How stars form in the smallest galaxies

It has been a long-standing puzzle how the smallest galaxies in the Universe have managed to continuously form stars at such a remarkably low rate – tiny galaxies like Leo P (pictured above) form approximately one star every 100,000 years! Making use of the DIRAC DIaL supercomputer, our research team has established how such low-levels of star formation arise because small dwarfs can stay quiescent for several billions of years, slowly accumulating gas in their centres. When this gas eventually collapses, star formation resumes at rates consistent with observations.

Figure 1: Despite its extremely low-mass, Leo P contains a significant number of blue stars. Our simulated galaxies also present these tell-tale signs of recent star formation activity. Image credit: K. McQuinn and Hubble Space Telescope.

Through high-resolution computer simulations, we demonstrate that star formation in the very smallest dwarf galaxies is shut down in the early Universe as a result of heating and ionisation from the strong light of new-born stars, exploding stars and stellar winds. However, we find that this quiescent state can be temporary. Some of our simulated dwarf galaxies are able to slowly accumulate fresh fuel in the form of hydrogen gas, over many billions of years. Once a sufficient amount of gas has piled-up to overcome any residual heating, star-formation re-ignites and proceeds continuously until today. This scenario explains, for the first time, the puzzlingly low star formation rates observed in faint dwarf galaxies such as Leo P, Leo T, and others. The computational power provided by DIRAC DIaL was crucial to establish this result, allowing us to afford detailed modelling of the star formation and heating processes in these tiny galaxies, over the full lifetime of the Universe.

By deepening our understanding of dwarf galaxies, we gain new insights into how astrophysical processes, such as stellar explosions and the heating and cooling of cosmic gas, regulate galaxy formation. This work also opens the door to harnessing these galaxies as a new test of the “Big Bang” cosmological model. Further work is currently underway to predict how many such star-forming dwarfs exist in our Universe and how many will be discovered by the next generation of astronomical telescopes.