196 research outputs found
Full spectral fitting of Milky Way and M31 globular clusters: ages and metallicities
Context: The formation and evolution of disk galaxies are long standing
questions in Astronomy. Understanding the properties of globular cluster
systems can lead to important insights on the evolution of its host galaxy.
Aims: We aim to obtain the stellar population parameters - age and metallicity
- of a sample of M31 and Galactic globular clusters. Studying their globular
cluster systems is an important step towards understanding their formation and
evolution in a complete way. Methods: Our analysis employs a modern
pixel-to-pixel spectral fitting technique to fit observed integrated spectra to
updated stellar population models. By comparing observations to models we
obtain the ages and metallicities of their stellar populations. We apply this
technique to a sample of 38 globular clusters in M31 and to 41 Galactic
globular clusters, used as a control sample. Results: Our sample of M31
globular clusters spans ages from 150 Myr to the age of the Universe.
Metallicities [Fe/H] range from -2.2 dex to the solar value. The
age-metallicity relation obtained can be described as having two components: an
old population with a flat age-[Fe/H] relation, possibly associated with the
halo and/or bulge, and a second one with a roughly linear relation between age
and metallicity, higher metallicities corresponding to younger ages, possibly
associated with the M31 disk. While we recover the very well known Galactic GC
metallicity bimodality, our own analysis of M31's metallicity distribution
function (MDF) suggests that both GC systems cover basically the same [Fe/H]
range yet M31's MDF is not clearly bimodal. These results suggest that both
galaxies experienced different star formation and accretion histories.Comment: A&A, in pres
Spectra of globular clusters in the Sombrero galaxy: evidence for spectroscopic metallicity bimodality
We present a large sample of over 200 integrated-light spectra of confirmed
globular clusters (GCs) associated with the Sombrero (M104) galaxy taken with
the DEIMOS instrument on the Keck telescope. A significant fraction of the
spectra have signal-to-noise levels high enough to allow measurements of GC
metallicities using the method of Brodie & Huchra (1990). We find a
distribution of spectroscopic metallicities ranging from -2.2 < [Fe/H] < +0.1
that is bimodal, with peaks at [Fe/H] ~ -1.4 and -0.6. Thus the GC system of
the Sombrero galaxy, like a few other galaxies now studied in detail, reveals a
bimodal spectroscopic metallicity distribution supporting the long-held belief
that colour bimodality reflects two metallicity subpopulations. This further
suggests that the transformation from optical colour to metallicity for old
stellar populations, such as GCs, is not strongly non-linear. We also explore
the radial and magnitude distribution with metallicity for GC subpopulations
but small number statistics prevent any clear trends in these distributions.Comment: 18 pages, 10 figures, 3 tables, MNRAS accepte
Plasmas and Controlled Nuclear Fusion
Contains research objectives and summary of research on six research projects.U.S. Atomic Energy Commission (Contract AT(11-1)-3070
Plasma Dynamics
Contains research objectives and summary of research on twenty-one projects split into three sections, with four sub-sections in the second section and reports on twelve research projects.National Science Foundation (Grant ENG75-06242)U.S. Energy Research and Development Administration (Contract E(11-1)-2766)U.S. Energy Research and Development Agency (Contract E(11-1)-3070)U.S. Energy Research and Development Administration (Contract E(11-1)-3070)Research Laboratory of Electronics, M.I.T. Industrial Fellowshi
Kinematics of Ten Early-Type Galaxies from HST and Ground-Based Spectroscopy
We present stellar kinematics for a sample of 10 early-type galaxies observed
using the STIS aboard the Hubble Space Telescope, and the Modular Spectrograph
on the MDM Observatory 2.4-m telescope. The spectra are used to derive
line-of-sight velocity distributions (LOSVDs) of the stars using a Maximum
Penalized Likelihood method. We use Gauss-Hermite polynomials to parameterize
the LOSVDs and find predominantly negative h4 values (boxy distributions) in
the central regions of our galaxies. One galaxy, NGC 4697, has significantly
positive central h4 (high tail weight). The majority of galaxies have a central
velocity dispersion excess in the STIS kinematics over ground-based velocity
dispersions. The galaxies with the strongest rotational support, as quantified
with v_MAX/sigma_STIS, have the smallest dispersion excess at STIS resolution.
