2,699 research outputs found
Masses, luminosities and dynamics of galactic molecular clouds
Star formation in galaxies takes place in molecular clouds and the Milky Way is the only galaxy in which it is possible to resolve and study the physical properties and star formation activity of individual clouds. The masses, luminosities, dynamics, and distribution of molecular clouds, primarily giant molecular clouds in the Milky Way are described and analyzed. The observational data sets are the Massachusetts-Stony Brook CO Galactic Plane Survey and the IRAS far IR images. The molecular mass and infrared luminosities of glactic clouds are then compared with the molecular mass and infrared luminosities of external galaxies
The Cool ISM in S0 Galaxies. I. A Survey of Molecular Gas
Lenticular galaxies remain remarkably mysterious as a class. Observations to
date have not led to any broad consensus about their origins, properties and
evolution, though they are often thought to have formed in one big burst of
star formation early in the history of the Universe, and to have evolved
relatively passively since then. In that picture, current theory predicts that
stellar evolution returns substantial quantities of gas to the interstellar
medium; most is ejected from the galaxy, but significant amounts of cool gas
might be retained. Past searches for that material, though, have provided
unclear results. We present results from a survey of molecular gas in a
volume-limited sample of field S0 galaxies, selected from the Nearby Galaxies
Catalog. CO emission is detected from 78 percent of the sample galaxies. We
find that the molecular gas is almost always located inside the central few
kiloparses of a lenticular galaxy, meaning that in general it is more centrally
concentrated than in spirals. We combine our data with HI observations from the
literature to determine the total masses of cool and cold gas. Curiously, we
find that, across a wide range of luminosity, the most gas rich galaxies have
about 10 percent of the total amount of gas ever returned by their stars. That
result is difficult to understand within the context of either monolithic or
hierarchical models of evolution of the interstellar medium.Comment: 26 pages of text, 15 pages of tables, 10 figures. Accepted for
publication in the Astrophysical Journa
Non-linear Dependence of L(B) on L(FIR) and M(H2) among Spiral Galaxies and Effects of Tidal Interaction
Through the study of a carefully selected sample of isolated spiral galaxies,
we have established that two important global physical quantities for tracing
star forming activities, L(FIR) and M(H2), have non-linear dependence on
another commonly cited global quantity L(B). Furthermore we show that simple
power law relations can effectively describe these non-linear relations for
spiral galaxies spanning four orders of magnitude in FIR and M(H2) and nearly
three orders of magnitude in L(B). While the existence of non-linear dependence
of M(H2) (assuming a constant CO-to-H2 conversion) and L(FIR) on optical
luminosity L(B) has been previously noted in the literature, an improper
normalization of simple scaling by L(B) has been commonly used in many previous
studies to claim enhanced molecular gas content and induced activities among
tidally interacting and other types of galaxies. We remove these non-linear
effects using the template relations derived from the isolated galaxy sample
and conclude that strongly interacting galaxies do not have enhanced molecular
gas content, contrary to previous claims. With these non-linear relations among
L(B), L(FIR) and M(H2) properly taken into account, we confirm again that the
FIR emission and the star formation efficiency L(FIR)/M(H2) are indeed enhanced
by tidal interactions. Virgo galaxies show the same level of M(H2) and L(FIR)
as isolated galaxies. We do not find any evidence for enhanced star forming
activity among barred galaxies.Comment: 19 pages and 5 figures, requires AAS style files, ApJ, accepte
Coherence of Nitrogen-Vacancy Electronic Spin Ensembles in Diamond
We present an experimental and theoretical study of electronic spin
decoherence in ensembles of nitrogen-vacancy (NV) color centers in bulk
high-purity diamond at room temperature. Under appropriate conditions, we find
ensemble NV spin coherence times (T_2) comparable to that of single NVs, with
T_2 > 600 microseconds for a sample with natural abundance of 13C and
paramagnetic impurity density ~10^15 cm^(-3). We also observe a sharp decrease
of the coherence time with misalignment of the static magnetic field relative
to the NV electronic spin axis, consistent with theoretical modeling of NV
coupling to a 13C nuclear spin bath. The long coherence times and increased
signal-to-noise provided by room-temperature NV ensembles will aid many
applications of NV centers in precision magnetometry and quantum information.Comment: 5 pages, 3 figures; v2 minor correction
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PPARÎł agonists negatively regulate αIIbÎČ3 integrin outside-in signalling and platelet function through upregulation of protein kinase A activity
BACKGROUND:
Agonists for the peroxisome proliferator activated receptor PPARÎł, have been shown to have inhibitory effects on platelet activity following stimulation by GPVI and GPCR agonists.
OBJECTIVES:
Profound effects on thrombus formation led us to suspect a role for PPARÎł agonists in the regulation of integrin αIIbÎČ3 mediated signalling. Both GPVI and GPCR signalling pathways lead to αIIbÎČ3 activation, and signalling through αIIbÎČ3 plays a critical role in platelet function and normal haemostasis.
METHODS:
The effects of PPARÎł agonists on the regulation of αIIbÎČ3 outside-in signalling was determined by monitoring the ability of platelets to adhere and spread on fibrinogen and undergo clot retraction. Effects on signalling components downstream of αIIbÎČ3 activation were also determined following adhesion to fibrinogen by western blotting.
