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Search Results: 1 - 10 of 385152 matches for " N. C. Hearn "
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Testing Strict Hydrostatic Equilibrium in Simulated Clusters of Galaxies: Implications to Abell 1689
S. M. Molnar,I. -N. Chiu,K. Umetsu,P. Chen,N. Hearn,T. Broadhurst,G. Bryan,C. Shang
Physics , 2010, DOI: 10.1088/2041-8205/724/1/L1
Abstract: Accurate mass determination of clusters of galaxies is crucial if they are to be used as cosmological probes. However, there are some discrepancies between cluster masses determined based on gravitational lensing, and X-ray observations assuming strict hydrostatic equilibirium (i.e., the equilibrium gas pressure is provided entirely by thermal pressure). Cosmological simulations suggest that turbulent gas motions remaining from hierarchical structure formation may provide a significant contribution to the equilibrium pressure in clusters. We analyze a sample of massive clusters of galaxies drawn from high resolution cosmological simulations, and find a significant contribution (20%-45%) from non-thermal pressure near the center of relaxed clusters, and, in accord with previous studies, a minimum contribution at about 0.1 Rvir, growing to about 30%-45% at the virial radius, Rvir. Our results strongly suggest that relaxed clusters should have significant non-thermal support in their core region. As an example, we test the validity of strict hydrostatic equilibirium in the well-studied massive galaxy cluster Abell 1689 using the latest high resolution gravitational lensing and X-ray observations. We find a contribution of about 40% from non-thermal pressure within the core region of A1689, suggesting an alternate explanation for the mass discrepancy: the strict hydrostatic equilibirium is not valid in this region.
Chandra Observations of Extended X-ray Emission in Arp 220
J. C. McDowell,D. L. Clements,S. A. Lamb,S. Shaked,N. C. Hearn,L. Colina,C. Mundell,K. Borne,A. C. Baker,S. Arribas
Physics , 2003, DOI: 10.1086/375289
Abstract: We resolve the extended X-ray emission from the prototypical ultraluminous infrared galaxy Arp 220. Extended, faint edge-brightened, soft X-ray lobes outside the optical galaxy are observed to a distance of 10 to 15 kpc on each side of the nuclear region. Bright plumes inside the optical isophotes coincide with the optical line emission and extend 11 kpc from end to end across the nucleus. The data for the plumes cannot be fit by a single temperature plasma, and display a range of temperatures from 0.2 to 1 keV. The plumes emerge from bright, diffuse circumnuclear emission in the inner 3 kpc centered on the Halpha peak, which is displaced from the radio nuclei. There is a close morphological correspondence between the Halpha and soft X-ray emission on all spatial scales. We interpret the plumes as a starburst-driven superwind, and discuss two interpretations of the emission from the lobes in the context of simulations of the merger dynamics of Arp 220.
Separate Populations of Neurons in Ventral Striatum Encode Value and Motivation
Gregory B. Bissonette, Amanda C. Burton, Ronny N. Gentry, Brandon L. Goldstein, Taylor N. Hearn, Brian R. Barnett, Vadim Kashtelyan, Matthew R. Roesch
PLOS ONE , 2013, DOI: 10.1371/journal.pone.0064673
Abstract: Neurons in the ventral striatum (VS) fire to cues that predict differently valued rewards. It is unclear whether this activity represents the value associated with the expected reward or the level of motivation induced by reward anticipation. To distinguish between the two, we trained rats on a task in which we varied value independently from motivation by manipulating the size of the reward expected on correct trials and the threat of punishment expected upon errors. We found that separate populations of neurons in VS encode expected value and motivation.
Arp 119: A High Speed Galaxy Collision with Episodic Star Formation
Nathan C. Hearn,Susan A. Lamb
Physics , 2000, DOI: 10.1086/320217
Abstract: Colliding galaxies are excellent laboratories for studying galactic evolution and global star formation. Computer simulations of galaxy collisions, in which at least one galaxy has a significant gaseous component, show the production of density enhancements and shock waves in the interstellar medium. These high-density regions coincide with the locations of recent, large-scale star formation in observations of some real colliding galaxies. Thus, combined n-body/hydrodynamic computer simulations can be used to explore the history and conditions of star forming regions in colliding galaxies. The galaxy system Arp 119 (CPG 29) contains a southern member, Arp 119S (Mrk 984), that has an extremely disturbed appearance, and a northern member, Arp 119N, a gas-poor elliptical. The morphology of both members can be fit well by a simulation in which a gas-rich disk galaxy has been impacted by an equal mass elliptical that had a trajectory approximately perpendicular to the plane of the disk and passed through the disk slightly off-center. We compare multi-wavelength observations of the Arp 119 system with a combined n-body/SPH simulation of colliding galaxies. We find that the progression of recent, large-scale star formation in this galaxy can be accounted for by a single, outwardly propagating collision-induced density wave in the gas.
