10,426 research outputs found
Simulating the impact of the Smith Cloud
We investigate the future evolution of the Smith Cloud by performing
hydrodynamical simulations of the cloud impact onto the gaseous Milky Way
Galactic disk. We assume a local origin for the cloud and thus do not include a
dark matter component to stabilize it. Our main focus is the cloud's influence
on the local and global star formation rate (SFR) of the Galaxy and whether or
not it leads to an observable event in the far future. Our model assumes two
extremes for the mass of the Smith Cloud, an upper mass limit of 10
M and a lower mass limit of 10 M, compared to the
observational value of a few 10 M. In addition, we also make the
conservative assumption that the entirety of the cloud mass of the extended
Smith Cloud is concentrated within the tip of the cloud. We find that the
impact of the low-mass cloud produces no noticeable change in neither the
global SFR nor the local SFR at the cloud impact site within the galactic disk.
For the high-mass cloud we find a short-term (roughly 5 Myr) increase of the
global SFR of up to 1 M yr, which nearly doubles the normal
Milky Way SFR. This highly localized starburst should be observable.Comment: 14 pages, 5 figure
Coherent polychotomous waves from an attractive well
A novel effect of a wave packet scattering off an attractive one- dimensional
well is found numerically and analytically. For a wave packet narrower than the
width of the well, the scattering proceeds through a quasi-bound state of
almost zero energy. The wave reflected from the well is a polychotomous
(multiple peak) monochromatic and coherent train. The transmitted wave is a
spreading smooth wave packet. The effect is strong for low average speeds of
the packet, and it disappears for wide packets.Comment: Latex, 8 eps figure
Wave packet scattering from an attractive well
Wave packet scattering off an attractive well is investigated in two spatial
dimensions numerically. The results confirm what was found previously for the
one dimensional case. The wave scattered at large angles is a polychotomous
(multiple peak) coherent train. Large angle scattering is extremely important
for low impinging velocities and at all impact parameters. The effect
disappears for packets more extended than the well. Experiments to detect the
polychotomous behavior are suggested.Comment: Final version, Journal of Physics A, to be publishe
The role of sign in students' modeling of scalar equations
We describe students revising the mathematical form of physics equations to
match the physical situation they are describing, even though their revision
violates physical laws. In an unfamiliar air resistance problem, a majority of
students in a sophomore level mechanics class at some point wrote Newton's
Second Law as F = -ma; they were using this form to ensure that the sign of the
force pointed in a direction consistent with the chosen coordinate system while
assuming that some variables have only positive value. We use one student's
detailed explanation to suggest that students' issues with variables are
context-dependent, and that much of their reasoning is useful for productive
instruction.Comment: 5 pages, 1 figure, to be published in The Physics Teache
Microjansky sources at 1.4 GHz
We present a deep 1.4 GHz survey made with the Australia Telescope Compact
Array (ATCA), having a background RMS of 9 microJy near the image phase centre,
up to 25 microJy at the edge of a 50' field of view. Over 770 radio sources
brighter than 45 microJy have been catalogued in the field. The differential
source counts in the deep field provide tentative support for the growing
evidence that the microjansky radio population exhibits significantly higher
clustering than found at higher flux density cutoffs. The optical
identification rate on CCD images is approximately 50% to R=22.5, and the
optical counterparts of the faintest radio sources appear to be mainly single
galaxies close to this optical magnitude limit.Comment: 6 pages, 4 figures, accepted by ApJ Letters 4 May 199
Mining cosmic dust from the blue ice lakes of Greenland
Extraterrestrial material, most of which invisible settles to Earth's surface as dust particles smaller than a millimeter in size were investigated. Particles of 1/10 millimeter size fall at a rate of one/sq m/yr collection of extraterrestrial dust is important because the recovered cosmic dust particles can provide important information about comets. Comets are the most important source of dust in the solar system and they are probably the major source of extraterrestrial dust that is collectable at the Earth's surface. A new collection site for cosmic dust, in an environment where degradation by weathering is minimal is reported. It is found that the blue ice lakes on the Greenland ice cap provide an ideal location for collection of extraterrestrial dust particles larger than 0.1 mm in size. It is found that the lakes contain large amounts of cosmic dust which is much better preserved than similar particles recovered from the ocean floor
Faraday cup monitor for the Iowa State College synchrotron electron beam
The design, construction, and testing of two high energy electron beam monitors are described. The one, a Faraday cup, is an absolute measuring device and the other is secondary emission monitor. The procedure and results necessary to calibrate the secondary emission monitor against the Faraday cup are included. Since the secondary emission monitor is portable, it is an adaptable instrument for the absolute measurement of high energy electron beams after it has been so calibrated
Generalized Swiss-cheese cosmologies: Mass scales
We generalize the Swiss-cheese cosmologies so as to include nonzero linear
momenta of the associated boundary surfaces. The evolution of mass scales in
these generalized cosmologies is studied for a variety of models for the
background without having to specify any details within the local
inhomogeneities. We find that the final effective gravitational mass and size
of the evolving inhomogeneities depends on their linear momenta but these
properties are essentially unaffected by the details of the background model.Comment: 10 pages, 14 figures, 1 table, revtex4, Published form (with minor
corrections
Environment, morphology and stellar populations of bulgeless low surface brightness galaxies
Based on the Sloan Digital Sky Survey DR 7, we investigate the environment,
morphology and stellar population of bulgeless low surface brightness (LSB)
galaxies in a volume-limited sample with redshift ranging from 0.024 to 0.04
and . The local density parameter is used to
trace their environments. We find that, for bulgeless galaxies, the surface
brightness does not depend on the environment. The stellar populations are
compared for bulgeless LSB galaxies in different environments and for bulgeless
LSB galaxies with different morphologies. The stellar populations of LSB
galaxies in low density regions are similar to those of LSB galaxies in high
density regions. Irregular LSB galaxies have more young stars and are more
metal-poor than regular LSB galaxies. These results suggest that the evolution
of LSB galaxies may be driven by their dynamics including mergers rather than
by their large scale environment.Comment: 12 pages, 13 figures, Accepted by A&
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