Galaxies in the nearby Universe come in a wide variety of shapes, colours and sizes, yet this diverse population is found to obey many complex relations between their different properties. A global picture emerges, where galaxies mostly fall in one of two categories: either they are gas-rich, blue, spiral-like and actively star-forming, or else they are red, gas-poor and featureless. If we want to understand how galaxies form and evolve such as to produce this picture, it is essential to pinpoint when, where and how stars are formed in any galaxy. While detailed observations of the stellar light of galaxies can already teach us a great deal, a major hurdle in our understanding is our comparatively poor knowledge of the gas contents of these galaxies. Since cold gas is the fuel for star formation, we need a better picture of how galaxies are supplied in gas, how they process this gas into stars, and how some of the gas is then returned to the outside environment. Only then can we hope to really understand how galaxies evolve.

To answer these questions, scientists at the Max Planck Institute for Astrophysics are part of an international team currently using some of the world's largest radio telescopes to map the gas contents of massive galaxies in the nearby Universe. Complementary information comes from the extensive multi-wavelength data set that is being assembled by the team, including data from the Sloan Digital Sky Survey (SDSS), the UV satellite GALEX, and the Arecibo radio-telescope.

The IRAM 30-metre telescope, atop Pico Veleta in southern Spain, is providing key information about how gas is transformed into stars. The telescope, which is funded and operated by the French CNRS, the German Max Planck Society and the Spanish IGN, can observe at millimetre wavelengths and trace the presence of molecular gas in galaxies - allowing the scientists a glimpse at the last step along the cycle of gas: Once gas is accreted onto galaxies, it cools in regions where it is compressed, leading to the formation of molecules and then stars. The COLD GASS survey team, led by Amelie Saintonge from MPE and Guinevere Kauffmann from MPA, has used the IRAM 30-m telescope to perform a census of this molecular gas in a sample of 350 galaxies representative of the population of massive galaxies in the nearby Universe.

The survey so far has been used to trace the abundance of molecular gas in galaxies as a function of their mass, colour and morphology. This showed impressively the existence of sharp divides in the galaxy population based on their gas contents. With the IRAM observations, the scientists could identify a population of galaxies with no more than trace amounts of molecular gas. These galaxies are the most passive at the present epoch, with very little to no on-going star formation.

A surprising result emerges when the IRAM data are combined with the extensive multi-wavelength data set: While the IRAM observations find this population of "red and dead" galaxies, with no significant trace of either molecular gas or star formation, the Arecibo radio telescope shows that these galaxies can contain a significant amount of atomic gas, which under normal circumstances would cool and ultimately form stars.

Why can these galaxies be rich in atomic gas and yet not be able to form stars? This observation alone tells us about the existence of a bottleneck in the star formation process in massive galaxies: it is not enough for a galaxy to have a large reservoir of atomic gas; the conditions have to be just right for this gas to reach the low temperatures and high densities required for star formation. The results of large numerical simulations, also conducted by the MPA team and their collaborators, are now being tested against these observations. The combination of these detailed simulations and the large body of data provided by the COLD GASS survey will likely mark a step forward in our understanding of how galaxies evolve through the cycling of gas and the formation of stars.

Amelie Saintonge

Related Links

COLD GASS
GASS
IRAM

Publications

Amelie Saintonge, Guinevere Kauffmann, Carsten Kramer et al., "COLD GASS, an IRAM legacy survey of molecular gas in massive galaxies - I. Relations between H2, HI, stellar content and structural properties", 2011, MNRAS, 415, 32
http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2966.2011.18677.x/full

Amelie Saintonge, Guinevere Kauffmann, Jing Wang et al., "COLD GASS, an IRAM legacy survey of molecular gas in massive galaxies - II. The non-universality of the molecular gas depletion time-scale", 2011, MNRAS, 415, 61 http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2966.2011.18823.x/full

Barbara Catinella, David Schiminovich, Guinevere Kauffmann et al., "The GALEX Arecibo SDSS Survey - I. Gas fraction scaling relations of massive galaxies and first data release", 2010, MNRAS, 403, 683 http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2966.2009.16180.x/full

Silvia Fabello, Barbara Catinella, Riccardo Giovanelli et al., "ALFALFA HI data stacking - I. Does the bulge quench ongoing star formation in early-type galaxies?", 2011, MNRAS, 411, 993 http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2966.2010.17742.x/full