Galaxy Collisions and Star Formation
Susan A. Lamb,Nathan C. Hearn
Physics , 2000,
Abstract: We present a brief overview of some recent observations of colliding galaxies and relevant numerical simulations. These are compared, and details of the locations and history of collision induced star formation are explored, with possible application to star formation at earlier epochs.
Collisions and Mergers of Disk Galaxies: Hydrodynamics of Star Forming Gas
Susan A. Lamb,Nathan C. Hearn
Physics , 2003, DOI: 10.1023/A:1024048009270
Abstract: We summarize the results of numerical simulations of colliding gas-rich disk galaxies in which the impact velocity is set parallel to the spin axes of the two galaxies. The effects of varying the impact speed are studied with particular attention to the resulting gaseous structures and shockwave patterns, and the time needed to produce these structures. The simulations employ an N-body treatment of the stars and dark matter, together with an SPH treatment of the gas, in which all components of the models are gravitationally active. The results indicate that for such impact geometries, collisions can lead to the very rapid formation of a central, rapidly rotating, dense gas disk, and that in all cases extensive star formation is predicted by the very high gas densities and prevalence of shocks, both in the nucleus and out in the galactic disks. As the dense nucleus is forming, gas and stars are dispersed over very large volumes, and only fall back towards the nucleus over long times. In the case of low impact velocities, this takes an order of magnitude more time than that needed for the formation of a dense nucleus.
Alien Maps of an Ocean-Bearing World
N. B. Cowan,E. Agol,V. S. Meadows,T. Robinson,T. A. Livengood,D. Deming,C. M. Lisse,M. F. A'Hearn,D. D. Wellnitz,S. Seager,D. Charbonneau
Physics , 2009, DOI: 10.1088/0004-637X/700/2/915
Abstract: [Abridged] To simulate the kinds of observations that will eventually be obtained for exoplanets, the Deep Impact spacecraft obtained light curves of Earth at seven wavebands spanning 300-1000 nm as part of the EPOXI mission of opportunity. In this paper we analyze disc-integrated light curves, treating Earth as if it were an exoplanet, to determine if we can detect the presence of oceans and continents. We present two observations each spanning one day, taken at gibbous phases. The rotation of the planet leads to diurnal albedo variations of 15-30%, with the largest relative changes occuring at the reddest wavelengths. To characterize these variations in an unbiased manner we carry out a principal component analysis of the multi-band light curves; this analysis reveals that 98% of the diurnal color changes of Earth are due to only 2 dominant eigencolors. We use the time-variations of these two eigencolors to construct longitudinal maps of the Earth, treating it as a non-uniform Lambert sphere. We find that the spectral and spatial distributions of the eigencolors correspond to cloud-free continents and oceans; this despite the fact that our observations were taken on days with typical cloud cover. We also find that the near-infrared wavebands are particularly useful in distinguishing between land and water. Based on this experiment we conclude that it should be possible to infer the existence of water oceans on exoplanets with time-resolved broadband observations taken by a large space-based coronagraphic telescope.
