Away from the lights of the city, a vast, bright band of stars dominate the night sky, outlining the plane of our Galaxy (see Fig. 1). Even with the naked eye, closer inspection reveals ‘dark lanes’ silhouetted against the stellar illumination. These are in fact clouds of gas and dust, dense enough to shield themselves from the highly destructive interstellar radiation field. This self-shielding means that the kinetic temperature of the gas remains very low (<15 degrees above absolute zero), allowing the gas to eventually condense into cores. As the collapse continues, the density and temperature of each core increases until nuclear fusion ignites and a star is born. The initial cold phase is required to prevent the gas being too agitated or dispersed, and deep in this cold, dark gas, the atoms can bind into molecules, giving the 100 plus species already identified in interstellar space.

Although such Giant Molecular Clouds, which are over 100 light-years in diameter and 100,000 times as massive as the Sun, are ubiquitous throughout our Galaxy, no pure molecular cloud has ever been detected in an extragalactic source. There are many examples of molecular gas being detected in other near-by galaxies and even in emission (gas warm enough to glow) at the limits of the observable Universe, but these are always accompanied by atomic gas emission, indicating that the gas is not purely molecular. However, we may have detected the first pure dark molecular cloud that is not only outside our own galaxy, but at a cosmological distance (1.6 billion light-years).

This is detected through the absorption of the radio emission from a background radio galaxy by the hydroxyl (OH) radical (Fig. 2). Although the detection is tentative, the two features are consistent with the OH ground state transition: the observed frequencies of 1478.66 and 1480.44 MHz match the rest frequencies of 1665 and 1667 MHz redshifted by z=0.1263, and the ratio of the line strengths is consistent with the theoretical 5:9 ratio.

This spectrum was obtained after 6.5 hours with the Australia Telescope Compact Array and no absorption due to atomic hydrogen at z=0.1263 was detected during the same observing run. Our results therefore suggest that the molecular hydrogen (which cannot be observed directly from the ground), is over 100 times as abundant as the atomic hydrogen, meaning that the gas is very nearly 100% molecular.  We will soon be undertaking further observations with the world's largest steerable radio telescope, at Green Bank in the USA, to confirm this result.

Fig. 1: Dusty molecular gas within our own Galaxy blocking the visible light from the stars behind. Fig. 2: The possible OH absorption features towards the radio galaxy PKS 2300-189. The ordinate shows the flux density relative to 1.05 Jy and the abscissa the observed frequency in MHz. The best two-component Gaussian fit is shown.

Steve Longmore, Steve Curran & Matthew Whiting