4,125 research outputs found
Flight-test evaluation of two electronic display formats for approach to landing under instrument conditions
The results of a flight evaluation of two electronic display formats for the approach to landing under instrument conditions are presented. The evaluation was conducted for a base-line electronic display format and for the same format with runway symbology and track information added. The evaluation was conducted during 3 deg, manual straight-in approaches with and without initial localizer offsets. Flight path tracking performance data and pilot subjective comments were examined with regard to the pilot's ability to capture and maintain localizer and glide slope by using both display formats
Accuracy of Mesh Based Cosmological Hydrocodes: Tests and Corrections
We perform a variety of tests to determine the numerical resolution of the
cosmological TVD eulerian code developed by Ryu et al (1993). Tests include
512^3 and 256^3 simulations of a Pk=k^{-1} spectrum to check for
self-similarity and comparison of results with those from higher resolution SPH
and grid-based calculations (Frenk et al 1998). We conclude that in regions
where density gradients are not produced by shocks the code degrades resolution
with a Gaussian smoothing (radius) length of 1.7 cells. At shock caused
gradients (for which the code was designed) the smoothing length is 1.1 cells.
Finally, for \beta model fit clusters, we can approximately correct numerical
resolution by the transformation R^2_{core}\to R^2_{core}-(C\Delta l)^2, where
\Delta l is the cell size and C=1.1-1.7. When we use these corrections on our
previously published computations for the SCDM and \Lambda CDM models we find
luminosity weighted, zero redshift, X-ray cluster core radii of (210\pm 86,
280\pm 67)h^{-1}kpc, respectively, which are marginally consistent with
observed (Jones & Forman 1992) values of 50-200h^{-1}kpc. Using the corrected
core radii, the COBE normalized SCDM model predicts the number of bright
L_x>10^{43}erg/s clusters too high by a factor of \sim 20 and the \Lambda CDM
model is consistent with observations.Comment: ApJ in press (1999
The Milky Way and Andromeda galaxies in a constrained hydrodynamical simulation: morphological evolution
We study the two main constituent galaxies of a constrained simulation of the
Local Group as candidates for the Milky Way (MW) and Andromeda (M31). We focus
on the formation of the stellar discs and its relation to the formation of the
group as a rich system with two massive galaxies, and investigate the effects
of mergers and accretion as drivers of morphological transformations. We use a
state-of-the-art hydrodynamical code which includes star formation, feedback
and chemical enrichment to carry out our study. We run two simulations, where
we include or neglect the effects of radiation pressure from stars, to
investigate the impact of this process on the morphologies and star formation
rates of the simulated galaxies. We find that the simulated M31 and MW have
different formation histories, even though both inhabit, at z=0, the same
environment. These differences directly translate into and explain variations
in their star formation rates, in-situ fractions and final morphologies. The
M31 candidate has an active merger history, as a result of which its stellar
disc is unable to survive unaffected until the present time. In contrast, the
MW candidate has a smoother history with no major mergers at late times, and
forms a disc that grows steadily; at z=0 the simulated MW has an extended,
rotationally-supported disc which is dominant over the bulge. Our two feedback
implementations predict similar evolution of the galaxies and their discs,
although some variations are detected, the most important of which is the
formation time of the discs: in the model with weaker/stronger feedback the
discs form earlier/later. In summary, by comparing the formation histories of
the two galaxies, we conclude that the particular merger/accretion history of a
galaxy rather than its environment at the LG-scales is the main driver of the
formation and subsequent growth or destruction of galaxy discs.Comment: 12 pages, 7 figures, accepted for publication in A&
The Effect of Environment on Milky Way-mass galaxies in a Constrained Simulation of the Local Group
In this letter we present, for the first time, a study of star formation
rate, gas fraction and galaxy morphology of a constrained simulation of the
Milky Way (MW) and Andromeda (M31) galaxies, compared to other MW-mass
galaxies. By combining with unconstrained simulations we cover a sufficient
volume to compare these galaxies environmental densities ranging from the field
to that of the Local Group (LG). This is particularly relevant as it has been
shown that, quite generally, galaxy properties depend intimately upon their
environment, most prominently when galaxies in clusters are compared to those
in the field. For galaxies in loose groups such as the LG, however,
environmental effects have been less clear. We consider the galaxy's
environmental density in spheres of 1200 kpc (comoving) and find that whilst
environment does not appear to directly affect morphology, there is a positive
trend with star formation rates. This enhancement in star formation occurs
systematically for galaxies in higher density environments, regardless whether
they are part of the LG or in filaments. Our simulations suggest that the
richer environment at Mpc-scales may help replenish the star-forming gas,
allowing higher specific star formation rates in galaxies such as the MW.Comment: 6 pages, 4 figures, accepted to ApJ
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