ALBERT

Description: We discuss physical conditions in Galactic neutral hydrogen based on deep, high-velocity resolution interferometric H i -21 cm absorption spectroscopy towards 33 compact extragalactic radio sources. The H i -21 cm optical depth spectra have root-mean-square noise values 10 –3 per 1 km s –1 velocity channel, i.e. sufficiently sensitive to detect H i -21 cm absorption by the warm neutral medium (WNM). Comparing these spectra with H i -21 cm emission spectra from the Leiden–Argentine–Bonn (LAB) survey, we show that some of the absorption detected on most sightlines must arise in gas with temperatures higher than that in the stable cold neutral medium (CNM). A multi-Gaussian decomposition of 30 of the H i -21 cm absorption spectra yielded very few components with linewidths in the temperature range of stable WNM, with no such WNM components detected for 16 of the 30 sightlines. We find that some of the detected H i -21 cm absorption along 13 of these sightlines must arise in gas with spin temperatures larger than the CNM range. For these sightlines, we use very conservative estimates of the CNM spin temperature and the non-thermal broadening to derive strict upper limits to the gas column densities in the CNM and WNM phases. Comparing these upper limits to the total H i column density, we find that typically at least 28 per cent of the gas must have temperatures in the thermally unstable range (200–5000 K). Our observations hence robustly indicate that a significant fraction of the gas in the Galactic interstellar medium has temperatures outside the ranges expected for thermally stable gas in two-phase models.