79 research outputs found
Gas-rich dwarf galaxies in dense and sparse environments
Dwarf irregular galaxies (generically labelled Im for the present purposes) pose an enigma to students of galaxy evolution. In nearby groups and the Virgo cluster, Im galaxies are at least as abundant as spiral galaxies, and their low surface brightnesses and high gas-to-stars ratios suggest that (at least in the stochastic self-propagating star formation scenario) there should be significant numbers of HI clouds with masses approaching 10(exp 8) solar mass which have undergone very little or no star formation. To date, however, no clouds with so little star formation that they would not be recognized as Im galaxies on high-quality photographic plates have been identified. There have been suggestions that such dwarfs may be tidally disrupted in regions of high galactic density, but may be prevalent in low density regions. We offer data from three parallel programs relevant to this issue. (1) A large number of Im galaxies throughout the Local Supercluster have been mapped in the HI spectral line using the Arecibo Radiotelescope, and we can establish the frequency with which HI disks much more extended than their optically visible portions are found. (2) Our extensive mapping of spiral and dwarf galaxies in the Virgo cluster allows us to set stringent limits on the density of star-free Hi clouds in that cluster. (3) We have conducted a sampling of the void in the distribution of galaxies toward the super galactic pole, optimized for finding low-mass HI clouds at redshifts out to approximately 2000 km/s
Mini High Velocity Clouds
Neutral hydrogen mapping of several small, very low column density High
Velocity Clouds (HVC), using the Arecibo telescope, is reported. Some were
found serendipitously superimposed at distinct velocities on largerHVCs; others
were found to comprise the four most isolated low column density sources in the
observations of Lockman et al. but proved to be individually much smaller than
the Green Bank 140 Foot telescope beam. We call these clouds ``mini-HVC'' to
distinguish them from the larger and denser Compact High Velocity Clouds
(CHVC). Peak column densities are typically a few X 10^18 cm^-2 averaged over
the Arecibo 3.2' beam, and diameters to our detection limit ~ 10^18 cm^-2 range
from 9' to 35'. These column densities and angular diameters overlap with those
for CHVC, but are typically smaller. We consider three possiblities: (1) that
most mini-HVC are related to the Magellanic Stream, (2) that most mini-HVC are
part of M31's retinue of CHVC, or (3) that the mini-HVC are simply the low
column density tail of the distribution of CHVC. None of these possibilities
can be rejected as yet, given the selection biases in our sample. We also
discuss controversies about the amount of ionized hydrogen in CHVC and
mini-HVC, which may be mainly ionized, and the implications of these small
clouds for Lyman Limit Systems.Comment: Astronomical Journal, in pres
High Velocity Cloud Edges and Mini-HVCs
Arecibo mapping is reported of the neutral hydrogen distribution along
selected directions out from the centers of two small High Velocity Clouds
(HVC), W486 and W491. Both HVCs have a small inner region where the neutral
hydrogen column density N_HI decreases slowly and a larger outer region where
N_HI declines more rapidly, smoothly and exponentially from ~ 2 X 10^19 atoms
cm^-2 down to < 10^18 atoms cm^-2. Line widths, and presumably temperature and
turbulence, do not increase in the outermost regions. Therefore pressure
decreases smoothly, making confinement by dark matter gravity more likely than
confinement by external pressure.
The more extended HVC, W491, has a superimposed small cloud (which we dub a
``mini-HVC''), offset by 66 km s^-1 in velocity along the line of sight with
peak column density about 5 X 10^18 atoms cm^-2. Preliminary data toward future
mapping of two more HVCs reveals two more mini-HVCs of similarly small size and
central column density a bit less than 1 X 10^19 atoms cm^-2. We suggest that
these three mini-HVCs are not physically associated with the HVCs on which they
are superimposed, but are either very small outlyers of the extended Magellanic
Stream HVC complex or more distant and/or smaller isolated CHVCs.
