Discovery of molecular hydrogen in a high-velocity cloud of the Galactic halo.

The Milky Way's halo contains clouds of neutral hydrogen with high radial velocities which do not follow the general rotational motion of the Galaxy. Few distances to these high-velocity clouds are known, so even gross properties such as total mass are hard to determine. As a consequence, there is no generally accepted theory regarding their origin. One idea is that they result from gas that has cooled after being ejected from the Galaxy through fountain-like flows powered by supernovae; another is that they are composed of gas, poor in heavy elements, which is falling onto the disk of the Milky Way from intergalactic space. The presence of molecular hydrogen, whose formation generally requires the presence of dust (and therefore gas, enriched in heavy elements), could help to distinguish between these possibilities. Here we report the discovery of molecular hydrogen absorption in a high-velocity cloud along the line of sight to the Large Magellanic Cloud. We also derive for the same cloud an iron abundance which is half of the solar value. From these data, we conclude that gas in this cloud originated in the disk of the Milky Way.

A 30-cm objective grating for far-UV astronomy: theoretical study and laboratory tests.

We have performed theoretical determination and experimental calibrations of an objective grating designed for high resolution spectroscopy of astronomical faint sources in the EUV and far-UV wavelength ranges (500-1400 A). First through theoretical calculations we show the feasibility of the concept with an aspheric shape for the grating blank and determine its geometrical parameters. A grating of this large size has been manufactured and tested, associated with a photon counting detector, in a vacuum environment. Finally we demonstrate that a resolving power of 3 x 10(4), a total equivalent effective area of ~5-10 cm(2) can be achieved, together with a very low scattered light level (10(-4)-10(-5) of the peak value).

Emission-line variability and distance of quasars.

The radius of the line-emitting region in quasars is larger than 1 parsec for the cosmological hypothesis and smaller than 1 parsec for the local hypothesis. Variability in emission lines has been reported but not proved beyond doubt. Its time scale would be a direct observational indicator of this radius and would add an important element to the discussion of the origin of quasars. Objects that are variable in their optical continuum seem to be the most promising ones to look at for line variations, and they should be observed spectroscopically at regular intervals.

Inverse compton effect: some consequences for quasars.

The inverse Compton effect can transform enough energy of relativistic electrons into radiation so that an upper limit to the mean energy of the electrons is set. In quasars, the limit is too small to allow the production of any appreciable amount of synchrotron or inverse Compton radiation, unless either the distances are not cosmological or the lifetimes of the relativistic electrons are extremely short, of the order of hours.