1,179 research outputs found
An ultra-compact low temperature scanning probe microscope for magnetic fields above 30 T
We present the design of a highly compact High Field Scanning Probe
Microscope (HF-SPM) for operation at cryogenic temperatures in an extremely
high magnetic field, provided by a water-cooled Bitter magnet able to reach 38
T. The HF-SPM is 14 mm in diameter: an Attocube nano-positioner controls the
coarse approach of a piezo resistive AFM cantilever to a scanned sample. The
Bitter magnet constitutes an extreme environment for SPM due to the high level
of vibrational noise; the Bitter magnet noise at frequencies up to 300 kHz is
characterized and noise mitigation methods are described. The performance of
the HF-SPM is demonstrated by topographic imaging and noise measurements at up
to 30 T. Additionally, the use of the SPM as a three-dimensional dilatometer
for magnetostriction measurements is demonstrated via measurements on a
magnetically frustrated spinel sample.Comment: 6 pages, 5 figure
The Star Formation Threshold in NGC 6822
We investigate the star formation threshold in NGC 6822, a nearby Local Group
dwarf galaxy, on sub-kpc scales using high-resolution, wide-field, deep HI,
Halpha and optical data. In a study of the HI velocity profiles we identify a
cool and warm neutral component in the Interstellar Medium of NGC 6822. We show
that the velocity dispersion of the cool component (~4 km/s) when used with a
Toomre-Q criterion gives an optimal description of ongoing star formation in
NGC 6822, superior to that using the more conventional dispersion value of 6
km/s. However, a simple constant surface density criterion for star formation
gives an equally superior description. We also investigate the two-dimensional
distribution of Q and the star formation threshold and find that these results
also hold locally. The range in gas density in NGC 6822 is much larger than the
range in critical density, and we argue that the conditions for star formation
in NGC 6822 are fully driven by this density criterion. Star formation is
local, and in NGC 6822 global rotational or shear parameters are apparently not
important.Comment: Accepted for publication in The Astronomical Journal. Version with
high-resolution figures available at
http://www.mso.anu.edu.au/~edeblok/papers/6822_paper2.ps.g
On the Neutral Gas Content and Environment of NGC 3109 and the Antlia Dwarf Galaxy
As part of a continuing survey of nearby galaxies, we have mapped the neutral
gas content of the low surface brightness, Magellanic-type galaxy NGC 3109 ---
and its environment, including the Antlia dwarf galaxy --- at unprecedented
velocity resolution and brightness sensitivity. The HI mass of NGC 3109 is
measured to be (3.8 +/- 0.5) x 10^8 Msun. A substantial warp in the disk of NGC
3109 is detected in the HI emission image in the form of an extended low
surface brightness feature. We report a positive detection in HI of the nearby
Antlia dwarf galaxy, and measure its total neutral gas mass to be (6.8 +/- 1.4)
x 10^5 Msun. We show the warp in NGC 3109 to lie at exactly the same radial
velocity as the gas in the Antlia dwarf galaxy and speculate that Antlia
disturbed the disk of NGC 3109 during a mild encounter ~1 Gyr in the past. HI
data for a further eight galaxies detected in the background are presented.Comment: Accepted for publication in A
The Baryonic Tully-Fisher Relation
We explore the Tully-Fisher relation over five decades in stellar mass in
galaxies with circular velocities ranging over 30 < Vc < 300 km/s. We find a
clear break in the optical Tully-Fisher relation: field galaxies with Vc < 90
km/s fall below the relation defined by brighter galaxies. These faint galaxies
are however very gas rich; adding in the gas mass and plotting baryonic disk
mass Md = M* + Mg in place of luminosity restores a single linear relation. The
Tully-Fisher relation thus appears fundamentally to be a relation between
rotation velocity and total baryonic mass of the form Md = A Vc^4.Comment: 10 pages including 1 color figure. Accepted for publication in ApJ
Letter
Spectroscopy of Low Surface Brightness Galaxies with the Hobby-Eberly Telescope
We have obtained low resolution spectra of nineteen red and blue low surface
brightness galaxies, using the Marcario Low Resolution Spectrograph on the 9.2m
Hobby-Eberly Telescope. These galaxies form a very heterogeneous class, whose
spectra qualitatively resemble those of high surface brightness galaxies
covering the full range of spectra seen in galaxies of Hubble types from E to
Irr. We use a combination of emission line (EW(Halpha), NII/Halpha) and
absorption line (Mgb, Hbeta, ) based diagnostics to investigate the
star-formation and chemical enrichment histories of these galaxies. These are
diverse, with some galaxies having low metallicity and very young mean stellar
ages, and other galaxies showing old, super-solar metallicity stellar
populations. In contrast with some previous studies which found a strong trend
of decreasing metallicity with decreasing central surface brightness, we find a
population of galaxies with low surface brightness and near-solar metallicity.
