431 research outputs found
Two-fluid gravitational instabilities in a galactic disk
We forlulate and solve the hydrodynamic equations describing an azimuthally symmetric galactic disk as a. two-fluid system. The stars and the gas are treated as two different isothermal fluids of different velocity dispersions (Cs >> Cg ), which interact gravitationally with each other. The disk is supported by rotation and random motion. The formulation of the equations closely follows the one-fluid treatment by Toomre. We solve the lineraized perturbation equations by the method of modes, and study the stability of the galactic disk agaInst the growth of axisymmetric two-fluid gravitational instabilities. We find that even when both the fluids in a two-fluid system are separately stable, the joint two-fluid system, because of the gravitational interaction between the two fluids, may be unstable. The ratio of the gas contnbution to the stellar contribution toward the formation of two-fluid instabilities is substantially greater than μg/μs, the ratio of their respective surface densities; this is due to the lower gas velocity dispersion as compared to the stellar velocity dispersion (Cg << Cs). The two contributions are comparable when the gas fraction (μg/μs) IS only ~ 0.10-0.25. Therefore, a galactic disk is a meaningful two-fluid system even when the gas constitutes only 1O %-20 % of the total surface density. The ratio of the amplitude in the gas to the amplItude In the stars is an increasIng function of the wavenumber of the two-fluid perturbation. The wavelength and the time of growth of a typical two-fluid instability in the inner galaxy, for μg/μs = 0.1-0.2, and ~2-3 kpc and ~2-4 × 107 years, respectively, and each of these contains gas of mass 4 × 107-108 M. The two-fluid analysis presented here is applocable to any general disk galaxy consisting of stars and gas
Deposition of polymer bilayer configuration by pulsed laser ablation and its use for study of polymer-polymer interface
Thin films of polyphenylene sulphide (PPS) and polyethylene (PE) polymers have been deposited in a bilayer configuration using pulsed excimer laser ablation. Such bilayer specimens have been annealed at different temperatures, up to a maximum of 120°C, and for different time intervals, up to a maximum of 110 min, to investigate the evolution of the interface. By employing the technique of spectroscopic ellipsometry, the nature and the degree of thermally induced polymeric transport across the interface are brought out
Generation of rotationally dominated galaxies by mergers of pressure-supported progenitors
Through the analysis of a set of numerical simulations of major mergers
between initially non-rotating, pressure supported progenitor galaxies with a
range of central mass concentrations, we have shown that: (1) it is possible to
generate elliptical-like galaxies, with v/sigma > 1 outside one effective
radius, as a result of the conversion of orbital- into internal-angular
momentum; (2) the outer regions acquire part of the angular momentum first; (3)
both the baryonic and the dark matter components of the remnant galaxy acquire
part of the angular momentum, the relative fractions depend on the initial
concentration of the merging galaxies. For this conversion to occur the initial
baryonic component must be sufficiently dense and/or the encounter should take
place on a orbit with high angular momentum. Systems with these hybrid
properties have been recently observed through a combination of stellar
absorption lines and planetary nebulae for kinematic studies of early-type
galaxies. Our results are in qualitative agreement with such observations and
demonstrate that even mergers composed of non-rotating, pressure-supported
progenitor galaxies can produce early-type galaxies with significant rotation
at large radii.Comment: 5 pages, 6 figures, 2 tables. Accepted for publication in A&A Letter
Global Star Formation Rates in Disk Galaxies and Circumnuclear Starbursts from Cloud Collisions
We invoke star formation triggered by cloud-cloud collisions to explain
global star formation rates of disk galaxies and circumnuclear starbursts.
Previous theories based on the growth rate of gravitational perturbations
ignore the dynamically important presence of magnetic fields. Theories based on
triggering by spiral density waves fail to explain star formation in systems
without such waves. Furthermore, observations suggest gas and stellar disk
instabilities are decoupled. Following Gammie, Ostriker & Jog (1991), the cloud
collision rate is set by the shear velocity of encounters with initial impact
parameters of a few tidal radii, due to differential rotation in the disk.