The value of N_HI at the point where the neutral and ionized column densities
are equal is ~ 2 X 10^19 atoms cm^-2 for the two mapped HVCs. Therefore the
angular scalelength of the total hydrogen is appreciably larger than the
observed HI scalelength. Previous distance estimates, related to absolute size
and mass of the total hydrogen cloud, may have to be scaled down because of the
undetected, more extended ionized hydrogen.Comment: 24 pages, 8 figures, ApJ in pres
The Fine Structure and Outskirts of DDO 154
Mapping of the HI disk of the isolated irregular galaxy DDO 154 with the C
array of the Very Large Array and with the 3.2' upgraded Arecibo beam is
presented. Our results show a truncation (or temporary drop) of the HI disk at
a column density around 10^19 atoms cm^-2, consistent with theoretical
expectations for the truncation produced by the extragalactic UV field. We also
detect a marginally significant levelling off of the HI distribution along the
continuation of the major axis at a column density near 2X10^18 atoms cm^-2.
The VLA results show that the gas beyond ~6' in radius must be relatively
smooth, with no structure larger in size than ~300 pc exhibiting a density
contrast of a factor of 10 or more. However, there is considerable
few-hundred-parsec scale structure in the gas disk at smaller radii, even well
outside the regions where there are visible stars. Two prominent cavities well
removed from any significant stellar populations are studied. While the
energies required for evacuation are consistent with those produced by multiple
supernovae, there is no visible trace of stars within a kpc of the center of
the larger cavity, and the smaller of the two cavities is centered just outside
the 26.5 mag arcsec^-2 B isophote. The velocity dispersion of the gas, measured
within our 270 pc beam, is 7 to 8 km s^-1 throughout the disk (to 6' radius).
This translates to a scaleheight of ~700 pc at the point where the rotation
curve flattens, at a radius of ~4.5 kpc. Velocity profiles are well fit by
single gaussians at all points.Comment: 14 pages, 2 tables, 13 figures, accepted for AJ Nov. 200
Neutral Hydrogen Mapping of Virgo Cluster Blue Compact Dwarf Galaxies
A new installment of neutral hydrogen mappings of Blue Compact Dwarf
galaxies, as defined by optical morphology, in and near the Virgo cluster is
presented. The primary motivation was to search for outlying clouds of HI as
potential interactive triggers of the enhanced star formation, and therefore
the mapped galaxies were selected for large HI} mass, large optical diameter,
and large velocity profile width. Approximately half the sample proved to have
one or more small, low column density star-free companion clouds, either
detached or appearing as an appendage in our maps, at resolution of order 4
kpc. Comparison is made to a sample of similarly mapped field BCD galaxies
drawn from the literature; however, the Virgo cluster sample of mapped BCDs is
still too small for conclusive comparisons to be made.
We found, on the one hand, little or no evidence for ram pressure stripping
nor, on the other, for extremely extended low column density HI envelopes. The
HI rotation curves in most cases rise approximately linearly, and slowly, as
far out as we can trace the gas.Comment: To appear in AJ, Dec. 200
The Arecibo Legacy Fast ALFA Survey: VIII. HI Source Catalog of the Anti-Virgo Region at dec = +25 deg
We present a fourth catalog of HI sources from the Arecibo Legacy Fast ALFA
(ALFALFA) Survey. We report 541 detections over 136 deg2, within the region of
the sky having 22h < R.A. < 03h and 24 deg < Dec. < 26 deg . This complements a
previous catalog in the region 26 deg < Dec. < 28 deg (Saintonge et al. 2008).
