We present the first results of the MusE GAs FLOw and Wind (MEGAFLOW)
survey obtained of 2 quasar fields which have 8 MgII absorbers of which 3 have
rest-equivalent width greater than 0.8 \AA. With the new Multi Unit
Spectroscopic Explorer (MUSE) spectrograph on the Very Large Telescope (VLT),
we detect 6 (75) MgII host galaxy candidates withing a radius of 30 arcsec
from the quasar LOS. Out of these 6 galaxy--quasar pairs, from geometrical
arguments, one is likely probing galactic outflows, two are classified as
"ambiguous", two are likely probing extended gaseous disks and one pair seems
to be a merger. We focus on the windpair and constrain the outflow using a
high resolution quasar spectra from Ultraviolet and Visual Echelle Spectrograph
(UVES). Assuming the metal absorption to be due to gas flowing out of the
detected galaxy through a cone along the minor axis, we find outflow velocities
of the order of 150 km/s (i.e. smaller than the escape velocity) with
a loading factor, SFR, of 0.7. We see
evidence for an open conical flow, with a low-density inner core. In the
future, MUSE will provide us with about 80 multiple galaxyquasar pairs in
two dozen fields.Comment: 20 pages, 16 figures, accepted for publication in Ap
Magnetic field evolution of the quasiparticle interference in a d-wave superconductor
Quasiparticle interference in a d-wave superconductor with weak disorder
produces distinctive peaks in the Fourier-transformed local density of states
measured by scanning tunneling spectroscopy. We predict that amplitudes of
these peaks can be enhanced or suppressed by applied magnetic field according
to a very specific pattern governed by the symmetry of the superconducting
order parameter. This calculated pattern agrees with the recent experimental
measurement and suggests that the technique could be useful for probing the
underlying normal state at high fields.Comment: 4 pages, 2 figures; version to appear in PRB/R
Probing background ionization: Positive streamers with varying pulse repetition rate and with a radioactive admixture
Positive streamers need a source of free electrons ahead of them to
propagate. A streamer can supply these electrons by itself through
photo-ionization, or the electrons can be present due to external background
ionization. Here we investigate the effects of background ionization on
streamer propagation and morphology by changing the gas composition and the
repetition rate of the voltage pulses, and by adding a small amount of
radioactive Krypton 85.
We find that the general morphology of a positive streamer discharge in high
purity nitrogen depends on background ionization: at lower background
ionization levels the streamers branch more and have a more feather-like
appearance. This is observed both when varying the repetition rate and when
adding Krypton 85, though side branches are longer with the radioactive
admixture. But velocities and minimal diameters of streamers are virtually
independent of the background ionization level. In air, the inception cloud
breaks up into streamers at a smaller radius when the repetition rate and
therefore the background ionization level is higher. When measuring the effects
of the pulse repetition rate and of the radioactive admixture on the discharge
morphology, we found that our estimates of background ionization levels are
consistent with these observations; this gives confidence in the estimates.
Streamer channels generally do not follow the paths of previous discharge
channels for repetition rates of up to 10 Hz. We estimate the effect of
recombination and diffusion of ions and free electrons from the previous
discharge and conclude that the old trail has largely disappeared at the moment
of the next voltage pulse; therefore the next streamers indeed cannot follow
the old trail.Comment: 30 pages, 13 figure
Probing photo-ionization: simulations of positive streamers in varying N2:O2 mixtures
Photo-ionization is the accepted mechanism for the propagation of positive
streamers in air though the parameters are not very well known; the efficiency
of this mechanism largely depends on the presence of both nitrogen and oxygen.
But experiments show that streamer propagation is amazingly robust against
changes of the gas composition; even for pure nitrogen with impurity levels
below 1 ppm streamers propagate essentially with the same velocity as in air,
but their minimal diameter is smaller, and they branch more frequently.
Additionally, they move more in a zigzag fashion and sometimes exhibit a
feathery structure. In our simulations, we test the relative importance of
photo-ionization and of the background ionization from pulsed repetitive
discharges, in air as well as in nitrogen with 1 ppm O2 . We also test
reasonable parameter changes of the photo-ionization model. We find that photo-
ionization dominates streamer propagation in air for repetition frequencies of
at least 1 kHz, while in nitrogen with 1 ppm O2 the effect of the repetition
frequency has to be included above 1 Hz. Finally, we explain the feather-like
structures around streamer channels that are observed in experiments in
nitrogen with high purity, but not in air.Comment: 12 figure
Probing isospin- and momentum-dependent nuclear effective interactions in neutron-rich matter
The single-particle potentials for nucleons and hyperons in neutron-rich
matter generally depends on the density and isospin asymmetry of the medium as
well as the momentum and isospin of the particle. It further depends on the
temperature of the matter if the latter is in thermal equilibrium. We review
here the extension of a Gogny-type isospin- and momentum-dependent interaction
in several aspects made in recent years and their applications in studying
intermediate-energy heavy ion collisions, thermal properties of asymmetric
nuclear matter and properties of neutron stars. The importance of the isospin-
and momentum-dependence of the single-particle potential, especially the
momentum dependence of the isovector potential, is clearly revealed throughout
these studies.Comment: 27 pages, 19 figures, 1 table, accepted version to appear in EPJA
special volume on Nuclear Symmetry Energ
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