This, together with the effective confinement of cloud orbits to a two
dimensional plane, enhances the collision rate above that for particles in a
three dimensional box. We predict Sigma_{SFR}(R) proportional to Sigma_{gas}
Omega (1-0.7 beta). For constant circular velocity (beta = 0), this is in
agreement with recent observations (Kennicutt 1998). We predict a B-band
Tully-Fisher relation: L_{B} proportional to v_{circ}^{7/3}, also consistent
with observations. As additional tests, we predict enhanced star formation in
regions with relatively high shear rates, and lower star formation efficiencies
in clouds of higher mass.Comment: 27 pages including 3 figures and 2 tables. Accepted to ApJ. Expanded
statistical analysis of cloud SF efficiency test. Stylistic changes. Data for
figures available electronically at
http://astro.berkeley.edu/~jt/disksfr.htm
Rotating Nuclear Rings and Extreme Starbursts in Ultraluminous Galaxies
New high resolution interferometer data of 10 IR ultraluminous galaxies shows
the molecular gas is in rotating nuclear rings or disks with radii 300 to 800
pc. Most of the CO flux comes from a moderate-density, warm, intercloud medium
rather than self-gravitating clouds. Gas masses of ~ 5 x 10^9 Msun, 5 times
lower than the standard method are derived from a model of the molecular disks.
The ratio of molecular gas to dynamical mass, is M_gas/M_dyn ~ 1/6 with a
maximum ratio of gas to total mass surface density of 1/3. For the galaxies
VIIZw31, Arp193, and IRAS 10565+24, there is good evidence for rotating
molecular rings with a central gap. In addition to the rotating rings a new
class of star formation region is identified which we call an Extreme
Starburst. They have a characteristic size of only 100 pc., about 10^9 Msun of
gas and an IR luminosity of ~3 x 10^11 Lsun. Four extreme starbursts are
identified in the 3 closest galaxies in the sample Arp220, Arp193 and Mrk273.
They are the most prodigious star formation events in the local universe, each
representing about 1000 times as many OB stars as 30 Doradus. In Arp220, the CO
and 1.3 mm continuum maps show the two ``nuclei'' embedded in a central ring or
disk and a fainter structure extending 3 kpc to the east, normal to the nuclear
disk. There is no evidence that these sources really are the pre-merger nuclei.
They are compact, extreme starburst regions containing 10^9 Msun of dense
molecular gas and new stars, but no old stars. Most of the dust emission and
HCN emission arises in the two extreme starbursts. The entire bolometric
luminosity of Arp~220 comes from starbursts, not an AGN. In Mrk231, the disk
geometry shows that the molecular disk cannot be heated by the AGN; the far IR
luminosity of Mrk~231 is powered by a starburst, not the AGN. (Abridged)Comment: 97 pages Latex with aasms.sty, including 29 encapsulated Postscript
figures. Figs 18 and 23 are GIFs. 31 figures total. Text and higher quality
versions of figures available at
http://sbastk.ess.sunysb.edu/www/RINGS_ESB_PREPRINT.html To be published in
Ap. J., 10 Nov. 199
Star cluster formation and star formation: the role of environment and star-formation efficiencies
“The original publication is available at www.springerlink.com”. Copyright Springer. DOI: 10.1007/s10509-009-0088-5By analyzing global starburst properties in various kinds of starburst and post-starburst galaxies and relating them to the properties of the star cluster populations they form, I explore the conditions for the formation of massive, compact, long-lived star clusters. The aim is to determine whether the relative amount of star formation that goes into star cluster formation as opposed to field star formation, and into the formation of massive long-lived clusters in particular, is universal or scales with star-formation rate, burst strength, star-formation efficiency, galaxy or gas mass, and whether or not there are special conditions or some threshold for the formation of star clusters that merit to be called globular clusters a few billion years later.Peer reviewe
Amplification, Saturation, and Q Thresholds for Runaway: Growth of Self-Gravitating Structures in Models of Magnetized Galactic Gas Disks
We investigate the susceptibility of gaseous, magnetized galactic disks to
formation of self-gravitating condensations using two-dimensional, local
models. We focus on two issues: (1) determining the threshold condition for
gravitational runaway, taking into account nonlinear effects, and (2)
distinguishing the magneto-Jeans instability (MJI) that arises under
inner-galaxy rotation curves from the modified swing amplification (MSA) that
arises under outer-galaxy rotation curves. For axisymmetric density
fluctuations, instability is known to require a Toomre parameter Q<1. For
nonaxisymmetric fluctuations, any nonzero shear winds up wavefronts such that in linear theory amplification saturates. Any
Q threshold for nonaxisymmetric gravitational runaway must originate from
nonlinear effects. We use numerical magnetohydrodynamic simulations to
demonstrate the anticipated threshold phenomenon, to analyze the nonlinear
processes involved, and to evaluate the critical value for stabilization.
