1,134 research outputs found
A census of Meddies tracked by floats
Recent subsurface float measurements in 27 Mediterranean Water eddies (Meddies) in the Atlantic are grouped together to reveal new information about the pathways of these energetic eddies and how they are often modified and possibly destroyed by collisions with seamounts. Twenty Meddles were tracked in the Iberian Basin west of Portugal, seven in the Canary Basin. During February 1994 14 Meddles were simultaneously observed, 11 of them in the Iberian Basin. Most (69%) of the newly formed Meddles in the Iberian Basin translated southwestward into the vicinity of the Horseshoe Seamounts and probably collided with them. Some Meddles (31%) passed around the northern side of the seamounts and translated southwestward at a typical velocity of 2.0 cm/s into the Canary Basin. Some Meddles observed there were estimated to be up to similar to 5 yr old. Four Meddles in the Canary Basin collided with the Great Meteor Seamounts and three Meddles were inferred to have been destroyed by the collision. Overall an estimated 90% of Meddles collided with major seamounts. The mean time from Meddy formation to a collision with a major seamount was estimated to be around 1.7 yr. Combined with the estimated Meddy formation rate of 17 Meddies/yr from previous work, this suggests that around 29 Meddles co-exist in the North Atlantic. Therefore during February 1994 we observed about half of the population of Meddle
An Unpublished Act of David II, 1359
This is a transcription, translation and commentary on an overlooked act of David II of Scotland from a 1359 treaty of alliance with France
Evidence of strong quasar feedback in the early Universe
Most theoretical models invoke quasar driven outflows to quench star
formation in massive galaxies, this feedback mechanism is required to account
for the population of old and passive galaxies observed in the local universe.
The discovery of massive, old and passive galaxies at z=2, implies that such
quasar feedback onto the host galaxy must have been at work very early on,
close to the reionization epoch. We have observed the [CII]158um transition in
SDSSJ114816.64+525150.3 that, at z=6.4189, is one of the most distant quasars
known. We detect broad wings of the line tracing a quasar-driven massive
outflow. This is the most distant massive outflow ever detected and is likely
tracing the long sought quasar feedback, already at work in the early Universe.
The outflow is marginally resolved on scales of about 16 kpc, implying that the
outflow can really affect the whole galaxy, as required by quasar feedback
models. The inferred outflow rate, dM/dt > 3500 Msun/yr, is the highest ever
found. At this rate the outflow can clean the gas in the host galaxy, and
therefore quench star formation, in a few million years.Comment: 5 pages, 3 figures, accepted for publication in MNRAS Letter
Simultaneous X-ray, radio, near-infrared, and optical monitoring of Young Stellar Objects in the Coronet cluster
Multi-wavelength (X-ray to radio) monitoring of Young Stellar Objects (YSOs)
can provide important information about physical processes at the stellar
surface, in the stellar corona, and/or in the inner circumstellar disk regions.
While coronal processes should mainly cause variations in the X-ray and radio
bands, accretion processes may be traced by time-correlated variability in the
X-ray and optical/infrared bands. Several multi-wavelength studies have been
successfully performed for field stars and approx. 1-10 Myr old T Tauri stars,
but so far no such study succeeded in detecting simultaneous X-ray to radio
variability in extremely young objects like class I and class 0 protostars.
Here we present the first simultaneous X-ray, radio, near-infrared, and optical
monitoring of YSOs, targeting the Coronet cluster in the Corona Australis
star-forming region, which harbors at least one class 0 protostar, several
class I objects, numerous T Tauri stars, and a few Herbig AeBe stars. [...]
Seven objects are detected simultaneously in the X-ray, radio, and
optical/infrared bands; they constitute our core sample. While most of these
sources exhibit clear variability in the X-ray regime and several also display
optical/infrared variability, none of them shows significant radio variability
on the timescales probed. We also do not find any case of clearly
time-correlated optical/infrared and X-ray variability. [...] The absence of
time-correlated multi-wavelength variability suggests that there is no direct
link between the X-ray and optical/infrared emission and supports the notion
that accretion is not an important source for the X-ray emission of these YSOs.
No significant radio variability was found on timescales of days.Comment: 11 pages, 11 figures, accepted for publication in A&A (06 Dec 2006
Detection of fast radio transients with multiple stations: a case study using the Very Long Baseline Array
Recent investigations reveal an important new class of transient radio
phenomena that occur on sub-millisecond timescales. Often transient surveys'
data volumes are too large to archive exhaustively. Instead, an on-line
automatic system must excise impulsive interference and detect candidate events
in real-time. This work presents a case study using data from multiple
geographically distributed stations to perform simultaneous interference
excision and transient detection. We present several algorithms that
incorporate dedispersed data from multiple sites, and report experiments with a
commensal real-time transient detection system on the Very Long Baseline Array
(VLBA). We test the system using observations of pulsar B0329+54. The
multiple-station algorithms enhanced sensitivity for detection of individual
pulses. These strategies could improve detection performance for a future
generation of geographically distributed arrays such as the Australian Square
Kilometre Array Pathfinder and the Square Kilometre Array.Comment: 12 pages, 14 figures. Accepted for Ap
The Canada-UK Deep Submillimetre Survey: First Submillimetre Images, the Source Counts, and Resolution of the Background
We present the first results of a deep unbiased submillimetre survey carried
out at 450 and 850 microns. We detected 12 sources at 850 microns, giving a
surface density of sources with 850-micron flux densities > 2.8mJy of of
0.49+-0.16 per square arcmin. The sources constitute 20-30% of the background
radiation at 850 microns and thus a significant fraction of the entire
background radiation produced by stars. This implies, through the connection
between metallicity and background radiation, that a significant fraction of
all the stars that have ever been formed were formed in objects like those
detected here. The combination of their large contribution to the background
radiation and their extreme bolometric luminosities make these objects
excellent candidates for being proto-ellipticals. Optical astronomers have
recently shown that the UV-luminosity density of the universe increases by a
factor of about 10 between z=0 and z=1 and then decreases again at higher
redshifts. Using the results of a parallel submillimetre survey of the local
universe, we show that both the submillimetre source density and background can
be explained if the submillimetre luminosity density evolves in a similar way
to the UV-luminosity density. Thus, if these sources are ellipticals in the
process of formation, they may be forming at relatively modest redshifts.Comment: 8 pages (LATEX), 6 postscript figures, submitted to ApJ Letter
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