5,877 research outputs found
The Temperature and Opacity of Atomic Hydrogen in Spiral Galaxies
We analyze the resolved neutral hydrogen emission properties of a sample of
eleven of the nearest spiral galaxies. Between 60 and 90% of the total HI line
flux within the optical disk is due to a high brightness network (HBN) of
emission features which are marginally resolved in their narrow dimension at
about 150 pc and have a face-on covering factor of about 15%. Averaged line
profiles of this component are systematically non-Gaussian with a narrow core
(less than about 6 km/s FWHM) superposed on broad Lorentzian wings (30 km/s
FWHM). An upper limit to the gas temperature of 300 K follows directly from the
narrow line profiles, while simple modeling suggests kinetic temperatures equal
to the peak emission brightness temperature (80-200 K) in all cases but the
outer disks of low mass galaxies, where the HBN becomes optically thin to the
21 cm line. Positive radial gradients in the derived kinetic temperature are
found in all spiral galaxies. The distributions of brightness temperature with
radius in our sample form a nested system with galaxies of earlier
morphological type systematically displaced to lower temperature at all radii.
The fractional line flux due to the HBN plummets abruptly near the edge of the
optical disk where a diffuse outer gas disk takes over. We identify the HBN
with the Cool Neutral Medium.Comment: 22 page LaTeX requires aastex, 10 PS figures. Accepted for
publication in the Ap
Understanding synthesis imaging dynamic range
We develop a general framework for quantifying the many different
contributions to the noise budget of an image made with an array of dishes or
aperture array stations. Each noise contribution to the visibility data is
associated with a relevant correlation timescale and frequency bandwidth so
that the net impact on a complete observation can be assessed. All quantities
are parameterised as function of observing frequency and the visibility
baseline length. We apply the resulting noise budget analysis to a wide range
of existing and planned telescope systems that will operate between about 100
MHz and 5 GHz to ascertain the magnitude of the calibration challenges that
they must overcome to achieve thermal noise limited performance. We conclude
that calibration challenges are increased in several respects by small
dimensions of the dishes or aperture array stations. It will be more
challenging to achieve thermal noise limited performance using 15 m class
dishes rather than the 25 m dishes of current arrays. Some of the performance
risks are mitigated by the deployment of phased array feeds and more with the
choice of an (alt,az,pol) mount, although a larger dish diameter offers the
best prospects for risk mitigation. Many improvements to imaging performance
can be anticipated at the expense of greater complexity in calibration
algorithms. However, a fundamental limitation is ultimately imposed by an
insufficient number of data constraints relative to calibration variables. The
upcoming aperture array systems will be operating in a regime that has never
previously been addressed, where a wide range of effects are expected to exceed
the thermal noise by two to three orders of magnitude. Achieving routine
thermal noise limited imaging performance with these systems presents an
extreme challenge. The magnitude of that challenge is inversely related to the
aperture array station diameter.Comment: 27 pages, 24 figures, accepted in A&A, final versio
Tiny HI Clouds in the Local ISM
Very sensitive HI absorption spectra (tau RMS about 10^-4 over 1 km/s) toward
high latitude QSOs have revealed a population of tiny discrete features in the
diffuse ISM with peak tau of 0.1 - 2% and core line-widths corresponding to
temperatures as low as 20 K. Imaging detections confirm linear dimensions of a
few 1000 AU. We suggest these structures may be formed by the stellar winds of
intermediate mass stars. A more speculative origin might involve molecular
"dark matter".Comment: 6 pages, 8 figures, to appear in "The IMF at 50", eds. E. Corbelli,
F. Palla, and H. Zinnecker, ASSL (Kluwer
Conic bundles in projective fourspace
P. Ellia and G.Sacchiero have shown that if is a smooth surface in \Pn
4 which is ruled in conics, then has degree 4 or 5. In this paper we give
a proof of this result combining the ideas of Ellia and Sacchiero as they are
used in the paper of the second author on plane curve fibrations and the recent
work of G. Fl\o ystad and the first author bounding the degree of smooth
surfaces in \Pn 4 not of general type.Comment: 7 pages, Plain-Te
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