215 research outputs found
Virial Sequences for Thick Discs and Haloes: Flattening and Global Anisotropy
The virial theorem prescribes the ratio of the globally-averaged equatorial
to vertical velocity dispersion of a tracer population in spherical and
flattened dark haloes. This gives sequences of physical models in the plane of
global anisotropy and flattening. The tracer may have any density, though there
are particularly simple results for power-laws and exponentials. We prove the
flattening theorem: for a spheroidally stratified tracer density with axis
ratio q in a dark density potential with axis ratio g, the ratio of globally
averaged equatorial to vertical velocity dispersion depends only on q/g. As the
stellar halo density and velocity dispersion of the Milky Way are accessible to
observations, this provides a new method for measuring the flattening of the
dark matter. If the kinematics of the local halo subdwarfs are representative,
then the Milky Way's dark halo is oblate with a flattening in the potential of
g ~ 0.85, corresponding to a flattening in the dark matter density of ~ 0.7.
The fractional pressure excess for power-law populations is roughly
proportional to both the ellipticity and the fall-off exponent. Given the same
pressure excess, if the density profile of one stellar population declines more
quickly than that of another, then it must be rounder. This implies that the
dual halo structure claimed by Carollo et al. (2007) for the Galaxy, a flatter
inner halo and a rounder outer halo, is inconsistent with the virial theorem.
For the thick disc, we provide formulae for the virial sequences of
double-exponential discs in logarithmic and Navarro-Frenk-White (NFW) haloes.
There are good matches to the observational data on the flattening and
anisotropy of the thick disc if the thin disc is exponential with a short
scalelength ~ 2.6 kpc and normalisation of 56 solar masses per square parsec,
together with a logarithmic dark halo.Comment: MNRAS, submitted, 13 pages, 7 figures, small changes to made to
correspond to final accepted versio
The development of a program analysis environment for Ada
A unit level, Ada software module testing system, called Query Utility Environment for Software Testing of Ada (QUEST/Ada), is described. The project calls for the design and development of a prototype system. QUEST/Ada design began with a definition of the overall system structure and a description of component dependencies. The project team was divided into three groups to resolve the preliminary designs of the parser/scanner: the test data generator, and the test coverage analyzer. The Phase 1 report is a working document from which the system documentation will evolve. It provides history, a guide to report sections, a literature review, the definition of the system structure and high level interfaces, descriptions of the prototype scope, the three major components, and the plan for the remainder of the project. The appendices include specifications, statistics, two papers derived from the current research, a preliminary users' manual, and the proposal and work plan for Phase 2
Line Profiles from Discrete Kinematic Data
We develop a method to extract the shape information of line profiles from
discrete kinematic data. The Gauss-Hermite expansion, which is widely used to
describe the line of sight velocity distributions extracted from absorption
spectra of elliptical galaxies, is not readily applicable to samples of
discrete stellar velocity measurements, accompanied by individual measurement
errors and probabilities of membership. We introduce two parameter families of
probability distributions describing symmetric and asymmetric distortions of
the line profiles from Gaussianity. These are used as the basis of a maximum
likelihood estimator to quantify the shape of the line profiles. Tests show
that the method outperforms a Gauss-Hermite expansion for discrete data, with a
lower limit for the relative gain of approx 2 for sample sizes N approx 800. To
ensure that our methods can give reliable descriptions of the shape, we develop
an efficient test to assess the statistical quality of the obtained fit. As an
application, we turn our attention to the discrete velocity datasets of the
dwarf spheroidals of the Milky Way. In Sculptor, Carina and Sextans the
symmetric deviations are consistent with velocity distributions more peaked
than Gaussian. In Fornax, instead, there is an evolution in the symmetric
deviations of the line profile from a peakier to more flat-topped distribution
on moving outwards. These results suggest a radially biased orbital structure
for the outer parts of Sculptor, Carina and Sextans. On the other hand,
tangential anisotropy is favoured in Fornax. This is all consistent with a
picture in which Fornax may have had a different evolutionary history to
Sculptor, Carina and Sextans.Comment: MNRAS, accepted for publication, minor change
Recommended from our members
SURFACE PREPARATION OF STEEL SUBSTRATES USING GRIT-BLASTING