We present here the detections falling into three classes: (a) extragalactic
sources with S/N > 6.5, where the reliability of the catalog is better than
95%; (b) extragalactic sources 5.0 < S/N < 6.5 and a previously measured
optical redshift that corroborates our detection; or (c) High Velocity Clouds
(HVCs), or subcomponents of such clouds, in the periphery of the Milky Way. Of
the 541 objects presented here, 90 are associated with High Velocity Clouds,
while the remaining 451 are identified as extragalactic objects. Optical
counterparts have been matched with all but one of the extragalactic objects.Comment: 26 pages, 5 figures, 1 table, accepted for publication in
Astrophysical Journal Supplement Serie
MCG+00-32-16: An Irregular Galaxy Close to the Lowest Redshift Absorber on the 3C 273 Line of Sight
MCG+00-32-16 is the galaxy closest in position-velocity space to the lowest
redshift Ly absorber along the line-of-sight to the quasar 3C 273. Its
projected separation is 204 (d/19 Mpc) kpc, where d is the distance from the
Milky Way to the galaxy, and the redshift difference is only 94 km/s; HI
1225+01 is slightly closer in projected separation to the absorber, but has a
greater redshift difference. We present HI synthesis array mapping and CCD
photometry in B and R for MCG+00-32-16. The HI disk is rotating in such a way
that the side of the galaxy closer to the sight-line to the quasar has the
larger velocity difference from the absorber. The absorber may be a ``failed
dwarf'' member of a poor galaxy group of which MCG+00-32-16 and HI 1225+01 are
the only members to have formed stars.Comment: 14 pages, 9 figures, accepted by Astrophysical Journa
Dark Matter and Gas Concentrations in the NGC 4532 / DDO 137 System
Neutral hydrogen synthesis mapping of NGC 4532 and DDO 137, a pair of Sm galaxies on the edge of the Virgo cluster, is used to determine rotation curves for each of the galaxies and to resolve the structure and kinematics of three distinct HI concentrations embedded in an extended envelope of diffuse HI discovered in earlier Arecibo studies of the system. The HI masses of the three concentrations do not appear to be sufficient for them to be self-gravitating; however, their HImasses and dynamical masses are very similar to those of faint Im galaxies in the Virgo cluster. The peak HI column density of each of the clouds is close to the star formation threshold, but CCD images in B and R reveal no trace of stars nor star formation. If the system is gravitationally bound and in Virial equilibrium, we find its total mass to be ten times that within the outermost HI contours of the individual galaxies and clouds. Thirty-seven percent of the total HI lies outside the boundaries of the individual galaxies as determined by the VLA mapping; 28% cannot be ascribed to either galaxy nor to the discrete concentrations. One possible way to account for the velocity field and the large quantity of diffuse gas is to assume that DDO 137, the gas concentrations and other apparent tidal features are due to gas infall into clumps within the dark matter potential of the group, and that prior to the interaction the group consisted of an irregular galaxy (NGC 4532) and a large, star-poor HI cloud like HI 1225+01 (Giovanelli et al. 1991)
Repetitive arm functional tasks after stroke (RAFTAS): a pilot randomised controlled trial
Background
Repetitive functional task practise (RFTP) is a promising treatment to improve upper limb recovery following stroke. We report the findings of a study to determine the feasibility of a multi-centre randomised controlled trial to evaluate this intervention.
Methods
A pilot randomised controlled trial was conducted. Patients with new reduced upper limb function were recruited within 14 days of acute stroke from three stroke units in North East England. Participants were randomised to receive a four week upper limb RFTP therapy programme consisting of goal setting, independent activity practise, and twice weekly therapy reviews in addition to usual post stroke rehabilitation, or usual post stroke rehabilitation. The recruitment rate; adherence to the RFTP therapy programme; usual post stroke rehabilitation received; attrition rate; data quality; success of outcome assessor blinding; adverse events; and the views of study participants and therapists about the intervention were recorded.
Results
Fifty five eligible patients were identified, 4-6% of patients screened at each site. Twenty four patients participated in the pilot study. Two of the three study sites met the recruitment target of 1-2 participants per month. The median number of face to face therapy sessions received was 6 [IQR 3-8]. The median number of daily repetitions of activities recorded was 80 [IQR 39-80]. Data about usual post stroke rehabilitation were available for 18/24 (75%). Outcome data were available for 22/24 (92%) at one month and 20/24 (83%) at three months. Outcome assessors were unblinded to participant group allocation for 11/22 (50%) at one month and 6/20 (30%) at three months. Four adverse events were considered serious as they resulted in hospitalisation. None were related to study treatment. Feedback from patients and local NHS therapists about the RFTP programme was mainly positive.
Conclusions
A multi-centre randomised controlled trial to evaluate an upper limb RFTP therapy programme provided early after stroke is feasible and acceptable to patients and therapists, but there are issues which needed to be addressed when designing a Phase III study. A Phase III study will need to monitor and report not only recruitment and attrition but also adherence to the intervention, usual post stroke rehabilitation received, and outcome assessor blinding
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