We find for a wide variety of conditions, with the largest
values corresponding to nonzero but subthermal mean magnetic fields. Our
findings for are similar to those inferred from thresholds for active
star formation in the outer regions of spiral galaxies. MJI is distinct from
MSA in that opposition to Coriolis forces by magnetic tension, rather than
cooperation of epicyclic motion with kinematic shear, enables nonaxisymmetric
density perturbations to grow. We suggest that under low-shear inner-disk
conditions, will be larger than that in outer disks by a factor , where and are the respective Alfven and sound
speeds.Comment: 45 pages, 15 figures, Accepted for publication in ApJ; better
postscript figures available from http://www.astro.umd.edu/~kimwt/FIGURES/ ;
for associated Animated GIF movies, see
http://www.astro.umd.edu/~kimwt/MOVIES
Gravitational Runaway and Turbulence Driving in Star-Gas Galactic Disks
Galactic disks consist of both stars and gas. The gas is more dynamically responsive than the stars, and strongly nonlinear structures and velocities can develop in the ISM even while stellar surface density perturbations remain fractionally small. We use 2D numerical simulations to explore formation of bound clouds and turbulence generation in the gas of two-component galactic disks. We represent the stars with collisionless particles and follow their orbits using a PM method, and treat the gas as an isothermal, unmagnetized fluid. The two components interact through a combined gravity. Using stellar parameters typical of mid-disk conditions, we find that models with gaseous Toomre parameter Q_g 1-2. The bound gaseous clouds that form have mass 6x10^7 Msun each; these represent superclouds that would subsequently fragment into GMCs. Self-gravity and sheared rotation also interact to drive turbulence in the gas when Q_g > Q_c. This turbulence is anisotropic, with more power in sheared than compressive motions. The gaseous velocity dispersion is ~ 0.6 times the thermal speed when Q_g ~ Q_c. This suggests that gravity is important in driving ISM turbulence in many spiral galaxies, since the low efficiency of star formation naturally leads to a state of marginal instability
Screening for Parkinson's disease with response time batteries: A pilot study
BACKGROUND: Although significant response time deficits (both reaction time and movement time) have been identified in numerous studies of patients with Parkinson's disease (PD), few attempts have been made to evaluate the use of these measures in screening for PD. METHODS: Receiver operator characteristic curves were used to identify cutoff scores for a unit-weighted composite of two choice response tasks in a sample of 40 patients and 40 healthy participants. These scores were then cross-validated in an independent sample of 20 patients and 20 healthy participants. RESULTS: The unit-weighted movement time composite demonstrated high sensitivity (90%) and specificity (90%) in the identification of PD. Movement time was also significantly correlated (r = 0.59, p < 0.025) with the motor score of the Unified Parkinson's Disease Rating Scale (UPDRS). CONCLUSIONS: Measures of chronometric speed, assessed without the use of biomechanically complex movements, have a potential role in screening for PD. Furthermore, the significant correlation between movement time and UPDRS motor score suggests that movement time may be useful in the quantification of PD severity
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