4,404 research outputs found
The dependence of HII region properties on global and local surface brightness within galaxy discs
Using B, R, and H-alpha images of roughly equal-sized samples of low surface
brightness (LSB) and high surface brightness (HSB) galaxies (~40 galaxies
apiece), we have explored the dependence of HII region properties on local and
global disc surface brightness. We have done this by constructing co-added HII
region luminosity functions (LFs) according to local and central disc surface
brightness and fitting Schechter functions to these LFs. The results show that
the shape of the HII region LF within LSB galaxies does not change noticeably
as different limiting (i.e., mu>mu_lim) local surface brightness values are
used. However, the LFs for HSB galaxies have larger values of L_* and are less
steep at the faint-end than those of LSB galaxies for limiting B-band local
surface brightness values as faint as mu_B,lim~23-24. Both the LFs and the data
for individual HII regions show that luminous (log L>39 ergs/s) HII regions are
much more common within HSB discs than within LSB discs, implying that the
newly formed star clusters are also larger. Taking this into account along with
the results of Monte Carlo simulations, the shapes of the LFs imply that the
regions within LSB discs and those within the LSB areas of HSB discs are
relatively old (~5 Myr) while the regions within HSB discs for mu_B<24 are
significantly younger (<1 Myr). Since the majority of the LSB galaxies do not
have noticeable spiral arms and the majority of the HSB galaxies do, this may
indicate a transition within HSB discs from spiral arm-driven star formation to
a more locally driven, possibly sporadic form of star formation at mu_B~24, a
transition that does not appear to occur within LSB discs.Comment: Accepted to MNRA
Degenerate mixing of plasma waves on cold, magnetized single-species plasmas
In the cold-fluid dispersion relation ω = ω_p/[1+(k_⊥/k_z)^(2]1/2) for Trivelpiece-Gould waves on an infinitely long magnetized plasma cylinder, the transverse and axial wavenumbers appear only in the combination k_⊥/k_z. As a result, for any frequency ω<ω_p, there are infinitely many degenerate waves, all having the same value of k_⊥/k_z. On a cold finite-length plasma column, these degenerate waves reflect into one another at the ends; thus, each standing-wave normal mode of the bounded plasma is a mixture of many degenerate waves, not a single standing wave as is often assumed. A striking feature of the many-wave modes is that the short-wavelength waves often add constructively along resonance cones given by dz/dr = ±(ω_p^2/ω^2-1)^(1/2). Also, the presence of short wavelengths in the admixture for a predominantly long-wavelength mode enhances the viscous damping beyond what the single-wave approximation would predict. Here, numerical solutions are obtained for modes of a cylindrical plasma column with rounded ends. Exploiting the fact that the modes of a spheroidal plasma are known analytically (the Dubin modes), a perturbation analysis is used to investigate the mixing of low-order, nearly degenerate Dubin modes caused by small deformations of a plasma spheroid
Supporting sit-to-stand rehabilitation using smartphone sensors and arduino haptic feedback modules
The aim of this project is to design and build a system to aid patients in their rehabilitation after suffering a stroke. A stroke is one of the most serious conditions that an individual can suffer from, and the rehabilitation is often a long and difficult process. For many with movement effects, the sit-to-stand exercise is an important step in rehabilitation. The focus of this ongoing project is to create a system to assist sit-to-stand rehabilitation through the use of haptic feedback on balance. In this poster we present our initial prototype using standard smartphone accelerometers linked wirelessly to Arduino based vibration feedback modules mounted on the patients' legs. Initial feedback on the prototype is promising
Response to Comment on `Undamped electrostatic plasma waves' [Phys. Plasmas 19, 092103 (2012)]
Numerical and experimental evidence is given for the occurrence of the
plateau states and concomitant corner modes proposed in \cite{valentini12}. It
is argued that these states provide a better description of reality for small
amplitude off-dispersion disturbances than the conventional
Bernstein-Greene-Kruskal or cnoidal states such as those proposed in
\cite{comment
Low Surface Brightness Galaxies around the HDF-S: II. Distances and volume densities
With this study we aim at the spectroscopic verification of a photometrically
selected sample of Low Surface Brightness (LSB) galaxy candidates in a field
around the Hubble Deep Field-South (HDF-S). The sample helps to extend the
parameter space for LSB galaxies to lower central surface brightnesses and to
provide better estimates on the volume densities of these objects. To derive
redshifts for the LSB candidates, long-slit spectra were obtained covering a
spectral range from 3400{\AA} to 7500{\AA}. The observations have been obtained
using the ESO 3.6m telescope, equipped with the EFOSC2 spectrograph. From the
measured radial velocities, distances could be estimated. With this distance
information, it is possible to differentiate between true LSB galaxies and
higher redshift High Surface Brightness (HSB) galaxies which may contaminate
the sample. A correction for the surface brightnesses can then be applied,
accounting for the cosmological dimming effect (``Tolman Dimming''). We show
that ~70% of the LSB candidates, selected based on their location in the
color-color space, are real LSB galaxies. Their position in the color-color
diagrams, therefore, indicate that the LSB galaxies have a different stellar
population mix resulting from a different star formation history compared to
HSBs. Our LSB galaxy sample consists only of large disk galaxies with
scale-length between 2.5kpc and 7.3kpc. We confirm the flat central surface
brightness distribution of previous surveys and extend this distribution down
to central surface brightnesses of 27 B mag arcsec^-2.Comment: 12 pages, 20 figures, accepted by A&
Prospects for Measuring Vtb via s-channel Single Top at ATLAS
The production of single top quarks via the electroweak interaction promises to provide new opportunities to both test the Standard Model and search for new physics. In particular, electroweak top production provides the only means to directly measure the CKM matrix element Vtb at ATLAS. The s-channel has the lowest rate, but is the best theoretically understood mechanism of electroweak top production. An evaluation of the potential for background suppression and Vtb measurement in this channel is presented. It is found that significant background suppression can be achieved and Vtb can be measured in the s-channel to a statistical precision of 2.8% after 30 inverse femtobarns of integrated luminosity at the LHC
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