Taking the temperature of the cosmic background
BY DR EMILY BALDWIN
ASTRONOMY NOW
Posted: May 13, 2008

Using ESO’s Very Large Telescope, astronomers have made the first detection of a carbon monoxide molecule in a galaxy almost 11 billion light years away, a feat that has resulted in the most precise measurement of the cosmic temperature at such a remote epoch.

The only way the galaxy can be observed is through the imprint its interstellar gas leaves on the spectrum of a remote quasar, which is used as a beacon in the very distant Universe. “Interstellar clouds of gas in galaxies, located between the quasars and us on the same line of sight, absorb parts of the light emitted by the quasars. The resulting spectrum consequently presents dark 'valleys' that can be attributed to well-known elements and possibly molecules," explains Raghunathan Srianand who led the research.

Well-hidden galaxies are identified through the imprint their interstellar gases leave on the spectrum of a remote quasar. In this diagram, the VLT observes the features associated with three systems (spectrum D), located at different distances (A-C) and whose light is therefore shifted by different amounts. The quasar is shown as the bright object at the left of the image. Image: ESO.

Thanks to the power of the VLT, the team was able to discover the presence of normal and deuterated molecular hydrogen and carbon monoxide molecules in the interstellar medium of this remote galaxy. "This is the first time that these three molecules have been detected in absorption in front of a quasar, a detection that has remained elusive for more than a quarter of a century," says Cédric Ledoux of the European Southern Observatory (ESO). The same team had already broken the record for the most distant detection of molecular hydrogen in a young, 1.5 billion year old galaxy.

Even more impressively, the team also measured with the best ever precision yet, the temperature of the cosmic background radiation in the remote Universe. "Unlike other methods, measuring the temperature of the cosmic background using the carbon monoxide molecule involves very few assumptions," says Pasquier Noterdaeme, also of the ESO. According to Big Bang theory, the glow of this primeval fireball should have been warmer in the past, which is exactly what the new measurements confirm. "Given the current measured temperature of 2.725 Kelvin (K), one would expect that the temperature 11 billion years ago was about 9.3 K," says Patrick Petitjean. "Our unique set of VLT observations allows us to deduce a temperature of 9.15 K, plus or minus 0.7 K, in excellent agreement with the theory."

The spectrum of a very distant quasar on which the footprints from a galaxy 11 billion light years away has been seen. Various bands of carbon monoxide and hydrogen were identified. The different intensities of the carbon monoxide (CO) bands allows astronomers to infer the temperature of the cosmic background radiation at this remote epoch. Image: ESO.

Based on the new observations, the astronomers have shown that the physical conditions prevailing in the interstellar gas in this remote galaxy are similar to what is seen in our Milky Way Galaxy. This pioneering analysis demonstrates that it is possible, combined with the detection of molecules such as molecular hydrogen and carbon monoxide, to use interstellar chemistry to tackle important cosmological issues, such as the formation and evolution of galaxies.