2,811 research outputs found
Bank reserve requirements and their enforcement: a comparison across streets
Bank reserves ; Money supply
SCUBA imaging of NGC 7331 dust ring
We present observations of the spiral galaxy NGC 7331 using the
Sub-millimetre Common User Bolometer Array (SCUBA) on the James Clark Maxwell
Telescope. We have detected a dust ring of 45 arcsec radius (3.3 kpc) at
wavelengths of 450 and 850-micron. The dust ring is in good correspondence with
other observations of the ring in the mid-infrared (MIR), CO and
radio-continuum, suggesting that the observed dust is associated with the
molecular gas and star formation. A B-K colour map shows an analogous ring
structure with an asymmetry about the major axis, consistent with the
extinction being produced by a dust ring. The derived temperature of the dust
lies between 16 and 31 K and the gas-to-dust ratio between 150 and 570,
depending on the assumed dust emission efficiency index (beta=1.5 or 2.).Comment: 5 pages, 6 figures, to be published in MNRA
Abundant dust found in intergalactic space
Galactic dust constitutes approximately half of the elements more massive
than helium produced in stellar nucleosynthesis. Notwithstanding the formation
of dust grains in the dense, cool atmospheres of late-type stars, there still
remain huge uncertainties concerning the origin and fate of galactic stardust.
In this paper, we identify the intergalactic medium (i.e. the region between
gravitationally-bound galaxies) as a major sink for galactic dust. We discover
a systematic shift in the colour of background galaxies viewed through the
intergalactic medium of the nearby M81 group. This reddening coincides with
atomic, neutral gas previously detected between the group members. The
dust-to-HI mass ratio is high (1/20) compared to that of the solar neighborhood
(1/120) suggesting that the dust originates from the centre of one or more of
the galaxies in the group. Indeed, M82, which is known to be ejecting dust and
gas in a starburst-driven superwind, is cited as the probable main source.Comment: 5 pages, 3 figures, 1 table. ApJ Letters in pres
Dust properties of external galaxies; NGC 891 revisited
We compare 850um SCUBA images of NGC 891 with the corresponding V-band
optical depth predicted from radiation transfer simulations. These two tracers
of dust show a very similar distribution along the minor axis and a reasonable
agreement along the major axis. Assuming that the grains responsible for
optical extinction are also the source of 850um emission we derive a
submillimeter emissivity (emission efficiency) for dust in the NGC 891 disk.
This quantity is found to be a factor of 2-3 higher than the generally-accepted
(but highly uncertain) values adopted for the Milky Way. It should be stated,
however, that if a substantial fraction of dust in NGC 891 is clumped, the
emissivity in the two galaxies may be quite similar. We use our newly-acquired
emissivity to convert our 850um images into detailed maps of dust mass and,
utilizing 21cm and CO-emission data for NGC 891, derive the gas-to-dust ratio
along the disk. We compute an average ratio of 260 -- a value consistent with
the Milky Way and external spirals within the uncertainties in deriving both
the dust mass and the quantity of molecular gas. The bulk of dust in NGC 891
appears to be closely associated with the molecular gas phase although it may
start to follow the distribution of atomic hydrogen at radii >9 kpc (i.e. >0.5
R_25). Using the optical depth of the NGC 891 disk, we quantify how light
emitted at high redshift is attenuated by dust residing in foreground spirals.
For B-band observations of galaxies typically found in the Hubble Deep Field,
the amount of light lost is expected to be small (~ 5%). This value depends
critically on the maximum radial extent of cold dust in spiral disks (which is
poorly known). It may also represent a lower limit if galaxies expel dust over
time into the intergalactic medium.Comment: 22 pages, 7 figures, A&A accepte
Star Formation Rate from Dust Infrared Emission
We examine what types of galaxies the conversion formula from dust infrared
(IR) luminosity into the star formation rate (SFR) derived by
Kennicutt (1998) is applicable to. The ratio of the observed IR luminosity,
, to the intrinsic bolometric luminosity of the newly (\la 10
Myr) formed stars,
, of a galaxy can be determined by a mean dust opacity in the
interstellar medium and the activity of the current star formation. We find
that these parameters area being is very large, and many nearby normal and active star-forming
galaxies really fall in this area. It results from offsetting two effects of a
small dust opacity and a large cirrus contribution of normal galaxies relative
to starburst galaxies on the conversion of the stellar emission into the dust
IR emission. In conclusion, the SFR determined from the IR luminosity under the
assumption of like Kennicutt (1998) is reliable within
a factor of 2 for all galaxies except for dust rich but quiescent galaxies and
extremely dust poor galaxies.Comment: Accepted by ApJL: 6 pages (emulateapj5), 2 figures (one is an extra
figure not appeared in ApJL
A Mini-survey of X-ray Point Sources in Starburst and Non-Starburst Galaxies
We present a comparison of X-ray point source luminosity functions of 3
starburst galaxies (the Antennae, M82, and NGC 253) and 4 non-starburst spiral
galaxies (NGC 3184, NGC 1291, M83, and IC 5332). We find that the luminosity
functions of the starbursts are flatter than those of the spiral galaxies; the
starbursts have relatively more sources at high luminosities. This trend
extends to early-type galaxies which have steeper luminosity functions than
spirals. We show that the luminosity function slope is correlated with 60
micron luminosity, a measure of star formation. We suggest that the difference
in luminosity functions is related to the age of the X-ray binary populations
and present a simple model which highlights how the shape of the luminosity
distribution is affected by the age of the underlying X-ray binary population.Comment: 8 pages, 4 figures. accepted for publication in Ap
3D structures for silicon carbide transistors utilising Al2O3 as a gate dielectric
This paper reports on the first investigation of the characteristics of 3D structures formed in silicon carbide for the realisation of ultra-high performance nanoscale transistors, based on the FINFET topology. Capacitance–voltage characteristics show evidence of a second flatband voltage, located at a higher bias than that seen for purely planar devices. Two distinct peaks in the conductance–voltage characteristics are observed, centred at the flatband voltages, where the amplitude of the high voltage peak correlates with the sidewall area. This suggests that the chemical behaviour of the sidewalls differ from those of the (0001) wafer surface. The breakdown electric field of the dielectric film grown on the 3D structure is in excess of 3 MV cm−1. It is demonstrated that 3D transistors (FINFETs) do not utilise the gate voltage range where the abnormal characteristics exist and so this work reports for the first time the possibility of high performance nanoscale transistors in silicon carbide that can operate at high temperatures
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