The LITTLE THINGS Science

The processes that lead to star formation on galactic scales are poorly understood, even in the simplest systems in the universe, dwarf galaxies. At best we have incomplete knowledge of certain processes in certain environments. We are addressing this situation, starting with the simplest systems Nature can provide, local dwarf irregular (dIm) galaxies. Our approach begins with assembling a complete dataset on a sample of dIm galaxies, tracing their stellar populations, gas content, dynamics, and star formation indicators. The LITTLE THINGS HI-line maps will reveal the gas from which star-forming clouds are created and their kinematical context. We will combine the HI data with our optical, UV, and IR data to answer the following questions:

What regulates star formation in small galaxies?

The standard large-scale gravitational instability model does not work in dwarfs or the outer disks of spirals where the gas density is below the critical threshold and is stable to spontaneous perturbations. Why do giant cloud complexes and stars form at all in sub-critical gas?

What is the relative importance of sequential triggering for star formation in small galaxies?

One generation of stars can trigger the formation of the next by rearranging the gas through winds and supernovae explosions, but how important is this process? HI and optical observations of dwarfs show a better correlation between the star formation rate and the V-band surface brightness, which emphasizes ~Gyr old stars, than any other measure. This suggests that the existing stars are somehow important for triggering new stars.

What is the relative importance of triggering by random turbulence compression in dwarf galaxies?

Turbulence can account for various phenomena that are indirectly related to star formation. Is turbulence the key to allowing star formation to proceed in a normal fashion in dwarfs even though the gas density is sub-critical? And, what regulates turbulence?

What is happening in the far outer parts of dwarf galaxies, where star formation continues in gravitationally stable gas?

Densities of gas in outer disks of dwarfs and spirals are sub-critical by more than an order of magnitude. Yet star formation is on-going out there. Is turbulence the only remaining process to trigger star formation out there?

What happens to the star formation process at breaks in the exponential light profiles?

There is a change in slope in the exponential surface brightness light profiles of the outer disks of 20--40% of dIm and spiral galaxies and disks at high z. This break implies some sort of transition in the star formation process at the break radius.

What happens in Blue Compact Dwarfs (BCD)?

BCDs are dIm galaxies in which the central star formation rates jump by a factor of ~10, but not all BCDs show signs of a galaxy-galaxy interaction on which to blame the starburst.