32,689 research outputs found
Has Blending Compromised Cepheid-Based Determinations of the Extragalactic Distance Scale?
We examine the suggestion that half of the HST Key Project- and
Sandage/Saha-observed galaxies have had their distances systematically
underestimated, by 0.1-0.3 mag in the distance modulus, due to the
underappreciated influence of stellar profile blending on the WFC chips. The
signature of such an effect would be a systematic trend in (i) the Type Ia
supernovae corrected peak luminosity and (ii) the Tully-Fisher residuals, with
increasing calibrator distance, and (iii) a differential offset between PC and
WFC distance moduli, within the same galaxy. The absence of a trend would be
expected if blending were negligible (as has been inherently assumed in the
analyses of the aforementioned teams). We adopt a functional form for the
predicted influence of blending that is consistent with the models of Mochejska
et al. and Stanek & Udalski, and demonstrate that the expected correlation with
distance predicted by these studies is not supported by the data. We conclude
that the Cepheid-based extragalactic distance scale has not been severely
compromised by the neglect of blending.Comment: 14 pages, 2 figures, 1 table, LaTeX, accepted for publication in
Astrophysical Journal Letters, also available at
http://casa.colorado.edu/~bgibson/publications.htm
Advanced Gas Turbine (AGT) power-train system development
Technical work on the design and component testing of a 74.5 kW (100 hp) advanced automotive gas turbine is described. Selected component ceramic component design, and procurement were tested. Compressor tests of a modified rotor showed high speed performance improvement over previous rotor designs; efficiency improved by 2.5%, corrected flow by 4.6%, and pressure ratio by 11.6% at 100% speed. The aerodynamic design is completed for both the gasifier and power turbines. Ceramic (silicon carbide) gasifier rotors were spin tested to failure. Improving strengths is indicated by burst speeds and the group of five rotors failed at speeds between 104% and 116% of engine rated speed. The emission results from combustor testing showed NOx levels to be nearly one order of magnitude lower than with previous designs. A one piece ceramic exhaust duct/regenerator seal platform is designed with acceptable low stress levels
On Dwarf Galaxies as the Source of Intracluster Gas
Recent observational evidence for steep dwarf galaxy luminosity functions in
several rich clusters has led to speculation that their precursors may be the
source of the majority of gas and metals inferred from intracluster medium
(ICM) x-ray observations. Their deposition into the ICM is presumed to occur
through early supernovae-driven winds, the resultant systems reflecting the
photometric and chemical properties of the low luminosity dwarf spheroidals and
ellipticals we observe locally. We consider this scenario, utilising a
self-consistent model for spheroidal photo-chemical evolution and gas ejection
via galactic superwinds. Insisting that post-wind dwarfs obey the observed
colour-luminosity-metallicity relations, we conclude that the bulk of the ICM
gas and metals does not originate within their precursors.Comment: 43 pages, 8 figures, LaTeX, also available at
http://msowww.anu.edu.au/~gibson/publications.html, to appear in ApJ, Vol
473, 1997, in pres
Weighing the galactic disc using the Jeans equation: lessons from simulations
Using three-dimensional stellar kinematic data from simulated galaxies, we examine the efficacy of a Jeans equation analysis in reconstructing the total disk surface density, including the dark matter, at the ‘Solar’ radius. Our simulation data set includes galaxies formed in a cosmological context using state-of-the-art high-resolution cosmological zoom simulations, and other idealized models. The cosmologically formed galaxies have been demonstrated to lie on many of the observed scaling relations for late-type spirals, and thus offer an interesting surrogate for real galaxies with the obvious advantage that all the kinematical data are known perfectly. We show that the vertical velocity dispersion is typically the dominant kinematic quantity in the analysis, and that the traditional method of using only the vertical force is reasonably effective at low heights above the disk plane. At higher heights the inclusion of the radial force becomes increasingly important. We also show that the method is sensitive to uncertainties in the measured disk parameters, particularly the scalelengths of the assumed double exponential density distribution, and the scalelength of the radial velocity dispersion. In addition, we show that disk structure and low number statistics can lead to significant errors in the calculated surface densities. Finally, we examine the implications of our results for previous studies of this sort, suggesting that more accurate measurements of the scalelengths may help reconcile conflicting estimates of the local dark matter density in the literature
On the Physical Origin of OVI Absorption-Line Systems
We present a unified analysis of the O{\sc vi} absorption-lines seen in the
disk and halo of the Milky Way, high velocity clouds, the Magellanic Clouds,
starburst galaxies, and the intergalactic medium. We show that these disparate
systems define a simple relationship between the O{\sc vi} column density and
absorption-line width that is independent of the Oxygen abundance over the
range O/H 10% to twice solar. We show that this relation is exactly that
predicted theoretically as a radiatively cooling flow of hot gas passes through
the coronal temperature regime - independent of its density or metallicity (for
O/H 0.1 solar). Since most of the intregalactic O{\sc vi} clouds obey
this relation, we infer that they can not have metallicities less than a few
percent solar. In order to be able to cool radiatively in less than a Hubble
time, the intergalactic clouds must be smaller than 1 Mpc in size. We
show that the cooling column densities for the O{\sc iv}, O{\sc v}, Ne{\sc v},
and Ne{\sc vi} ions are comparable to those seen in O{\sc vi}. This is also
true for the Li-like ions Ne{\sc viii}, Mg{\sc x}, and Si{\sc xii} (if the gas
is cooling from K). All these ions have strong resonance lines
in the extreme-ultraviolet spectral range, and would be accessible to at
0.2 to 0.8. We also show that the Li-like ions can be used to probe
radiatively cooling gas at temperatures an order-of-magnitude higher than where
their ionic fraction peaks. We calculate that the H-like (He-like) O, Ne, Mg,
Si, and S ions have cooling columns of cm. The O{\sc vii},
O{\sc viii}, and Ne{\sc ix} X-ray absorption-lines towards PKS 2155-304 may
arise in radiatively cooling gas in the Galactic disk or halo.Comment: 25 pages, 5 figure
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