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Throughout cosmic evolution, although dwarf galaxies are individually small, they are numerous and serve as the fundamental building blocks shaping the large-scale structure of today's universe. These small galaxies are also the ancestors of our own Milky Way.

Dwarf galaxies are not only the "raw materials" for constructing massive galaxies but may also harbor secrets about the formation of the earliest stars and black holes in the universe. However, due to their low mass, shallow gravitational wells, and complex gas interactions, scientists have long struggled to understand the formation mechanisms of dwarf galaxies and their evolutionary relationship with surrounding gas.

Recently, Research Assistant Pei-Cheng Tung and Assistant Research Fellow Ke-Jung Chen from the Institute of Astronomy and Astrophysics, Academia Sinica, successfully developed a breakthrough cosmological simulation. For the first time, using unprecedented spatial and mass resolution, they systematically studied the co-evolution process of dwarf galaxies and their surrounding circumgalactic medium (CGM). The results have been in the latest issue of the Astrophysical Journal.

Technical innovation

This research used the well-known large cosmological simulation IllustrisTNG as the initial condition and employed the GIZMO simulation code combined with the "Particle Splitting" technique to increase the resolution of a single galaxy simulation by a factor of 100. The simulation successfully captured the multiphase gas structure and metal transport details inside and around dwarf galaxies.

This is among the highest resolution simulations worldwide focused on low-mass galaxies. The simulations relied on the large-scale supercomputing facilities at the National Energy Research Scientific Computing Center (NERSC) of Lawrence Berkeley National Laboratory (LBNL). These simulations typically require millions of CPU hours and represent a technical threshold achieved by only a few groups globally.

The simulation results show that star formation in dwarf galaxies is closely linked to the supply of cold gas in the CGM. More than 40% to 70% of the galaxy's mass originates from gas inflows, and nearly half of the cold gas needed for star formation is recycled from the CGM, indicating that the galaxy and its gaseous environment form an interacting "ecosystem."

In addition, the study quantitatively traced how metals released from stars are transported into the CGM by galactic winds, subsequently cooling and recycling in the environment, further influencing the future star formation efficiency of the galaxy.

More notably, the simulation discovered that even at the high-redshift epochs of the early universe, black holes at the centers of dwarf galaxies exhibited significant intermittent accretion activities. This supports the theoretical hypothesis that dwarf galaxies may serve as the cradles of supermassive black hole seeds and aligns with recent James Webb Space Telescope observations of high-redshift active galactic nuclei formation processes.

Future predictions

This study not only establishes a new benchmark for dwarf galaxy evolution models but also provides concrete, testable predictions for next-generation observational facilities such as JWST, Atacama Large Millimeter/submillimeter Array, and the forthcoming Nancy Grace Roman Space Telescope. These include the temperature and metal distribution in the CGM, the impact of black hole accretion history on galaxy luminosity, and the timescales of metal recycling inside and outside low-mass galaxies.

Dr. Ke-Jung Chen stated, "This work marks a new era where theoretical simulations have reached the precision needed to match observational details, allowing us for the first time to quantitatively explore the true nature of small galaxies in the early universe. By combining high-resolution simulations with observational data, we are approaching a critical point in answering fundamental questions such as 'How did the earliest galaxies form?' and 'How did the first black holes grow?'