Correlations between several of the gas phase and stellar population age and
metallicity indicators are used to place contraints on plausible evolutionary
scenarios for LSB galaxies. The redshift range spanned by these galaxies is
broad, with radial velocities from 3400 km/s to more than 65000 km/s. A subset
of the sample galaxies have published HI redshifts and gas masses based on
observations with the Arecibo 305m single-dish radio telescope, which place
these galaxies far off of the mean Tully-Fisher relation. Our new optical
redshifts do not agree with the published HI redshifts for these galaxies. Most
of the discrepancies can be explained by beam confusion in the Arecibo
observations, causing erroneous HI detections for some of the galaxies.Comment: 31 pages, 12 figures, 7 tables. Uses emulateapj5.sty and
onecolfloat5.sty, which are included. Accepted for publication in the
Astronomical Journa
Ontwikkelen met kwaliteit : een verkenning van evaluatiecriteria
WOT Natuur & Milieu achtergrondsdocument rond invulling van ontwikkelingsplanologie, zoals gepresenteerd in de Nota Ruimte. Visie op grond van gesprekken met: gemeenten, provincies, LNV, Landschapsbeheer Nederland, Stichting Instandhouding Kleine Landschapselementen in Limburg, Ruimtelijk Planburea
CO Detection and Millimeter Continuum Emission from Low Surface Brightness Galaxies
We present BIMA and IRAM CO(1--0) observations of seven low surface brightness (LSB) galaxies, including three large spirals with faint disks but prominent bulges, and four relatively small LSB galaxies with irregular disks. The giant LSB galaxies are UGC 5709, UGC 6614 and F568-6 (Malin2). The smaller LSB galaxies are NGC 5585, UGC 4115, UGC 5209 and F583-1. The galaxies were selected based on their relatively high metallicity and apparent signs of star formation in their disks. The BIMA maps suggested the presence of molecular gas in 2 of the giant LSB galaxies, F568-6 and UGC 6614. Using the 30m IRAM telescope we detected CO (1--0) emission in the disks of both galaxies and in the nucleus of F568-6. The molecular gas in these galaxies is clearly offset from the nucleus and definitely associated with the LSB disk. In addition we also detected a millimeter continuum source in the center of UGC 6614. When compared with VLA 1.5 GHz observations of the galaxy, the emission was found to have a flat spectrum indicating that the millimeter continuum emission is most likely due to an active galactic nucleus (AGN) in the galaxy. Our results show that giant LSB spirals may contain significant quantities of molecular gas in their disks and also harbor radio bright AGN in their centers
Galaxies on the Blue Edge
We have successfully constructed a catalog of HI-rich galaxies selected from
the Minnesota Automated Plate Scanner Catalog of the Palomar Observatory Sky
Survey (POSS I) based solely on optical criteria. We identify HI-rich
candidates by selecting the bluest galaxies at a given apparent magnitude,
those galaxies on the "blue edge" of POSS I color-magnitude parameter space.
Subsequent 21-cm observations on the upgraded Arecibo 305m dish detected over
50% of the observed candidates. The detected galaxies are HI-rich with HI
masses comparable to "normal" high surface brightness disk galaxies and they
have gas mass-to-light ratios ranging from 0.1 to 4.8 (in solar units).
Comparison of our candidate galaxies with known low surface brightness galaxies
(hereafter LSBs) shows that they exhibit similar optical and HI properties to
that population. We also show that previously identified LSBs, including
several LSBs with red B-V colors, preferentially occupy the "blue edge" of POSS
I color-magnitude parameter space. Their presence on the "blue edge" appears to
be a selection effect due to differing plate limits in the two POSS I
bandpasses. This suggests the POSS I is a good filter for separating galaxies
on the higher surface brightness end of the LSB population from the general
population of galaxies in the night sky.Comment: 56 pages, 19 figures, to be published in the Astronomical Journal
(July 1, 2002
Effects Of Increasing The Involvement Of Community-Dwelling Frail Older Adults In A Proactive Assessment Service:A Pragmatic Trial
Contains fulltext :
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