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

Conic bundles in projective fourspace

P. Ellia and G.Sacchiero have shown that if $S$ is a smooth surface in \Pn 4 which is ruled in conics, then $S$ 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

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