arXiv subjects
Publications and source records attributed to Robert Braun.
We discuss the current observational and theoretical issues concerning cold gas at high redshift and present simulations showing how a number of observational issues can be resolved with planned future instrumentation.
High quality data are presented of neutral hydrogen emission and absorption in the fields of eleven of the nearest spiral galaxies. Multi-configuration VLA observations have provided angular resolution of 6~arcsec (corresponding to about 100~pc at the average galaxy distance of 3.5~Mpc) and velocity resolution of 6~km~s$^{-1}$, while accurately recovering the total line flux detected previously with filled apertures. Previous experience suggests that this physical resolution is sufficient to at least marginally resolve the \ion{H}{1} super-cloud population which delineates regions of active star formation. A high brightness filamentary network of \ion{H}{1} super-clouds is seen in each galaxy. Emission brightness temperatures in excess of 200~Kelvin are sometimes detected at large radii, even in relatively face-on systems. All galaxies display a systematic increase in the observed brightness temperature of super-clouds with radius, followed by a flattening and subsequent decline. In the few instances where background continuum sources allow detection of \ion{H}{1} absorption, the indicative spin temperatures are consistent with the super-cloud brightness temperature seen in emission at similar radii. These data suggest substantial opacity of the \ion{H}{1} in the super-cloud network.
We briefly review the beginnings of HI astronomy, proceed to an assessment of our current capabilities in this area, and continue by considering what will be necessary to push back the frontier to cosmological distances. We then consider how such a leap in performance might be realized.
The scientific motivation is reviewed for a next generation radio observatory operating at frequencies between about 200 MHz and 2 GHz with about 2 orders of magnitude greater sensitivity than that which is currently available, together with sub-arcsecond angular resolution. Instrumental concepts for the telescope are discussed, highlighting the role of mass produced receiver elements and digital electronics in increasing cost-effectiveness while actively reducing the instrument's sensitivity to radio frequency interference.
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. Fløystad and the first author bounding the degree of smooth surfaces in $\Pn 4$ not of general type.