1,847 research outputs found
An Observational Limit on the Dwarf Galaxy Population of the Local Group
We present the results of an all-sky, deep optical survey for faint Local
Group dwarf galaxies. Candidate objects were selected from the second Palomar
survey (POSS-II) and ESO/SRC survey plates and follow-up observations performed
to determine whether they were indeed overlooked members of the Local Group.
Only two galaxies (Antlia and Cetus) were discovered this way out of 206
candidates. Based on internal and external comparisons, we estimate that our
visual survey is more than 77% complete for objects larger than one arc minute
in size and with a surface brightness greater than an extremely faint limit
over the 72% of the sky not obstructed by the Milky Way. Our limit of
sensitivity cannot be calculated exactly, but is certainly fainter than 25
magnitudes per square arc second in R, probably 25.5 and possibly approaching
26. We conclude that there are at most one or two Local Group dwarf galaxies
fitting our observational criteria still undiscovered in the clear part of the
sky, and a roughly a dozen hidden behind the Milky Way. Our work places the
"missing satellite problem" on a firm quantitative observational basis. We
present detailed data on all our candidates, including surface brightness
measurements.Comment: 58 pages in AJ manuscript format; some figures at slightly reduced
quality; accepted by the Astronomical Journa
Concurrent Magnetic and Metal-Insulator Transitions in (Eu,Sm)B_6 Single Crystals
The effects of magnetic doping on a EuB_6 single crystal were investigated
based on magnetic and transport measurements. A modest 5% Sm substitution for
Eu changes the magnetic and transport properties dramatically and gives rise to
concurrent antiferromagnetic and metal-insulator transitions (MIT) from
ferromagnetic MIT for EuB6. Magnetic doping simultaneously changes the
itinerant carrier density and the magnetic interactions. We discuss the origin
of the concurrent magnetic MIT in (Eu,Sm)B_6.Comment: 13 pages, 3 figures, final version to appear in Appl. Phys. Lett
New Consequences of Induced Transparency in a Double-Lambda scheme: Destructive Interference In Four-wave Mixing
We investigate a four-state system interacting with long and short laser
pulses in a weak probe beam approximation. We show that when all lasers are
tuned to the exact unperturbed resonances, part of the four-wave mixing (FWM)
field is strongly absorbed. The part which is not absorbed has the exact
intensity required to destructively interfere with the excitation pathway
involved in producing the FWM state. We show that with this three-photon
destructive interference, the conversion efficiency can still be as high as
25%. Contrary to common belief,our calculation shows that this process, where
an ideal one-photon electromagnetically induced transparency is established, is
not most suitable for high efficiency conversion. With appropriate
phase-matching and propagation distance, and when the three-photon destructive
interference does not occur, we show that the photon flux conversion efficiency
is independent of probe intensity and can be close to 100%. In addition, we
show clearly that the conversion efficiency is not determined by the maximum
atomic coherence between two lower excited states, as commonly believed. It is
the combination of phase-matching and constructive interference involving the
two terms arising in producing the mixing wave that is the key element for the
optimized FWM generation. Indeed, in this scheme no appreciable excited state
is produced, so that the atomic coherence between states |0> and |2> is always
very small.Comment: Submitted to Phys. Rev. A, 7 pages, 4 figure
GMOS Integral Field Spectroscopy of a Merging System with Enhanced Balmer Absorption
In this paper we present the three dimensional dynamics of the galaxy SDSS
J101345.39+011613.66, selected for its unusually strong Balmer absorption lines
(Wo(H-delta)=7.5A). Using the GMOS-South IFU in Nod & Shuffle mode we have
mapped the continuum and optical absorption lines of this z=0.1055 field
galaxy. This galaxy has a disturbed morphology, with a halo of diffuse material
distributed asymmetrically toward the north. Using the [OII] emission line
(Wo([OII])=4.1A) we find that the gas and hot OB stars are offset from the
older stars in the system. The gas also has a spatially extended and elongated
morphology with a velocity gradient of 100+/-20km/s across 6kpc in projection.