The best-fitting, general, axisymmetric dynamical models (described in a
companion paper) require black holes in all cases, with masses ranging from
10^6.5 to 10^9.3 Msun. We replot these updated masses on the BH/sigma relation,
and show that the fit to only these 10 galaxies has a slope consistent with the
fits to larger samples. The greatest outlier is NGC 2778, a dwarf elliptical
with relatively poorly constrained black hole mass. The two best candidates for
pseudobulges, NGC 3384 and 7457, do not deviate significantly from the
established relation between black hole and sigma. Neither do the three
galaxies which show the most evidence of a recent merger, NGC 3608, 4473, and
4697.Comment: 43 pages, accepted for publication in the Astrophysical Journal, high
resolution version found at http://hoku.as.utexas.edu/~gebhardt/pinkney.p
Xanthine oxidase inhibition and white matter hyperintensity progression following ischaemic stroke and transient ischaemic attack (XILO-FIST): a multicentre, double-blinded, randomised, placebo-controlled trial
Acknowledgments This work was supported by the Stroke Association and British Heart Foundation [grant number TSA BHF 2013/01]. The work of Dr David Dickie and Dr Terry Quinn is funded by the Stroke Association. We would like to thank Christine McAlpine, Ruth Graham, Glasgow Royal Infirmary, UK; Lauren Pearce, Royal United Hospital, UK; Caroline Fornolles, Louise Tate, Frances Justin, Luton and Dunstable University Hospital, UK; Dean Waugh, Leeds Teaching Hospitals NHS Trust, UK; Donal Concannon, Altnagelvin Hospital, UK; Sharon Tysoe, Nina Francia, Nisha Menon, Raji Prabakaran, Southend University Hospital, UK; Amy Ashton, Caroline Watchurst, Marilena Marinescu, Sabaa Obarey, Scheherazade Feerick, University College London NHS Foundation Trust, UK; and Janice Irvine, Sandra Williams, and German Guzman Gutierrez, Aberdeen Royal Infirmary, UK; Caroline Fox and Joanne Topliffe, Broomfield Hospital, Essex, UK.Peer reviewedPublisher PD
Xanthine oxidase inhibition and white matter hyperintensity progression following ischaemic stroke and transient ischaemic attack (XILO-FIST):a multicentre, double-blinded, randomised, placebo-controlled trial
The Centers of Early-Type Galaxies with HST. V. New WFPC2 Photometry
We analyze HST+WFPC2 images of 77 early-type galaxies. Brightness profiles
are classed into "core" or "power-law" forms. Cores are typically rounder than
power-law galaxies. Nearly all power-laws with central ellipticity >=0.3 have
stellar disks, implying that disks are present in power-laws with epsilon <0.3,
but are not visible due to unfavorable geometry. A few low-luminosity core
galaxies also have disks; these may be transition forms from power-laws. Cores
and power-laws both have twisting isophotes at small radii. Core galaxies have
somewhat weaker color gradients than power-laws. Nuclei are found in 29% of the
cores and 60% of the power-laws. Nuclei are typically bluer than the
surrounding galaxy. NGC 4073 and 4382 have central minima in their intrinsic
starlight distributions; NGC 4382 resembles the double nucleus of M31. In
general, the peak brightness location is coincident with the photocenter of the
core to <1 pc. Five galaxies, however, have centers significantly displaced
from their cores; these may be unresolved asymmetric double nuclei. Central
dust is visible in half of the galaxies. The presence and strength of dust
correlates with nuclear emission. The prevalence of dust and its morphology
suggests that dust clouds form, settle to the center, and disappear repeatedly
on ~10^8 yr timescales. We discuss the hypothesis that cores are created by the
decay of a massive black hole binary. Apart from their brightness profiles,
there are no strong differences between cores and power-laws that demand this
scenario; however, the rounder shapes of cores, their lack of disks, and their
reduced color gradients may be consistent with it.Comment: 76 pages, 40 figures, submitted to the Astronomical Journal. The
complete postscript version of the paper is available at
http://www.noao.edu/noao/staff/lauer/nuker.html (the Nuker Team page
A developmental approach to predicting neuronal connectivity from small biological datasets: a gradient-based neuron growth model.
PMCID: PMC3931784 Open Access article: BB/G006652/1 and BB/G006369/1.Relating structure and function of neuronal circuits is a challenging problem. It requires demonstrating how dynamical patterns of spiking activity lead to functions like cognitive behaviour and identifying the neurons and connections that lead to appropriate activity of a circuit. We apply a "developmental approach" to define the connectome of a simple nervous system, where connections between neurons are not prescribed but appear as a result of neuron growth. A gradient based mathematical model of two-dimensional axon growth from rows of undifferentiated neurons is derived for the different types of neurons in the brainstem and spinal cord of young tadpoles of the frog Xenopus. Model parameters define a two-dimensional CNS growth environment with three gradient cues and the specific responsiveness of the axons of each neuron type to these cues. The model is described by a nonlinear system of three difference equations; it includes a random variable, and takes specific neuron characteristics into account. Anatomical measurements are first used to position cell bodies in rows and define axon origins. Then a generalization procedure allows information on the axons of individual neurons from small anatomical datasets to be used to generate larger artificial datasets. To specify parameters in the axon growth model we use a stochastic optimization procedure, derive a cost function and find the optimal parameters for each type of neuron. Our biologically realistic model of axon growth starts from axon outgrowth from the cell body and generates multiple axons for each different neuron type with statistical properties matching those of real axons. We illustrate how the axon growth model works for neurons with axons which grow to the same and the opposite side of the CNS. We then show how, by adding a simple specification for dendrite morphology, our model "developmental approach" allows us to generate biologically-realistic connectomes
Progress report no. 1
Statement of responsibility on title-page reads: Editors: I.A. Forbes, M.J. Driscoll, D.D. Lanning, I. Kaplan, N.C. Rasmussen; Contributors: S.A. Ali, S.T. Brewer, D.K. Choi, F.M. Clikeman, W.R. Corcoran, M.J. Driscoll, I.A. Forbes, C.W. Forsberg, S.L. Ho, C.S. Kang, I. Kaplan, J.L. Klucar, D.D. Lanning, T.C. Leung, E.L. McFarland P.G. Mertens, N.R. Ortiz, A. Pant, N.A. Passman, N.C. Rasmussen, M.K. Sheaffer, D.A. Shupe, G.E. Sullivan, A.T. Supple, J.W. Synan, C.P. Tzanos, W.J. Westlake"MIT-4105-3."Includes bibliographical referencesProgress report; June 30, 1970U.S. Atomic Energy Commission contracts: AT(30-1)410
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