MPA :: Current Research Highlight

Galaxies are well known to divide into two large families: red, old ellipticals and blue, star-forming spirals (Figure 1). While this distinction has been known for a long time, recent work based on the Sloan Digital Sky Survey (SDSS) has shown that, in the local universe, the division into these two large families happens at a stellar mass of ~3 10¹⁰ Msun (a value very close to the stellar mass of our own Galaxy, the Milky Way). Galaxies with stellar mass smaller than the threshold typically have young stellar populations, low surface mass densities and the low concentrations characteristic of disks. On the other hand, galaxies with higher stellar mass are characterized by old stellar populations, high surface mass densities and the high concentrations typical of bulges. New theoretical work has led to a diverse set of possible mechanisms to explain this characteristic mass scale where galaxies transition from young to old, with nearly all operating via quenching or regulation of the gas supply. New insight into this problem will come from studying gas in massive galaxies on both sides of the characteristic stellar mass, where the transition seems to take place. The cold gas (i.e. neutral hydrogen, HI) is the source of the material that will eventually form stars, thus it is clearly a key ingredient to understand what sets this transition in galaxy properties and test our theoretical models. In order to obtain an unbiased picture, it is also very important that the sample be selected according to stellar mass only, as opposed to optical morphology, star formation properties, gas richness, colour or other parameters.

This motivated a team of international astronomers, which includes Barbara Catinella, Guinevere Kauffmann, Silvia Fabello, Jing Wang at the MPA and several collaborators in other institutions, to undertake a new programme, the GALEX Arecibo SDSS Survey (GASS). We target 1000 of the largest and most massive galaxies in the local universe. These massive galaxies very likely started forming when the universe was quite young, but in the present day, some appear to have stopped forming stars. Has star formation stalled in these galaxies because the gas supply has been fully consumed? Or has the gas been pushed to the outskirts of these galaxies, or heated to temperatures that inhibit the gravitational collapse needed to form new stars? GASS is designed to answer these questions.

GASS probes the relationship between stars and gas by linking several new large galaxy surveys in visible and ultraviolet light to observations from Arecibo, the largest radio telescope in the world. The ground-based visible (SDSS) and space-based ultraviolet (Galaxy Evolution Explorer, GALEX) surveys measure the young and old stars in each galaxy, while the Arecibo survey that we are currently conducting measures the neutral hydrogen content of each galaxy. The Arecibo observations started in 2008 and are on-going. We observe the targets until we either detect them or we reach a low gas mass fraction limit (Figure 2). The first results show that, even in the high stellar mass regime, ~60% of the galaxies possess a significant amount of HI gas. We used our initial data set of ~200 galaxies to explore how the gas fraction depends on structure and stellar population properties of the sample. We find that the gas fraction of massive galaxies correlates strongly with stellar mass, stellar surface mass density and NUV-r colour, but correlates only weakly with bulge-to-disk ratio (Figure 3).

One of the key goals of GASS is to identify and quantify the incidence of transition objects, which might be moving between the blue, star-forming cloud and the red sequence of passively-evolving galaxies. Depending on their path to or from the red sequence, these objects should show signs of recent quenching of star formation or accretion of gas, respectively. Objects that deviate strongly from the average behavior of the sample are the best candidates for such transition galaxies. We have thus identified interesting objects that are anomalously gas-rich given their colours and densities, as well as gas-poor galaxies that are still forming stars. Objects in the first category might be accreting gas from recent encounters with other galaxies or from the surrounding environment, and in some cases might even be able to re-grow a star-forming disk. Objects in the second class may be systems where the HI gas has recently been stripped by tidal interactions or by ram-pressure exerted by intergalactic gas, or where other feedback processes have expelled the gas. In future work, we plan to investigate these different classes of transition galaxy in more detail.

In order to further our understanding of the physical processes causing the transition between blue and red sequence galaxies, we are also carrying out the COLD GASS ("CO Legacy Database for GASS") survey. This is an on-going large programme, carried out in collaboration with colleagues at the MPE and the IRAM observatory in Granada (Spain), which uses the IRAM 30m radio telescope to measure the molecular hydrogen content for a third of the GASS sample. Molecular hydrogen is another key ingredient needed to understand transition galaxies and constrain theoretical models, because it represents the intermediate step between the neutral hydrogen reservoir and the formation of new stars.

The GALEX Arecibo SDSS Survey

Further reading