Veilleux S.,University of Maryland University College |
Melendez M.,University of Maryland University College |
Melendez M.,NASA |
Melendez M.,Wyle |
And 4 more authors.
Astrophysical Journal | Year: 2016
New near- and far-ultraviolet (NUV and FUV) Hubble Space Telescope spectra of Mrk 231, the nearest quasar known, are combined with ground-based optical spectra to study the remarkable dichotomy between the FUV and NUV-optical spectral regions in this object. The FUV emission-line features are faint, broad, and highly blueshifted (up to ∼7000 km s-1), with no significant accompanying absorption. In contrast, the profiles of the NUV absorption features resemble those of the optical Na i D, He i, and Ca ii H and K lines, exhibiting broad blueshifted troughs that overlap in velocity space with the FUV emission-line features and indicate a dusty, high-density and patchy broad absorption line (BAL) screen covering ∼90% of the observed continuum source at a distance ≲2-20 pc. The FUV continuum emission does not show the presence of any obvious stellar features and is remarkably flat compared with the steeply declining NUV continuum. The NUV (FUV) features and continuum emission have not varied significantly over the past ∼22 (3) years and are unresolved on scales ∼40 (170) pc. These results favor an active galactic nucleus origin for the NUV-FUV line and continuum emission. The observed FUV line emission is produced in the outflowing BAL cloud system, while the Balmer lines arise primarily from the standard broad line region seen through the dusty BAL screen. Our data are inconsistent with the recently proposed binary black hole model. We argue instead that Mrk 231 is the nearest example of weak-lined "wind-dominated" quasars with high Eddington ratios and geometrically thick ("slim") accretion disks; these quasars are likely more common in the early universe. © 2016. The American Astronomical Society. All rights reserved..
Finlator K.,New Mexico State University |
Finlator K.,Copenhagen University |
Oppenheimer B.D.,University of Colorado at Boulder |
Dave R.,University of the Western Cape |
And 5 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2016
The sources that drove cosmological reionization left clues regarding their identity in the slope and inhomogeneity of the ultraviolet ionizing background (UVB): bright quasars (QSOs) generate a hard UVB with predominantly large-scale fluctuations while Population II stars generate a softer one with smaller scale fluctuations. Metal absorbers probe the UVB's slope because different ions are sensitive to different energies. Likewise, they probe spatial fluctuations because they originate in regions where a galaxy-driven UVB is harder and more intense. We take a first step towards studying the reionization-epoch UVB's slope and inhomogeneity by comparing observations of 12 metal absorbers at z ~ 6 versus predictions from a cosmological hydrodynamic simulation using three different UVBs: a soft, spatially inhomogeneous 'galaxies+QSOs' UVB; a homogeneous 'galaxies+QSOs' UVB, and a 'QSOs-only' model. All UVBs reproduce the observed column density distributions of C II, Si IV, and C IV reasonably well although high-column, high-ionization absorbers are underproduced, reflecting numerical limitations. With upper limits treated as detections, only a soft, fluctuating UVB reproduces both the observed Si IV/C IV and C II/C IV distributions. The QSOs-only UVB overpredicts both C IV/C II and C IV/Si IV, indicating that it is too hard. The Haardt & Madau (2012) UVB underpredicts C IV/Si IV, suggesting that it lacks amplifications near galaxies. Hence current observations prefer a soft, fluctuating UVB as expected from a predominantly Population II background although they cannot rule out a harder one. Future observations probing a factor of 2 deeper in metal column density will distinguish between the soft, fluctuating and QSOs-only UVBs. © 2016 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.
Veilleux S.,University of Maryland University College |
Veilleux S.,NASA |
Veilleux S.,Max Planck Institute for Extraterrestrial Physics |
Trippe M.,University of Maryland University College |
And 13 more authors.
