(left) A true-colour image of the sky surrounding PKS 1555-140. The radio galaxy itself is the large one located between the ends of the two dashed lines. (right) The Compact Array spectrum that reveals the presence of hydrogen gas.

Most matter in the universe is in the form of hydrogen, either inside stars, self-gravitating clouds or free-floating gas. When heated, for instance by nearby stars, the gas is easily seen by its own light. When cold, however, it can be hard to detect. A useful property is that hydrogen absorbs light and radio waves of particular wavelengths that pass through the cloud, leaving characteristic absorption lines in the spectra of the background radiation source. Detection of such lines can betray the presence of otherwise undetectable gas along the line-of-sight to the background source.

One such source of radiation is the radio galaxy PKS 1555-140, 1.3 billion light-years away in the constellation Libra. This galaxy has a super-massive black hole at its centre (with 20 million times the Sun’s mass), the presence of which accelerates electrons close to the speed of light, producing a strong radio source in the centre of the galaxy. This radio emission is absorbed by cold hydrogen undergoing the spin-flip transition at 1420 MHz. We observe this at 1294 MHz, giving the same redshift as the galaxy.

From the spectrum, obtained with Australia Telescope Compact Array at Narrabri in July 2004, we see that the gas is spread over 400 km/s, indicative of galactic rotation – further evidence for association with the galaxy. From the velocity width and the depth of the line, for ~100 atoms per cubic centimetre at a temperature of ~ -200 C, we can infer a cloud size of < 100 light years: relatively small compared to the size of the galaxy.
Interestingly, the absorption line appears to have at least two peaks, indicating that there are possibly several separate gas clouds present. Collisions between galaxies are common and we believe that interactions with the other nearby galaxies in this system may have stirred up the surrounding gas, creating the complex absorption line profile. This hypothesis should be confirmed by up-coming optical observations from Siding Spring Observatory.

Matthew Whiting, Steve Curran
and Steve Longmore