Final Report Summary - ECOGAL (Star Formation and the Galactic Ecology)
Star formation is the process that drives the evolution of galaxies, provides the birthplace for planetary systems, and populates our luminous universe. Understanding how stars form is a fundamental aspect of all areas of astronomy. The ECOGAL project was constituted to develop the first Galactic-scale models of star formation that include both the large-scale dynamics of a spiral galaxy and the small-scale physics where stars form and subsequently feedback energy in terms of ionising radiation, stellar winds and supernova explosions. We wished to understand the physical drivers behind star formation, the origin of galaxies star formation rates and how feedback can affect the star forming and galactic environments.
Galactic scale numerical simulations of the gas flows in a spiral galaxy were used as input to higher resolutions re-simulations of the gas compression in the shock as the gas converges in the spiral arms. The shock origin of the dense star forming clouds produces the characteristic structures and turbulent motions as observed in molecular clouds. The simulations also produced star formation rates scaling relations found in galaxies, but require a constant efficiency parameter due to magnetic fields or stellar feedback. Including ionising radiation reduces the local star formation rate, but not sufficient to account for the overall low efficiency of star formation. Isolated molecular cloud simulations also showed only modest decreases in star formation efficiencies due to ionising radiation. Ionising radiation could disrupt the cloud if the gas was of low mass and fairly large to make it only loosely gravitationally bound.
Supernova feedback was also found to have a limited effect on the star formation environment, especially after an earlier stage of stellar winds and ionising radiation feedback. The energy from the supernova is able to escape along the evacuated, or at least low density, channels. This allows the energetic feedback to impact at larger distances from the star forming region. The effect on gas at large scales would then contribute to limiting future star formation and driving galaxy evolution.
The zoom simulations provided models of how stellar clusters form with realistic initial conditions due to the galaxy's dynamics. We found that star formation commenced while the proto-cluster clump was still being assembled. This resulted in a hierarchical process where small clusters grow by gas accretion and later through mergers to become larger systems. Predictions of the cluster properties, including a mass-radius relation, angular momentum and stellar age spreads provide observational comparisons. The compressive nature of the spiral flows also provide an explanation for OB associations: they are failed clusters where insufficient energy is lost in the shock, and local tidal fields from nearby clusters result in unbound systems.
High-mass stars require high accretion rates to form, and likely require to be born in stellar clusters. Apparently isolated high-mass stars can occur due to later cluster merger events where a lower mass cluster is tidally disrupting, spilling its high-mass stars into the galactic environment. Accretion, together with magnetic fields, can explain why high-mass stars are commonly found in close binary systems. Magnetic braking of the accreting gas limits the angular momentum, and hence the orbital separation, gained through this process.
Our global galaxy models have produced realistic models of dense molecular. We have analysed the evolutionary processes that affect molecular clouds. Their streaming motions along spiral arms can confuse distance estimates based on their velocities that assume circular motions. These datasets will continue to provide the basis for many future studies of molecular clouds, and of star formation in clustered and dispersed environments.
Galactic scale numerical simulations of the gas flows in a spiral galaxy were used as input to higher resolutions re-simulations of the gas compression in the shock as the gas converges in the spiral arms. The shock origin of the dense star forming clouds produces the characteristic structures and turbulent motions as observed in molecular clouds. The simulations also produced star formation rates scaling relations found in galaxies, but require a constant efficiency parameter due to magnetic fields or stellar feedback. Including ionising radiation reduces the local star formation rate, but not sufficient to account for the overall low efficiency of star formation. Isolated molecular cloud simulations also showed only modest decreases in star formation efficiencies due to ionising radiation. Ionising radiation could disrupt the cloud if the gas was of low mass and fairly large to make it only loosely gravitationally bound.
Supernova feedback was also found to have a limited effect on the star formation environment, especially after an earlier stage of stellar winds and ionising radiation feedback. The energy from the supernova is able to escape along the evacuated, or at least low density, channels. This allows the energetic feedback to impact at larger distances from the star forming region. The effect on gas at large scales would then contribute to limiting future star formation and driving galaxy evolution.
The zoom simulations provided models of how stellar clusters form with realistic initial conditions due to the galaxy's dynamics. We found that star formation commenced while the proto-cluster clump was still being assembled. This resulted in a hierarchical process where small clusters grow by gas accretion and later through mergers to become larger systems. Predictions of the cluster properties, including a mass-radius relation, angular momentum and stellar age spreads provide observational comparisons. The compressive nature of the spiral flows also provide an explanation for OB associations: they are failed clusters where insufficient energy is lost in the shock, and local tidal fields from nearby clusters result in unbound systems.
High-mass stars require high accretion rates to form, and likely require to be born in stellar clusters. Apparently isolated high-mass stars can occur due to later cluster merger events where a lower mass cluster is tidally disrupting, spilling its high-mass stars into the galactic environment. Accretion, together with magnetic fields, can explain why high-mass stars are commonly found in close binary systems. Magnetic braking of the accreting gas limits the angular momentum, and hence the orbital separation, gained through this process.
Our global galaxy models have produced realistic models of dense molecular. We have analysed the evolutionary processes that affect molecular clouds. Their streaming motions along spiral arms can confuse distance estimates based on their velocities that assume circular motions. These datasets will continue to provide the basis for many future studies of molecular clouds, and of star formation in clustered and dispersed environments.