Astrophysical Journal | Year: 2013
Mrk 231, the nearest (z = 0.0422) quasar, hosts both a galactic-scale wind and a nuclear-scale iron low-ionization broad absorption line (FeLoBAL) outflow. We recently obtained a far-ultraviolet (FUV) spectrum of this object covering ≳1150-1470 Å with the Cosmic Origins Spectrograph on board the Hubble Space Telescope. This spectrum is highly peculiar, highlighted by the presence of faint (≲2% of predictions based on Hα), broad (≳10,000 km s-1 at the base), and highly blueshifted (centroid at ≳ -3500 km s-1) Lyα emission. The FUV continuum emission is slightly declining at shorter wavelengths (consistent with F λ∝λ1.7) and does not show the presence of any obvious photospheric or wind stellar features. Surprisingly, the FUV spectrum also does not show any unambiguous broad absorption features. It thus appears to be dominated by the AGN, rather than hot stars, and virtually unfiltered by the dusty FeLoBAL screen. The observed Lyα emission is best explained if it is produced in the outflowing BAL cloud system, while the Balmer lines arise primarily from the standard broad emission line region seen through the dusty (AV ≳ 7 mag) broad absorption line region. Two possible geometric models are discussed in the context of these new results. © 2013. The American Astronomical Society. All rights reserved.
Jeon S.-H.,Los Alamos National Laboratory |
Jeon S.-H.,University of Massachussetts |
Xu P.,Los Alamos National Laboratory |
Xu P.,Harbin Institute of Technology |
And 4 more authors.
Journal of Physical Chemistry C | Year: 2010
We report a facile synthesis of silver nanoparticles (AgNPs) by using a new reducing agent, pretreated N-methylpyrrolidone (NMP*). The resulting AgNPs are characterized by using UV-vis, TEM, and X-ray spectroscopy. These AgNPs exhibit strong surface enhanced Raman scattering response on addition of 4-mercaptobenzoic acid. A possible redox mechanism involving silver ion and NMP* was proposed. The oxidized species resulting from thermally treated NMP/O2 were analyzed by nuclear magnetic resonance and gas chromatography techniques, and it was determined that 5-hydroxy-N-methyl-2- pyrrolidone played the role of reducing agent. The facile synthesis of functional metal nanoparticles via an environmentally friendly procedure with control in particle size, and understanding of the reaction mechanisms pave the ways to further developing metal nanoparticles for chemical and biological detections. © 2010 American Chemical Society.
Jancar J.,Brno University of Technology |
Kalfus J.,Brno University of Technology |
Kalfus J.,University of Massachussetts |
Balkova R.,Brno University of Technology
Polymer Engineering and Science | Year: 2011
To understand the toughness enhancement of β-nucleated isotactic polypropylene (iPP) in comparison with iPP, the differences in the micro-deformation mechanisms between the neat iPP and β-nucleated iPP were visualized using the confocal laser scanning microscopy (CLSM). Structure of the α- and β-spherulites situated close to the tip of the sharp starter crack has been investigated during tensile deformation in the viewing field of the CLSM. In the α-spherulite of the neat iPP, highly localized inter- and intra-spherulitic micro-shear bands have been observed. In the β-nucleated iPP, relatively uniform distribution of diffuse shear bands has been observed in the β-phase, while the α-phase remained relatively undeformed exhibiting only narrow intra-spherulitic shear bands in the direction perpendicular to the loading direction. Delocalization of plastic deformation into diffuse shear bands in β-nucleated iPP can explain its enhanced crack resistance compared with the neat iPP exhibiting highly localized shear banding. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers.
