6,712 research outputs found
Mrk 1419 - a new distance determination
Water vapor megamasers from the center of active galaxies provide a powerful
tool to trace accretion disks at sub-parsec resolution and, through an entirely
geometrical method, measure direct distances to galaxies up to 200 Mpc. The
Megamaser Cosmology Project (MCP) is formed by a team of astronomers with the
aim of identifying new maser systems, and mapping their emission at high
angular resolution to determine their distance. Two types of observations are
necessary to measure a distance: single-dish monitoring to measure the
acceleration of gas in the disk, and sensitive VLBI imaging to measure the
angular size of the disk, measure the rotation curve, and model radial
displacement of the maser feature. The ultimate goal of the MCP is to make a
precise measurement of H0 by measuring such distances to at least 10 maser
galaxies in the Hubble flow. We present here the preliminary results from a new
maser system, Mrk 1419. Through a model of the rotation from the systemic
masers assuming a narrow ring, and combining these results with the
acceleration measurement from the Green Bank Telescope, we determine a distance
to Mrk 1419 of 81\pm10 Mpc. Given that the disk shows a significant warp that
may not be entirely traced by our current observations, more sensitive
observations and more sophisticated disk modeling will be essential to improve
our distance estimation to this galaxy.Comment: 5 pages, 3 figures, to appear in the proceedings of IAU Symposium 287
"Cosmic Masers- from OH to Ho", in Stellenbosch, S
The Submillimeter Array
The Submillimeter Array (SMA), a collaborative project of the Smithsonian
Astrophysical Observatory (SAO) and the Academia Sinica Institute of Astronomy
and Astrophysics (ASIAA), has begun operation on Mauna Kea in Hawaii. A total
of eight 6-m telescopes comprise the array, which will cover the frequency
range of 180-900 GHz. All eight telescopes have been deployed and are
operational. First scientific results utilizing the three receiver bands at
230, 345, and 690 GHz have been obtained and are presented in the accompanying
papers.Comment: 10 pages, 4 figure
Motility induced changes in viscosity of suspensions of swimming microbes in extensional flows
Suspensions of motile cells are model systems for understanding the unique
mechanical properties of living materials which often consist of ensembles of
self-propelled particles. We present here a quantitative comparison of theory
against experiment for the rheology of such suspensions. The influence of
motility on viscosities of cell suspensions is studied using a novel
acoustically-driven microfluidic capillary-breakup extensional rheometer.
Motility increases the extensional viscosity of suspensions of algal pullers,
but decreases it in the case of bacterial or sperm pushers. A recent model
[Saintillan, Phys. Rev. E, 2010, 81:56307] for dilute active suspensions is
extended to obtain predictions for higher concentrations, after independently
obtaining parameters such as swimming speeds and diffusivities. We show that
details of body and flagellar shape can significantly determine macroscale
rheological behaviour.Comment: 12 pages, 1 appendix, 7 figures, submitted to Soft Matter - under
revie
Linking Arrhythmias and Adipocytes: Insights, Mechanisms, and Future Directions
Obesity and atrial fibrillation have risen to epidemic levels worldwide and may continue to grow over the next decades. Emerging evidence suggests that obesity promotes atrial and ventricular arrhythmias. This has led to trials employing various strategies with the ultimate goal of decreasing the atrial arrhythmic burden in obese patients. The effectiveness of these interventions remains to be determined. Obesity is defined by the expansion of adipose mass, making adipocytes a prime candidate to mediate the pro-arrhythmogenic effects of obesity. The molecular mechanisms linking obesity and adipocytes to increased arrhythmogenicity in both the atria and ventricles remain poorly understood. In this focused review, we highlight areas of potential molecular interplay between adipocytes and cardiomyocytes. The effects of adipocytes may be direct, local or remote. Direct effect refers to adipocyte or fatty infiltration of the atrial and ventricular myocardium itself, possibly causing increased dispersion of normal myocardial electrical signals and fibrotic substrate of adipocytes that promote reentry or adipocytes serving as a direct source of aberrant signals. Local effects may originate from nearby adipose depots, specifically epicardial adipose tissue (EAT) and pericardial adipose tissue, which may play a role in the secretion of adipokines and chemokines that can incite inflammation given the direct contact and disrupt the conduction system. Adipocytes can also have a remote effect on the myocardium arising from their systemic secretion of adipokines, cytokines and metabolites. These factors may lead to mitochondrial dysfunction, oxidative stress, autophagy, mitophagy, autonomic dysfunction, and cardiomyocyte death to ultimately produce a pro-arrhythmogenic state. By better understanding the molecular mechanisms connecting dysfunctional adipocytes and arrhythmias, novel therapies may be developed to sever the link between obesity and arrhythmias
Circumnuclear Structures in Megamaser Host Galaxies
Using HST, we identify circumnuclear (- pc scale) structures in
nine new HO megamaser host galaxies to understand the flow of matter from
kpc-scale galactic structures down to the supermassive black holes (SMBHs) at
galactic centers. We double the sample analyzed in a similar way by Greene et
al. (2013) and consider the properties of the combined sample of 18 sources. We