The primary purpose of grit blasting for thermal spray applications is to ensure a strong mechanical bond between the substrate and the coating by the enhanced roughening of the substrate material. This study presents statistically designed experiments that were accomplished to investigate the effect of abrasives on roughness for A36/1020 steel. The experiments were conducted using a Box statistical design of experiment (SDE) approach. Three grit blasting parameters and their effect on the resultant substrate roughness were investigated. These include blast media, blast pressure, and working distance. The substrates were characterized for roughness using surface profilometry. These attributes were correlated with the changes in operating parameters. Twin-Wire Electric Arc (TWEA) coatings of aluminum and zinc/aluminum were deposited on the grit-blasted substrates. These coatings were then tested for bond strength. Bond strength studies were conducted utilizing a portable adhesion tester following ASTM standard D4541
Using dwarf satellite proper motions to determine their origin
The highly organised distribution of satellite galaxies surrounding the Milky
Way is a serious challenge to the concordance cosmological model. Perhaps the
only remaining solution, in this framework, is that the dwarf satellite
galaxies fall into the Milky Way's potential along one or two filaments, which
may or may not plausibly reproduce the observed distribution. Here we test this
scenario by making use of the proper motions of the Fornax, Sculptor, Ursa
Minor and Carina dwarf spheroidals, and trace their orbits back through several
variations of the Milky Way's potential and account for dynamical friction. The
key parameters are the proper motions and total masses of the dwarf galaxies.
Using a simple model we find no tenable set of parameters that can allow Fornax
to be consistent with filamentary infall, mainly because the 1 sigma error on
its proper motion is relatively small. The other three must walk a tightrope
between requiring a small pericentre (less than 20 kpc) to lose enough orbital
energy to dynamical friction and avoiding being tidally disrupted. We then
employed a more realistic model with host halo mass accretion, and found that
the four dwarf galaxies must have fallen in at least 5 Gyrs ago. This time
interval is longer than organised distribution is expected to last before being
erased by the randomisation of the satellite orbits.Comment: 9 pages, 5 figures, accepted for publication in MNRA
A Forum for Business Growth and Workforce Development: Findings and Recommendations
In the fall of 2008, Illinois State University – Extended University (EU) and the Economic Development Council of the Bloomington-Normal Area (EDC) initiated discussions about a community partnership project to identify workforce opportunities and challenges related to economic stabilization and growth in order to gain a better understanding of the state of workforce preparedness in the area. Rapidly changing dynamics in the economy made previous assessments obsolete. Organizations who work toward the promotion of a strong workforce were approached to participate in the project. EU and the EDC were joined in sponsoring a community event by Heartland Community College, Illinois Wesleyan University, Lincoln College – Normal, Regional Office of Education 17, McLean County Chamber of Commerce, CareerLink 16, and the Small Business Development Center at Illinois State University.
Project partners designed and developed a series of discussion forums for eight sectors: Agriculture and Energy, Manufacturing, Small Business Retail, Service, Financial Services, Information Technology, Healthcare, and Construction. The Forum for Business Growth and Workforce Development was held from June 8 – 12, 2009 at Illinois State University. Each sector panel discussion was moderated over a ninety minute period and included two to seven panelists from area businesses
The SWELLS survey. III. Disfavouring "heavy" initial mass functions for spiral lens galaxies
We present gravitational lens models for 20 strong gravitational lens systems
observed as part of the Sloan WFC Edge-on Late-type Lens Survey (SWELLS)
project. Fifteen of the lenses are taken from paper I while five are newly
discovered systems. The systems are galaxy-galaxy lenses where the foreground
deflector has an inclined disc, with a wide range of morphological types, from
late-type spiral to lenticular. For each system, we compare the total mass
inside the critical curve inferred from gravitational lens modelling to the
stellar mass inferred from stellar population synthesis (SPS) models, computing
the stellar mass fraction f* = M(SPS)/M(lens). We find that, for the lower mass
SWELLS systems, adoption of a Salpeter stellar initial mass function (IMF)
leads to estimates of f* that exceed 1. This is unphysical, and provides strong
evidence against the Salpeter IMF being valid for these systems. Taking the
lower mass end of the SWELLS sample sigma(SIE) < 230 km/s, we find that the IMF
is lighter (in terms of stellar mass-to-light ratio) than Salpeter with 98%
probability, and consistent with the Chabrier IMF and IMFs between the two.