Jancar J.,Brno University of Technology |
Hoy R.S.,University of South Florida |
Lesser A.J.,University of Massachussetts |
Jancarova E.,Brno University of Technology |
Zidek J.,Brno University of Technology
Macromolecules | Year: 2013
We present extensive studies of the nonlinear mechanics of neat, microparticle (MP) and/or nanoparticle (NP) filled PMMA. Rigid particles are used as "probes" that alter local chain packing, the degree of correlated molecular motion, and structural relaxation times. We show that the extent of these alterations are proportional to the specific particle-PMMA interface area and the spatial distribution of interparticle distances. Nanoparticles are used to maximize these probe effects because both their size (D) and the distance between them (DNP) are of order the radius of gyration of the polymer chains at low NP volume fractions. We analyze the effects of MPs and NPs on the elastic moduli and postyield response for samples prepared with different thermal histories and deformed at a range of temperatures and strain rates. In particular, we exploit qualitative differences between the response of PMMA composites deformed at Tlo = T g - 80 K and Thi = Tg - 20 K to illustrate key features of the relationship between the mechanically rejuvenated yield stress, σyr, and strain hardening modulus, GH. The observed trends are supported by kinetic analyses and exploited to test recently proposed models of plastic flow and strain hardening. Our results should lead to an improved understanding of the factors controlling the ultimate performance of polymer composites. © 2013 American Chemical Society.
Dave R.,University of Arizona |
Oppenheimer B.D.,Leiden University |
Katz N.,University of Massachussetts |
Kollmeier J.A.,Observatories of the Carnegie Institute of Washington |
Weinberg D.H.,Ohio State University
Monthly Notices of the Royal Astronomical Society | Year: 2010
The intergalactic medium (IGM) is the dominant reservoir of baryons at all cosmic epochs. In this paper, we investigate the evolution of the IGM from z= 2 → 0 in (48 h-1 Mpc)3, 110 million particle cosmological hydrodynamic simulations using three prescriptions for galactic outflows. We focus on the evolution of IGM physical properties, and how such properties are traced by Lyα absorption as detectable using Hubble's Cosmic Origins Spectrograph (COS). Our results broadly confirm the canonical picture that most Lyα absorbers arise from highly ionized gas tracing filamentary large-scale structure. Growth of structure causes gas to move from the diffuse photoionized IGM into other cosmic phases, namely stars, cold and hot gas within galaxy haloes, and the unbound and shock-heated warm-hot intergalactic medium (WHIM). By today, baryons are comparably divided between bound phases (35 per cent in our favoured outflow model), the diffuse IGM (41 per cent) and the WHIM (24 per cent). Here we (re)define the WHIM as gas with overdensities lower than that in haloes (today) and temperatures T > 105 K, to more closely align it with the 'missing baryons' that are not easily detectable in emission or Lyα absorption. Strong galactic outflows can have a noticeable impact on the temperature of the IGM, though with our favoured momentum-driven wind scalings they do not. When we (mildly) tune our assumed photoionizing background to match the observed evolution of the Lyα mean flux decrement, we obtain line count evolution statistics that broadly agree with available (pre-COS) observations. We predict a column density distribution slope of for our favoured wind model, in agreement with recent observational estimates, and it becomes shallower with redshift. Winds have a mostly minimal impact, but they do result in a shallower column density slope and more strong lines. With improved statistics, the frequency of strong lines can be a valuable diagnostic of outflows, and the momentum-driven wind model matches existing data significantly better than the two alternatives we consider. The relationship between column density and physical density broadens mildly from z= 2 → 0, and evolves as for diffuse absorbers, consistent with previous studies. Linewidth distributions are quite sensitive to spectral resolution; COS should yield significantly broader lines than higher resolution data. Thermal contributions to linewidths are typically subdominant, so linewidths only loosely reflect the temperature of the absorbing gas. This will hamper attempts to quantify the WHIM using broad Lyα absorbers, though it may still be possible to do so statistically. Together, COS data and simulations such as these will provide key insights into the physical conditions of the dominant reservoir of baryons over the majority of cosmic time. © 2010 The Authors. Journal compilation © 2010 RAS.