find that disk-like structure is virtually ubiquitous when we can resolve
pc scales, in support of the notion that non-axisymmetries on these
scales are a necessary condition for SMBH fueling. We perform an analysis of
the orientation of our identified nuclear regions and compare it with the
orientation of megamaser disks and the kpc-scale disks of the hosts. We find
marginal evidence that the disk-like nuclear structures show increasing
misalignment from the kpc-scale host galaxy disk as the scale of the structure
decreases. In turn, we find that the orientation of both the pc scale
nuclear structures and their host galaxy large-scale disks is consistent with
random with respect to the orientation of their respective megamaser disks.Comment: 24 pages, 16 figures, 4 tables; Resubmitted to ApJ after referee's
comment
Renovascular adaptive changes in chronic hypoxic polycythemia
Renovascular adaptive changes in chronic hypoxic polycythemia.BackgroundChronic hypoxia in rats produces polycythemia, and the plasma fraction falls, reducing renal plasma flow (RPF) relative to renal blood flow (RBF). Polycythemia also causes increased blood viscosity, which tends to reduce RBF and renal oxygen delivery. We studied how renal regulation of electrolyte balance and renal tissue oxygenation (which is crucial for erythropoietin regulation) are maintained in rats during hypoxic exposure.MethodsRats of two strains with differing polycythemic responses, with surgically implanted catheters in the urinary bladder, femoral artery, and left renal and right external jugular veins, were exposed to a simulated high altitude (0.5 atm) for 0, 1, 3, 14, and 30days, after which RPF (para-aminohippurate clearance), glomerular filtration rate (GFR, polyfructosan clearance), hematocrit and blood gases were measured, and RBF, renal vascular resistance and hindrance (resistance/viscosity), renal oxygen delivery, and renal oxygen consumption were calculated.ResultsDuring chronic hypoxia RBF increased, but RPF decreased because of the polycythemia. GFR remained normal because the filtration fraction (FF) increased. Renal vascular resistance decreased, and renal vascular hindrance decreased more markedly. Renal oxygen delivery and consumption both increased.ConclusionsDuring chronic hypoxia GFR homeostasis apparently took precedence over RBF autoregulation. The large decrease in renal vascular hindrance suggested that renal vascular remodeling contributes to GFR regulation. The reduced hindrance also prevented a vicious cycle of increasing polycythemia and blood viscosity, decreasing RBF, and increasing renal hypoxia and erythropoietin release
Megamaser Disks Reveal a Broad Distribution of Black Hole Mass in Spiral Galaxies
We use new precision measurements of black hole masses from water megamaser
disks to investigate scaling relations between macroscopic galaxy properties
and supermassive black hole (BH) mass. The megamaser-derived BH masses span
10^6-10^8 M_sun, while all the galaxy properties that we examine (including
stellar mass, central mass density, central velocity dispersion) lie within a
narrow range. Thus, no galaxy property correlates tightly with M_BH in ~L*
spiral galaxies. Of them all, stellar velocity dispersion provides the tightest
relation, but at fixed sigma* the mean megamaser M_BH are offset by -0.6+/-0.1
dex relative to early-type galaxies. Spiral galaxies with non-maser dynamical
BH masses do not show this offset. At low mass, we do not yet know the full
distribution of BH mass at fixed galaxy property; the non-maser dynamical
measurements may miss the low-mass end of the BH distribution due to inability
to resolve the spheres of influence and/or megamasers may preferentially occur
in lower-mass BHs.Comment: 6 pages, 4 figures, replaced to fix error: NGC 4594 is not a maser
galax
The effect of aspect ratio on the wake of the Ahmed body
This paper seeks to further elucidate the wake of the Ahmed body by investigating how the time-averaged flow structures vary with frontal aspect ratio. High-resolution particle image velocimetry results are provided for eight different width Ahmed geometries at Re = 3 × 10^4. It is shown that the narrower the body, the greater the downwash over the back slant, meaning the flow remains more attached. At a critical aspect ratio ( AR = 1.9), the flow downstream changes. The separation over the back slant is shown to be affected by the AR, and this in turn has a significant effect on the circulation in the c-pillar vortices
The nature of the vortical structures in the near wake of the Ahmed body
This study presents the results from high-spatial-resolution water-channel velocity-field measurements behind an Ahmed body with 25° rear slant angle. The Ahmed body represents a simplified generic model of a hatchback automobile that has been widely used to study near-wake flow dynamics. The results help clarify the unresolved question of whether the time-mean near-wake flow structure is topologically equivalent to a toroidal vortex or better described by a pair of horizontally aligned horseshoe vortices, with their legs pointing downstream. The velocimetry data presented allows the tracking of the vortical structures throughout the near wake through a set of orthogonal planes, as well as the measurement of their circulation. The spanwise vortices that form as the flow separates from the top and bottom rear edges are shown to tilt downstream at the sides of the body, while no evidence is found of a time-mean attached toroidal vortex, at least for the Reynolds number (based on the square root of the frontal area) of Re~30000 under consideration
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