In the News this month: the molecular content of early galaxies
The Plateau de Bure interferometer CREDIT: IRAM
A long-standing question in the study of star formation is whether the process was more efficient in the early universe than it is today. Stars form through the collapse of clouds of cold gas. As the collapse progresses, the core of the cloud gets denser and hotter until nuclear fusion begins and a star is born. In the local universe, however, cold molecular gas is relatively rare so star formation occurs slowly; the Milky Way forms new stars at a rate of only a few per year. More distant galaxies formed stars at a much higher rate, but in order to determine whether this is due to a more efficient star formation process or a more ready supply of molecular gas, it is necessary to investigate their gas content.
Star formation within these clouds is very difficult to observe directly since the gas absorbs much of the visible light produced by young proto-stars. Once they begin to shine, the radiation pressure of young stars begins to dispel the surrounding gas and the star becomes visible. The gas itself is hard to detect but some molecules, such as carbon monoxide, are visible through the radiation they emit at infrared wavelengths.
A team of researchers used the Plateau de Bure interferometer to examine the gas content of two samples of galaxies which are so distant that we see them as they were when the universe was only 40 and 24 percent of its current age. Because they are so distant, the infrared radiation from the carbon monoxide molecules in these galaxies is shifted into the part of the spectrum where wavelengths are measured in millimetres. Using new receivers recently installed on the antennas of the interferometer at the Plateau de Bure in France, Linda Tacconi and colleagues imaged the molecular gas content of these galaxies. Many previous studies have focused on highly extreme examples, galaxies forming stars at very high rates due to powerful central black holes or systems where galaxies are merging, but Tacconi's team studied more modest examples likely to be more typical of normal star forming galaxies.
Published in the journal Nature on February 11th, their results show that distant star forming galaxies were in fact gas rich, containing three to ten times more cold gas (as a fraction of the galaxy's total mass) than equivalent galaxies in the local universe today. Their results also show that the fraction of gas does not vary greatly with redshift: the galaxies in the more distant sample, seen when the universe was just three billion years old, contained 44 percent molecular gas while those in the closer sample, seen when the universe was 5.5 billion years old, contained 34 percent gas.
The results also suggest that there is a mechanism replenishing the molecular gas in these galaxies. The rate at which stars are forming can be used to estimate how long it would take to use up the entire supply of molecular gas, the timescale turns out to be less than the time interval between the two samples, suggesting that either the gas is replenished, or that the two galaxy populations studied have experienced different evolutionary paths.
This blog post is a news story from the Jodcast, aired in the March 2010 edition.
Tacconi, L., Genzel, R., Neri, R., Cox, P., Cooper, M., Shapiro, K., Bolatto, A., Bouché, N., Bournaud, F., Burkert, A., Combes, F., Comerford, J., Davis, M., Schreiber, N., Garcia-Burillo, S., Gracia-Carpio, J., Lutz, D., Naab, T., Omont, A., Shapley, A., Sternberg, A., & Weiner, B. (2010). High molecular gas fractions in normal massive star-forming galaxies in the young Universe Nature, 463 (7282), 781-784 DOI: 10.1038/nature08773