15,908 research outputs found
X-ray Isophote Shapes and the Mass of NGC 3923
We present analysis of the shape and radial mass distribution of the E4
galaxy NGC 3923 using archival X-ray data from the ROSAT PSPC and HRI. The
X-ray isophotes are significantly elongated with ellipticity e_x=0.15
(0.09-0.21) (90% confidence) for semi-major axis a\sim 10h^{-1}_70 kpc and have
position angles aligned with the optical isophotes within the estimated
uncertainties. Applying the Geometric Test for dark matter, which is
independent of the gas temperature profile, we find that the ellipticities of
the PSPC isophotes exceed those predicted if M propto L at a marginal
significance level of 85% (80%) for oblate (prolate) symmetry. Detailed
hydrostatic models of an isothermal gas yield ellipticities for the gravitating
matter, e_mass=0.35-0.66 (90% confidence), which exceed the intensity weighted
ellipticity of the R-band optical light, = 0.30 (e_R^max=0.39).
We conclude that mass density profiles with rho\sim r^{-2} are favored over
steeper profiles if the gas is essentially isothermal (which is suggested by
the PSPC spectrum) and the surface brightness in the central regions (r<~15")
is not modified substantially by a multi-phase cooling flow, magnetic fields,
or discrete sources. We argue that these effects are unlikely to be important
for NGC 3923. (The derived e_{mass} range is very insensitive to these issues.)
Our spatial analysis also indicates that the allowed contribution to the ROSAT
emission from a population of discrete sources with Sigma_x propto Sigma_R is
significantly less than that indicated by the hard spectral component measured
by ASCA.Comment: 14 pages (6 figures), To Appear in MNRA
X-ray Constraints on the Intrinsic Shape of the Lenticular Galaxy NGC 1332
We have analyzed ROSAT PSPC X-ray data of the optically elongated S0 galaxy
NGC 1332 with the purposes of constraining the intrinsic shape of its
underlying mass and presenting a detailed investigation of the uncertainties
resulting from the assumptions underlying this type of analysis. The X-ray
isophotes are elongated with ellipticity (90% confidence) for
semi-major axes 75\arcsec -90\arcsec and have orientations consistent with
the optical isophotes (ellipticity ). The spectrum is poorly
constrained by the PSPC data and cannot rule out sizeable radial temperature
gradients or an emission component due to discrete sources equal in magnitude
to the hot gas. Using (and clarifying) the "geometric test" for dark matter, we
determined that the hypothesis that mass-traces-light is not consistent with
the X-ray data at 68% confidence and marginally consistent at 90% confidence
independent of the gas temperature profile. Detailed modeling gives constraints
on the ellipticity of the underlying mass of \epsilon_{mass} = 0.47 - 0.72
(0.31 - 0.83) at 68% (90%) confidence for isothermal and polytropic models. The
total mass of the isothermal models within a=43.6 kpc (D = 20h^{-1}_{80} Mpc)
is M_{tot} = (0.38 - 1.7) \times 10^{12}M_{\sun} (90% confidence) corresponding
to total blue mass-to-light ratio \Upsilon_B = (31.9 - 143) \Upsilon_{\sun}.
Similar results are obtained when the dark matter is fit directly using the
known distributions of the stars and gas. When possible rotation of the gas and
emission from discrete sources are included flattened mass distributions are
still required, although the constraints on \epsilon_{mass}$, but not the
total mass, are substantially weakened.Comment: 45 pages (figures missing), PostScript, to appear in ApJ on January
20, 199
The Tactical and Strategic Value of Commodity Futures
Historically, commodity futures have had excess returns similar to those of equities. But what should we expect in the future? The usual risk factors are unable to explain the time-series variation in excess returns. In addition, our evidence suggests that commodity futures are an inconsistent, if not tenuous, hedge against unexpected inflation. Further, the historically high average returns to a commodity futures portfolio are largely driven by the choice of weighting schemes. Indeed, an equally weighted long-only portfolio of commodity futures returns has approximately a zero excess return over the past 25 years. Our portfolio analysis suggests that the a long-only strategic allocation to commodities as a general asset class is a bet on the future term structure of commodity prices, in general, and on specific portfolio weighting schemes, in particular. In contrast, we provide evidence that there are distinct benefits to an asset allocation overlay that tactically allocates using commodity futures exposures. We examine three trading strategies that use both momentum and the term structure of futures prices. We find that the tactical strategies provide higher average returns and lower risk than a long-only commodity futures exposure.
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