Using the strong H-gamma and H-delta absorption lines we find that the A- stars
are widely distributed across the system and are not centrally concentrated
arguing that the A-star population has formed in molecular clouds outside the
nucleus. By cross correlating the spectra from the datacube with an A-star
template we find evidence that the A-star population has a 40km/s shear in the
same direction as the gas. The disturbed morphology, strong colour gradients
and strong H-delta and H-gamma absorption lines in SDSS J101345.39 argue that
this is a recent tidal interaction/merger between a passive elliptical and
star-forming galaxy. Although based on a single object, these results show that
we can spatially resolve and constrain the dynamics of this short lived (yet
important) phase of galaxy formation in which the evolutionary process take
galaxies from star-forming to their quiescent end products.Comment: 7 pages, 7 figures. Accepted for publication in Ap
Relativistic Effects of Light in Moving Media with Extremely Low Group Velocity
A moving dielectric medium acts as an effective gravitational field on light.
One can use media with extremely low group velocities [Lene Vestergaard Hau et
al., Nature 397, 594 (1999)] to create dielectric analogs of astronomical
effects on Earth. In particular, a vortex flow imprints a long-ranging
topological effect on incident light and can behave like an optical black hole.Comment: Physical Review Letters (accepted
GASP II. A MUSE view of extreme ram-pressure stripping along the line of sight: kinematics of the jellyfish galaxy JO201
This paper presents a spatially-resolved kinematic study of the jellyfish
galaxy JO201, one of the most spectacular cases of ram-pressure stripping (RPS)
in the GASP (GAs Stripping Phenomena in Galaxies with MUSE) survey. By studying
the environment of JO201, we find that it is moving through the dense
intra-cluster medium of Abell 85 at supersonic speeds along our line of sight,
and that it is likely accompanied by a small group of galaxies. Given the
density of the intra-cluster medium and the galaxy's mass, projected position
and velocity within the cluster, we estimate that JO201 must so far have lost
~50% of its gas during infall via RPS. The MUSE data indeed reveal a smooth
stellar disk, accompanied by large projected tails of ionised (Halpha) gas,
composed of kinematically cold (velocity dispersion <40km/s) star-forming knots
and very warm (>100km/s) diffuse emission which extend out to at least ~50 kpc
from the galaxy centre. The ionised Halpha-emitting gas in the disk rotates
with the stars out to ~6 kpc but in the disk outskirts becomes increasingly
redshifted with respect to the (undisturbed) stellar disk. The observed
disturbances are consistent with the presence of gas trailing behind the
stellar component, resulting from intense face-on RPS happening along the line
of sight. Our kinematic analysis is consistent with the estimated fraction of
lost gas, and reveals that stripping of the disk happens outside-in, causing
shock heating and gas compression in the stripped tails.Comment: ApJ, revised version after referee comments, 15 pages, 16 figures.
The interactive version of Figure 9 can be viewed at
web.oapd.inaf.it/gasp/publications.htm
Tillage system effect on the epidemic of soybean brown spot.
Six field experiments were carried out during the summers of 1997 to 2003 to evaluate disease progress of soybean brown spot caused by Septoria glycines considering two tillage systems, conventional and no-tillage (wheat was cultivated during winter in all plots). Two logistic models were fitted to the disease progress data: (i) a logistic model with constant disease progress rate r [y=1/(1+(1/y0-1)exp(-rt)), where y0 is the initial disease level at time t=0] and (ii) a logistic model with an exponentially increasing progress rate r(t)=r0 ebt [y=1/(1+(1/y0-1) exp(r0/b(1-ebt))), where r0 is the initial progress rate at time t=0, b the rate increasing parameter, and y0 again the initial disease level]. The logistic model with constant rate underestimated disease incidence on the first disease assessment in nine out of twelve epidemics. The logistic model with an increasing rate gave a better fit to all disease progress curves (R2 between 0.90 and 0.99; no pattern in the residuals). According to this model, y0 was in most cases (four out of six) significantly smaller in the plots with no-tillage compared with conventional tillage, but no differences were detected in the initial rate parameter r0 and the rate increasing parameter b (with one exception). It is proposed that these results are due to higher susceptibility of old leaflets compared with young leaflets, as demonstrated by artificial inoculation: In four laboratory experiments the mean brown spot severity was 16.7 % on old leaflets but only 3.9 % on young leaflets
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