495 research outputs found
Casenotes: Corporations — Fiduciary Duty of Majority Shareholders — Has Spring Arrived for the Frozen out Minority? Twenty Seven Trust v. Realty Growth Investors & RGI Holding Co., 533 F. Supp. 1028 (D. Md. 1982)
Important Terms for Inclusion in Confidential Settlement Agreements for Financial Services Companies
Casenotes: Corporations — Fiduciary Duty of Majority Shareholders — Has Spring Arrived for the Frozen out Minority? Twenty Seven Trust v. Realty Growth Investors & RGI Holding Co., 533 F. Supp. 1028 (D. Md. 1982)
Cristi Elizabeth Watts and Bryan Keith Bolton in a Joint Senior Recital
This is the program for the joint senior recital of French horn player Cristi Elizabeth Watts, and bass-baritone Bryan Keith Bolton. Ms. Watts was accompanied by Kiri Tan, Pam Dennis, and Cindy Burks. Mr. Bolton was accompanied on the piano by Steven Cole. This recital took place on March 8, 1994, in the McBeth Recital Hall in the Mabee Fine Arts Center
Investigation of the Vibrational Modes of Edge-Constrained Fibrous Samples Placed in a Standing Wave Tube
The impact of helium reionization on the structure of the intergalactic medium
We examine the impact of helium reionization on the structure of the
intergalactic medium (IGM). We model the reionization using a radiative
transfer (RT) code coupled to the combined gravity hydrodynamics code Enzo.
Neutral hydrogen and helium are initially ionized by a starburst spectrum,
which is allowed to gradually evolve into a power law spectrum over the
redshift interval 3 < z < 4. The temperature-density relation of the gas is
found to fan out and flatten following HeII reionization, with an inversion for
overdensities above 5. Peculiar velocities of up to 10 km/s are induced by the
increased pressure, with the gas density field distorted over large coherent
regions by 10-20%, and the dark matter by levels of 1%. The
photoionization-induced flows may thus distort the matter power spectrum at
comoving wavenumbers k > 0.5 h/Mpc by a few percent by z = 2. Absorption
spectra for HI and HeII are drawn from the simulations, and absorption lines
are fit to the spectra. A median Doppler parameter of 35 km/s is obtained for
the HI absorption systems at z = 3. Dividing into subsamples optically thick
and optically thin at line centre reveals that the optically thick systems
undergo only mild evolution while the optically thin systems evolve rapidly
following HeII reionization. A comparison between HeII and HI absorption
features shows a broad distribution in the HeII and HI column density ratio,
peaking near the measured value and only slightly narrower than measured. A
comparison with approximate simulation methods shows moderately good agreement
in the absorption line properties, but not to the precision to which they may
be measured.Comment: 19 pages, 25 figures. Submitted to MNRA
Resolving the high redshift Lyman-alpha forest in smoothed particle hydrodynamics simulations
We use a large set of cosmological smoothed particle hydrodynamics (SPH)
simulations to examine the effect of mass resolution and box size on synthetic
Lya forest spectra at 2 \leq z \leq 5. The mass resolution requirements for the
convergence of the mean Lya flux and flux power spectrum at z=5 are
significantly stricter than at lower redshift. This is because transmission in
the high redshift Lya forest is primarily due to underdense regions in the
intergalactic medium (IGM), and these are less well resolved compared to the
moderately overdense regions which dominate the Lya forest opacity at z~2-3. We
further find that the gas density distribution in our simulations differs
significantly from previous results in the literature at large overdensities
(\Delta>10). We conclude that studies of the Lya forest at z=5 using SPH
simulations require a gas particle mass of M_gas \leq 2x10^5 M_sol/h, which is
>8 times the value required at z=2. A box size of at least 40 Mpc/h is
preferable at all redshifts.Comment: 5 pages, 5 figures, 2 tables, accepted by MNRA
The Lyman-alpha forest at redshifts 0.1 -- 1.6: good agreement between a large hydrodynamic simulation and HST spectra
We give a comprehensive statistical description of the Lyman-alpha absorption
from the intergalactic medium in a hydrodynamic simulation at redshifts
0.1-1.6, the range of redshifts covered by HST spectra of QSOs. We use the ENZO
code to make a 76 comoving Mpc cube simulation using 75 kpc cells, for a Hubble
constant of 71 km/s/Mpc. The best prior work, by \citet{dave99},used an SPH
simulation in a 15.6 Mpc box with an effective resolution of 245 kpc and
slightly different cosmological parameters. At redshifts z=2 this simulation is
different from data. \citet{tytler07b} found that the simulated spectra at z=2
have too little power on large scales, Lyman-alpha lines are too wide, there is
a lack high column density lines, and there is a lack of pixels with low flux.
Here we present statistics at z<1.6, including the flux distribution, the mean
flux, the effective opacity, and the power and correlation of the flux. We also
give statistics of the lyman alpha lines including the line width distribution,
the column density distribution, the number of lines per unit equivalent width
and redshift, and the correlation between the line width and column density. We
find that the mean amount of absorption in the simulated spectra changes
smoothly with redshift with DA(z)=0.01(1+z)^{2.25}. Both the trend and absolute
values are close to measurements of HST spectra by \citet{kirkman07a}. The
column density and line width distributions are also close to those measured
from HST spectra by \citet{janknecht06a}, except for the mode of the line width
distribution which is smaller in the HST spectra. Although some differences
that we saw at z=2 are too subtle to be seen in existing HST spectra, overall,
the simulation gives an good description of HST spectra at 0.1<z<1.6
The HeII Lyman alpha forest and the thermal state of the IGM
Recent analyses of the intergalactic UV background by means of the HeII Lyman
alpha forest assume that HeII and HI absorption features have the same line
widths. We omit this assumption to investigate possible effects of thermal line
broadening on the inferred HeII/HI ratio eta and to explore the potential of
intergalactic HeII observations to constrain the thermal state of the IGM.
Deriving a simple relation between the column density and the temperature of an
absorber we develop a procedure to fit the parameters of a power law
temperature-density relation and eta simultaneously. In an alternative approach
the temperature of an absorber, eta, and the redshift scale of eta variations
are estimated simultaneously. Tests with artificial data show that
well-constrained results can be obtained only if the signal-to-noise ratio in
the HeII forest is S/N > 20. Thus, it is impossible to give an estimate of the
temperature-density relation with the HeII data available at present (S/N ~5).
However, we find that only 45% of the lines in our sample favor turbulent line
widths. Furthermore, the inferred eta values are on average about 0.05 dex
larger if a thermal component is taken into account, and their distribution is
46% narrower in comparison to a purely turbulent fit. Therefore, variations of
eta on a 10% level may be related to the presence of thermal line broadening.
The apparent correlation between the strength of the HI absorption and the eta
value, which has been found in former studies, essentially disappears if
thermal broadening is taken into account. In the redshift range 2.58 < z < 2.74
towards the quasars HE2347-4342 and HS1700+6416 we obtain eta ~ 100. (abridged)Comment: accepted for publication by A&A, 11 pages, 13 figure
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