Herschel and IRAM-30m Observations of Comet C/2012 S1 (ISON) at 4.5 AU from the Sun
L. O'Rourke,D. Bockelée-Morvan,N. Biver,B. Altieri,D. Teyssier,L. Jorda,V. Debout,C. Snodgrass,M. Küppers,M. A'Hearn,T. G. Müller,T. Farnham
Physics , 2013, DOI: 10.1051/0004-6361/201322756
Abstract: Our goal was to characterize the distant gaseous and dust activity of comet C\2012 S1 (ISON), inbound, from observations of H2O, CO and the dust coma in the far-infrared and submillimeter domains. In this paper, we report observations undertaken with the Herschel Space Observatory on 8 & 13 March 2013 (rh = 4.54 - 4.47AU) and with the 30m telescope of Institut de Radioastronomie Millim\'etrique (IRAM) in March and April 2013 (rh = 4.45 - 4.18 AU). The HIFI instrument aboard Herschel was used to observe the H$_{2}$O $1_{10}-1_{01}$ line at 557 GHz, whereas images of the dust coma at 70 and 160 {\mu}m were acquired with the PACS instrument. Spectra acquired at the IRAM 30m telescope cover the CO J(2-1) line at 230.5 GHz. The spectral observations were analysed with excitation and radiative transfer models. A model of dust thermal emission taking into account a range of dust sizes is used to analyse the PACS maps. While H$_{2}$O was not detected in our 8 March 2013 observation, we derive a sensitive 3 $\sigma$ upper limit of QH$_{2}$O < 3.5 x 10$^{26}$ molecules/s for this date. A marginal 3.2 $\sigma$ detection of CO is found, corresponding to a CO production rate of QCO = 3.5 x 10$^{27}$ molecules/s. The Herschel PACS measurements show a clear detection of the coma and tail in both the 70 {\mu}m and 160 {\mu}m maps. Under the assumption of a 2 km radius nucleus, we infer dust production rates in the range 10 - 13 kg/s or 40 - 70 kg/s depending on whether a low or high gaseous activity from the nucleus surface is assumed. We constrain the size distribution of the emitted dust by comparing PACS 70 and 160 {\mu}m data, and considering optical data. Size indices between -4 and -3.6 are suggested. The morphology of the tail observed on 70 {\mu}m images can be explained by the presence of grains with ages older than 60 days.
Some Thoughts on Electrical Interventions for the Control of Tremor in Parkinson's Dinase
A. N. Hearn,Madeleine Lowery,Annraoi M de Paor
Advances in Electrical and Electronic Engineering , 2008,
Abstract: Ideas from control theory – state variable feedback, dither injection, the equivalent nonlinearity and the describing function-are applied to models developed to help understand the mechanisms of electrical interventions such as deep brain stimulation (DBS) for the alleviation of Parkinsonian tremor.
Diel use of a saltwater creek by white-tip reef sharks Triaenodon obesus (Carcharhiniformes: Carcharhinidae) in Academy Bay, Galapagos Islands
César Pe?aherrera,Alex R Hearn,Angela Kuhn
Revista de Biología Tropical , 2012,
Abstract: White-tip reef sharks are common inhabitants of the shallow waters surrounding the Galapagos Islands, where several known aggregation sites have become touristic attractions. With the aim to describe site fidelity and residency patterns of the white-tip reef sharks in a saltwater creek, we used the ultrasonic telemetry method. The study was undertaken in a saltwater channel South of Academy Bay, Santa Cruz Island, from May 2008-September 2009. A total of nine transmitters were attached to sharks and ultrasonic receivers were deployed at the inner and outside areas of the creek. From the total of fitted sharks, four lost their transmitters. The results obtained with the remaining sharks showed an elevated use of the inner area of the channel during the day, with more use of the external area during the night. However, none of the sharks were detected at the site every day, suggesting that they may have a number of preferred sites within their home range. More studies are needed to detail the home range and habitat use of this species, and to guide its protection level in the AcademyBay area. Rev. Biol. Trop. 60 (2): 735-743. Epub 2012 June 01. Los tiburones punta blanca de arrecife son habitantes comunes de las aguas que rodean las Islas Galápagos, por lo que muchos de sus sitios de agregación se han convertido en atractivos turísticos. Con el objetivo de describir la fidelidad del sitio y los patrones de residencia de nueve tiburones desde mayo 2008-septiembre 2009, se utilizó telemetría ultrasónica en un canal de agua salada en el sur de Bahía Academia, Isla Santa Cruz. A pesar de que cuatro tiburones perdieron sus transmisores, los restantes tiburones monitoreados mostraron un uso elevado del interior del canal durante el día y del exterior durante la noche. Sin embargo, ninguno de los tiburones fue detectado en el sitio diariamente, lo cual sugiere que deben tener un número mayor de sitios preferidos dentro de su área de vida.
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