This result is consistent with previous studies of spiral galaxies based on
independent techniques. In combination with recent studies of massive
early-type galaxies that have favoured a heavier Salpeter-like IMF, this result
strengthens the evidence against a universal stellar IMF.Comment: Accepted for publication in MNRAS. Some changes (none major) to
address the referee's comments. 18 pages, 8 figure
Balancing mass and momentum in the Local Group
In the rest frame of the Local Group (LG), the total momentum of the Milky
Way (MW) and Andromeda (M31) should balance to zero. We use this fact to
constrain new solutions for the solar motion with respect to the LG
centre-of-mass, the total mass of the LG, and the individual masses of M31 and
the MW. Using the set of remote LG galaxies at kpc from the MW and M31,
we find that the solar motion has amplitude in a direction pointing toward galactic longitude
and galactic latitude
. The velocities of M31 and the MW in
this rest frame give a direct measurement of their mass ratio, for which we
find . We combine these
measurements with the virial theorem to estimate the total mass within the LG
as .
Our value for is consistent with the sum of literature values
for and . This suggests that the mass of the LG is
almost entirely located within the two largest galaxies rather than being
dispersed on larger scales or in a background medium. The outskirts of the LG
are seemingly rather empty. Combining our measurement for and the
mass ratio, we estimate the individual masses of the MW and M31 to be and , respectively. Our analysis favours M31
being more massive than the MW by a factor of 2.3, and the uncertainties
allow only a small probability (9.8%) that the MW is more massive. This is
consistent with other properties such as the maximum rotational velocities,
total stellar content, and numbers of globular clusters and dwarf satellites,
which all suggest that .Comment: 16 pages, 11 figures, 3 tables. Accepted for publication in MNRA
The mass of the Milky Way from satellite dynamics
We present and apply a method to infer the mass of the Milky Way (MW) by comparing the dynamics of MW satellites to those of model satellites in the EAGLE cosmological hydrodynamics simulations. A distribution function (DF) for galactic satellites is constructed from EAGLE using specific angular momentum and specific energy, which are scaled so as to be independent of host halo mass. In this two-dimensional space, the orbital properties of satellite galaxies vary according to the host halo mass. The halo mass can be inferred by calculating the likelihood that the observed satellite population is drawn from this DF. Our method is robustly calibrated on mock EAGLE systems. We validate it by applying it to the completely independent suite of 30 AURIGA high-resolution simulations of MW-like galaxies: the method accurately recovers their true mass and associated uncertainties. We then apply it to 10 classical satellites of the MW with six-dimensional phase-space measurements, including updated proper motions from the Gaia satellite. The mass of the MW is estimated to be M200MW = 1.17+0.21−0.15 × 1012 M (68 per cent confidence limits). We combine our total mass estimate with recent mass estimates in the inner regions of the Galaxy to infer an inner dark matter (DM) mass fraction MDM(< 20 kpc)/MDM200 = 0.12, which is typical of ∼1012 M lambda cold dark matter haloes in hydrodynamical galaxy formation simulations. Assuming a Navarro, Frenk and White (NFW) profile, this is equivalent to a halo concentration of c200MW = 10.9+2.6−2.
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