The star formation rate  in galaxies varies greatly both across different galaxy types and over  galactic time scales. MPA astronomers have been trying to gain insight  into how the interstellar medium may change in different galaxies by  studying molecular gas in a wide variety of galaxies, ranging from  gas-poor, massive ellipticals to strongly star-forming irregulars, and  in environments ranging from inner bulges to outer disks. They find that  the gas depletion time depends both on the strength of the local  gravitational forces and the star formation activity inside the galaxy. 

  Molecular clouds are  clouds in galaxies consisting predominantly of molecular hydrogen. They  are stellar nurseries where the gas reaches high enough densities to  form new stars and planetary systems. Molecular clouds are highly  complex structures. Figure 1 shows a Hubble Space Telescope image of the  Eagle Nebula, a nearby molecular cloud with a highly filamentary and  irregular structure. 

  In the neighbourhood  of our Sun, molecular clouds make up only 1 % of the total volume of the  interstellar medium and form stars at modest rates of a few solar  masses per year. In the early Universe, however, there is mounting  evidence that galaxies contain much more molecular gas and therefore  they can form stars at rates up to a thousand times higher than in our  Milky Way. The densities and pressures in the interstellar media of  these early galaxies are also orders of magnitude higher than in the  solar neighbourhood, and it is unlikely that molecular clouds in these  systems are the same as the very well-studied Eagle nebula. 

Fig. 1:       Eagle Nebula imaged by Hubble Space Telescope. 

Credit: NASA, ESA/Hubble and the Hubble Heritage Team (STScI/AURA)

Fig. 2:        Top: This  plot is linking the depletion time and a specific combination of star  formation rate (SFR) and stellar surface density. Each data point  represents a grid cell of 1kpc x 1kpc size within different structures  of the galaxies analysed.       Bottom: The  optical image of one of the galaxies in the sample, NGC 5457. Coloured  squares show grids cells, with 1 kpc on a side, in the arm (green),  interarm (yellow) and bulge (red) regions. 

  In recent work, the  MPA group studied variations in the relation between the local density  of molecular gas and newly formed stars. They used this as a diagnostic  of changing conditions within the interstellar medium. According to  standard theory, molecular clouds exist in a balance between  gravitational forces, which work to collapse the cloud, and pressure  forces (primarily from the gas), which work to keep the cloud from  collapsing. When these forces fall out of balance, such as can happen in  a supernova shock wave, the cloud begins to collapse and fragment into  smaller and smaller pieces. The smallest of these fragments begin  contracting and become proto-stars. 

  Gravitational forces  vary significantly from one galaxy to the next, as well as in different  regions of the same galaxy. At the centre of a giant elliptical galaxy,  gravity is much higher than in the outskirts of a small dwarf irregular.  Likewise, the incidence of supernova explosions can differ drastically  between different galaxies and between different locations within the  same galaxy. Variations in the ratio of the density of molecular gas to  young stars (commonly referred to as the depletion time of the molecular  gas) may thus be expected as a consequence of these changing  conditions. 

  The main result (see  Figure 2) from the MPA group's analysis is that the rate at which  molecular gas forms new stars is set BOTH by gravity (as measured by the  local surface density of stars in the galaxy) and by the local star  formation activity level in the galaxy, which in turn will determine the  incidence of supernova-driven shock waves in the interstellar medium.  Molecular gas depletion times are shortest in regions where gravity is  strong and where the star formation activity is high, particularly in  galaxy bulges with gas and ongoing star formation. 

Reaching this  conclusion required very careful analysis of a variety of data sets at  different wavelengths. In particular, star formation rates derived from  the combination of infrared images that trace young stars embedded  inside dusty clouds and far-ultraviolet images that trace stars that  have migrated outside these clouds, are crucial for pinpointing these  relations as accurately as possible. In future, new state-of-the-art  interferometric radio telescopes, in particular the Atacama Large  Millimeter/submillimeter Array (ALMA), will allow us to understand the  detailed structure of molecular clouds in regions of high gravity in  much more detail.       

  For a little more insight into the project see this： 

   http://www.mpa-garching.mpg.de/mpa/research/current_research/hl2015-7/hl2015-7-en.html