Cohen E.,University of Massachussetts |
Vikas V.,Tufts University |
Trimmer B.,Tufts University |
McCarthy S.,University of Massachussetts
Proceedings of the ASME Design Engineering Technical Conference | Year: 2015
Soft material robots have gained interest in recent years due to the mechanical potential of non-rigid materials and technological development in the additive manufacturing (3D printing) techniques. The incorporation of soft materials provides robots with potential for locomotion in unstructured environments due to the conformability and deformability properties of the structure. Current additive manufacturing techniques allow multimaterial printing which can be utilized to build soft bodied robots with rigid-material inclusions/features in a single process, single batch (low manufacturing volumes) thus saving on both design prototype time and need for complex tools to allow multimaterial manufacturing. However, design and manufacturing of such deformable robots needs to be analyzed and formalized using state of the art tools. This work conceptualizes methodology for motor-tendon actuated soft-bodied robots capable of locomotion. The methodology relies on additive manufacturing as both a prototyping tool and a primary manufacturing tool and is categorized into body design & development, actuation and control design. This methodology is applied to design a soft caterpillar-like biomimetic robot with soft deformable body, motor-tendon actuators which utilizes finite contact points to effect locomotion. The versatility of additive manufacturing is evident in the complex designs that are possible when implementing unique actuation techniques contained in a soft body robot (Modulus discrepancy); For the given motor-tendon actuation, the hard tendons are embedded inside the soft material body which acts as both a structure and an actuator. Furthermore, the modular design of soft/hard component coupling is only possible due to this manufacturing technique and often eliminates the need for joining and fasteners. The multi-materials are also used effectively to manipulate friction by utilizing soft/hard material frictional interaction disparity. Copyright © 2015 by ASME.
Sharma U.,University of Massachussetts |
Shenoy P.,University of Massachussetts |
Towsley D.,University of Massachussetts
ICAC'12 - Proceedings of the 9th ACM International Conference on Autonomic Computing | Year: 2012
In this paper we present a simple and effective approach for resource provisioning to achieve a percentile bound on the end to end response time of a multi-tier application. We, at first, model the multi-tier application as an open tandem network of M/G/1-PS queues and develop a method that produces a near optimal application configuration, i.e, number of servers at each tier, to meet the percentile bound in a homogeneous server environment - using a single type of server. We then extend our solution to a K-server case and our technique demonstrates a good accuracy, independent of the variability of service-times. Our approach demonstrates a provisioning error of no more than 3% compared to a 140% worst case provisioning error obtained by techniques based on an M/M/1- FCFS queue model. In addition, we extend our approach to handle a heterogenous server environment, i.e., with multiple types of servers. We find that fewer high-capacity servers are preferable for high percentile provisioning. Finally, we extend our approach to account for the rental cost of each server-type and compute a cost efficient application configuration with savings of over 80%. We demonstrate the applicability of our approach in a real world system by employing it to provision the two tiers of the java implementation of TPC-W - a multi-tier transactional web benchmark that represents an e-commerce web application, i.e. an online bookstore. Copyright 2012 ACM.
Singh R.,University of Massachussetts |
Sharma U.,University of Massachussetts |
Cecchet E.,University of Massachussetts |
Shenoy P.,University of Massachussetts
Proceeding of the 7th International Conference on Autonomic Computing, ICAC '10 and Co-located Workshops | Year: 2010
Online Internet applications see dynamic workloads that fluctuate over multiple time scales. This paper argues that the non-stationarity in Internet application workloads, which causes the request mix to change over time, can have a significant impact on the overall processing demands imposed on data center servers. We propose a novel mix-aware dynamic provisioning technique that handles both the non-stationarity in the workload as well as changes in request volumes when allocating server capacity in Internet data centers. Our technique employs the k-means clustering algorithm to automatically determine the workload mix and a queuing model to predict the server capacity for a given workload mix. We implement a prototype provisioning system that incorporates our technique and experimentally evaluate its efficacy on a laboratory Linux data center running the TPC-W web benchmark. Our results show that our k-means clustering technique accurately captures workload mix changes in Internet applications. We also demonstrate that mix-aware dynamic provisioning eliminates SLA violations due to under-provisioning with non-stationary web workloads, and that it offers a better resource usage by reducing over-provisioning when compared to a baseline provisioning approach that only reacts to workload volume changes. We also present a case study of our provisioning approach on Amazon's EC2 cloud platform. © 2010 ACM.