11,031 research outputs found
On the Formation of Boxy and Disky Elliptical Galaxies
The origin of boxy and disky elliptical galaxies is investigated. The results
of two collisionless N-body simulations of spiral-spiral mergers with mass
ratios of 1:1 and 3:1 are discussed and the projected properties of the merger
remnants are investigated. It is shown that the equal-mass merger leads to an
anisotropic, slowly rotating system with preferentially boxy isophotes and
significant minor axis rotation. The unequal-mass merger results in the
formation of a rotationally supported elliptical with disky isophotes and small
minor axis rotation. The observed scatter in the kinematical and isophotal
properties of both classes of elliptical galaxies can be explained by
projection effects.Comment: 12 pages, incl. 5 figures, accepted by ApJ Letter
Constraints on galaxy formation from alpha-enhancement in luminous elliptical galaxies
We explore the formation of alpha-enhanced and metal-rich stellar populations
in the nuclei of luminous ellipticals under the assumption of two extreme
galaxy formation scenarios based on hierarchical clustering, namely a fast
clumpy collapse and the merger of two spirals. We investigate the parameter
space of star formation time-scale, IMF slope, and stellar yields. In
particular, the latter add a huge uncertainty in constraining time-scales and
IMF slopes. We find that -- for Thielemann, Nomoto & Hashimoto nucleosynthesis
-- in a fast clumpy collapse scenario an [alpha/Fe] overabundance of approx.
0.2 dex in the high metallicity stars can be achieved with a Salpeter IMF and
star formation time-scales of the order 10^9 yr. The scenario of two merging
spirals which are similar to our Galaxy, instead, fails to reproduce
alpha-enhanced abundance ratios in the metal-rich stars, unless the IMF is
flattened during the burst ignited by the merger. This result is independent of
the burst time-scale. We suggest that abundance gradients give hints to
distinguish between the two extreme formation scenarios considered in this
paper.Comment: Accepted for publication in MNRAS, LaTex 2.09 with mn.sty, 13 pages,
5 figure
TOPEX orbit determination using GPS signals plus a sidetone ranging system
The GPS orbit determination was studied to see how well the radial coordinate for altimeter satellites such as TOPEX could be found by on board measurements of GPS signals, including the reconstructed carrier phase. The inclusion on altimeter satellites of an additional high accuracy tracking system is recommended. It is suggested that a sidetone ranging system is used in conjunction with TRANET 2 beacons
Addendum to SSV Generic OFT First Stage Ascent Base Convective Heating Environments
Convective environments for OFT Mission C are presented in graphs for first stage convective heating to the internal surfaces of the OMS nozzle, to the aft facing 8 and 9 RCS nozzles, and to the base (trailing edge) of the vertical tail
The Stellar Kinematic Fields of NGC 3379
We have measured the stellar kinematic profiles of NGC 3379 along four
position angles using the MMT. The data extend 90" from the center, at
essentially seeing-limited resolution out to 17". The mean velocities and
dispersions have total errors better than 10 km/s (frequently better than 5
km/s) out to 55". We find very weak (3 km/s) rotation on the minor axis
interior to 12", and no detectable rotation above 6 km/s from 12" to 50" or
above 16 km/s out to 90" (95% confidence). However, a Fourier reconstruction of
the mean velocity field from all 4 sampled PAs does indicate a 5 degree twist
of the kinematic major axis, opposite to the known isophotal twist. The h_3 and
h_4 parameters are small over the entire observed region. The
azimuthally-averaged dispersion profile joins smoothly at large radii with the
dispersions of planetary nebulae. Unexpectedly, we find sharp bends in the
major-axis rotation curve, also visible (though less pronounced) on the
diagonal position angles. The outermost bend coincides in position with other
sharp kinematic features: an abrupt flattening of the dispersion profile, and
local peaks in h_3 and h_4. All of these features are in a region where the
surface brightness profile departs significantly from a de Vaucouleurs law.
Features such as these are not generally known in ellipticals owing to a lack
of data at comparable resolution; however, very similar behavior is seen the
kinematics of the edge-on S0 NGC 3115. We discuss the suggestion that NGC 3379
could be a misclassified S0; preliminary results from dynamical modeling
indicate that it may be a flattened, weakly triaxial system seen in an
orientation that makes it appear round.Comment: 31 pages incl. 4 tables, Latex, AASTeX v4.0, with 17 eps figures. To
appear in The Astronomical Journal, February 199
The evolution of the color gradients of early-type cluster galaxies
We investigate the origin of color gradients in cluster early-type galaxies
to probe whether pure age or pure metallicity gradients can explain the
observed data in local and distant (z approx 0.4) samples. We measure the
surface brightness profiles of the 20 brightest early-type galaxies of
CL0949+44 (hereafter CL0949) at redshift z=0.35-0.38 from HST WF2 frames taken
in the filters F555W, F675W, F814W. We determine the color profiles (V-R)(r),
(V-I)(r), and (R-I)(r) as a function of the radial distance r in arcsec, and
fit logarithmic gradients in the range -0.2 to 0.1 mag per decade. These values
are similar to what is found locally for the colors (U-B), (U-V), (B-V) which
approximately match the (V-R), (V-I), (R-I) at redshift approx 0.4. We analyse
the results with up to date stellar population models. We find that passive
evolution of metallicity gradients (approx 0.2 dex per radial decade) provides
a consistent explanation of the local and distant galaxies' data. Invoking pure
age gradients (with fixed metallicity) to explain local color gradients
produces too steep gradients at redshifts z approx 0.4. Pure age gradients are
consistent with the data only if large present day ages (>=15 Gyr) are assumed
for the galaxy centers.Comment: 23 pages, 19 figures, Accepted for publication in A&
Vector Casimir effect for a D-dimensional sphere
The Casimir energy or stress due to modes in a D-dimensional volume subject
to TM (mixed) boundary conditions on a bounding spherical surface is
calculated. Both interior and exterior modes are included. Together with
earlier results found for scalar modes (TE modes), this gives the Casimir
effect for fluctuating ``electromagnetic'' (vector) fields inside and outside a
spherical shell. Known results for three dimensions, first found by Boyer, are
reproduced. Qualitatively, the results for TM modes are similar to those for
scalar modes: Poles occur in the stress at positive even dimensions, and cusps
(logarithmic singularities) occur for integer dimensions . Particular
attention is given the interesting case of D=2.Comment: 20 pages, 1 figure, REVTe
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