RESULTS:
Treatment of platelets with PPARÎł agonists inhibited platelet adhesion and spreading on fibrinogen and diminished clot retraction. A reduction in phosphorylation of several components of αIIbÎČ3 signalling, including the integrin ÎČ3 subunit, Syk, PLCÎł2, FAK and Akt was also observed as a result of reduced interaction of the integrin ÎČ3 subunit with Gα13. Studies of VASP phosphorylation revealed that this was a due to an increase in PKA activity following treatment with PPARÎł receptor agonists.
CONCLUSIONS:
This study provides further evidence for anti-platelet actions of PPARÎł agonists, identifies a negative regulatory role for PPARÎł agonists in the control of integrin αIIbÎČ3 outside-in signalling, and provides a molecular basis by which the PPARÎł agonists negatively regulate platelet activation and thrombus formation
Bellybutton: Accessible and Customizable Deep-Learning Image Segmentation
The conversion of raw images into quantifiable data can be a major hurdle in
experimental research, and typically involves identifying region(s) of
interest, a process known as segmentation. Machine learning tools for image
segmentation are often specific to a set of tasks, such as tracking cells, or
require substantial compute or coding knowledge to train and use. Here we
introduce an easy-to-use (no coding required), image segmentation method, using
a 15-layer convolutional neural network that can be trained on a laptop:
Bellybutton. The algorithm trains on user-provided segmentation of example
images, but, as we show, just one or even a portion of one training image can
be sufficient in some cases. We detail the machine learning method and give
three use cases where Bellybutton correctly segments images despite substantial
lighting, shape, size, focus, and/or structure variation across the regions(s)
of interest. Instructions for easy download and use, with further details and
the datasets used in this paper are available at
pypi.org/project/Bellybuttonseg.Comment: 6 Pages 3 Figure
Deformation of a nearly hemispherical conducting drop due to an electric field: theory and experiment
We consider, both theoretically and experimentally, the deformation due to an electric field of a pinned nearly-hemispherical static sessile drop of an ionic fluid with a high conductivity resting on the lower substrate of a parallel plate capacitor. Using both numerical and asymptotic approaches we find solutions to the coupled electrostatic and augmented YoungâLaplace equations which agree very well with the experimental results. Our asymptotic solution for the drop interface extends previous work in two ways, namely to drops that have zero-field contact angles that are not exactly Ï/2 and to higher order in the applied electric field, and provides useful predictive equations for the changes in the height, contact angle and pressure as functions of the zero-field contact angle, drop radius, surface tension and applied electric field. The asymptotic solution requires some numerical computations, and so a surprisingly accurate approximate analytical asymptotic solution is also obtained
Multiwavelength Observations of the Low Metallicity Blue Compact Dwarf Galaxy SBS 0335-052
New infrared and millimeter observations from Keck, Palomar, ISO, and OVRO
and archival data from the NRAO VLA and IRAS are presented for the low
metallicity blue compact dwarf galaxy SBS 0335-052. Mid-infrared imaging shows
this young star-forming system is compact (0.31"; 80 pc) at 12.5 microns. The
large Br-gamma equivalent width (235 Angstroms) measured from integral field
spectroscopy is indicative of a ~5 Myr starburst. The central source appears to
be optically thin in emission, containing both a warm (~80 K) and a hot (~210
K) dust component, and the overall interstellar radiation field is quite
intense, about 10,000 times the intensity in the solar neighborhood. CO
emission is not detected, though the galaxy shows an extremely high global H I
gas-to-dust mass ratio, high even for blue compact dwarfs. Finally, the
galaxy's mid-infrared-to-optical and mid-to-near-infrared luminosity ratios are
quite high, whereas its far-infrared-to-radio and far-infrared-to-optical flux
ratios are surprisingly similar to what is seen in normal star-forming
galaxies. The relatively high bolometric infrared-to-radio ratio is more easily
understood in the context of such a young system with negligible nonthermal
radio continuum emission. These new lines of evidence may outline features
common to primordial galaxies found at high redshift.Comment: 28 pages including 6 figures; accepted for publication in the
Astronomical Journa
Depression in Luminous Starburst Mergers
It is known that the class of luminous starburst galaxies tends to have
higher integrated line intensity ratios
() than normal spiral galaxies (). Since most previous studies
investigated only , it remains uncertain whether the luminous starburst
galaxies are overabundant in CO or underabundant in CO. Here we
propose a new observational test to examine this problem. Our new test is to
compare far-infrared luminosities [(FIR)] with those of CO and
and , respectively]. It is shown that there
is a very tight correlation between and L(FIR), as found in many
previous studies. However, we find that the CO luminosities of the
high-R galaxies are lower by a factor of three on the average than those
expected from the correlation for the remaining galaxies with ordinary
values. Therefore, we conclude that the observed high values for the
luminous starburst galaxies are attributed to their lower CO line
intensities.Comment: 9 pages (aaspp4.sty), 3 postscript figures (embedded). Accepted for
publication in Astrophysical Journal Letter
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