The deficiency of star formation in dwarf galaxies
Face-on maps of the gas column density in a simulated galaxy at different spatial scales, highlighting the complex structure of the ISM. The central panel shows the entire star-forming region of the dwarf galaxy model. The details shown are a filamentary structure that is about 300 pc long (top left), a 200 pc bubble driven by supernova explosions (bottom left), a group of dense clouds (top right), and a further zoom-in of the dense clouds (bottom right). The effective spatial resolution is about 2 pc, so most of the clouds are well resolved.
In typical spiral galaxies, observations have shown a correlation between the surface density of the local star formation rate and the gas surface density, the so-called Kennicutt-Schmidt relation. The correlation is almost linear, i.e. the gas is converted into stars on a constant timescale of ~2 billion years. In the Milky-Way and other spiral galaxies star formation appears to happen exclusively in regions dominated by molecular gas.
However, this linear correlation breaks down in dwarf galaxies, where stars form very inefficiently on timescales that are much longer: 10-100 billion years. It is not yet clear whether the star forming gas in these dwarf galaxies consists mainly of molecules or atoms. Observations have not yet detected molecular gas but it has been speculated that an unseen molecular reservoir could dictate the star formation rate. This would provide an explanation for the longer star formation timescales in dwarf galaxies, which could be regulated by an inefficient transition from the atomic to molecular state.
This plot shows the gas temperature vs. the gas density in a simulated dwarf galaxy. If the gas was in thermal equilibrium it would follow the solid black curve. The dashed line indicates the resolution limit of the simulations below which the Jeans mass of gas is unresolved. Because the supernova explosions trigger turbulence and shocks, the gas is driven out of thermal equilibrium.
Recently, scientists at MPA have investigated the star formation in dwarf galaxies using numerical hydro-dynamical simulations, which incorporate a wealth of relevant physical processes. In particular it is assumed that molecular hydrogen forms on dust grains and that interstellar UV starlight can destroy the molecules. The simulations were conducted at an unprecedented high resolution (with a spatial resolution of 2 Parsec and matter particles of 4 solar masses). The impact of individual supernova explosions is numerically resolved. Fig. 1 shows a snapshot of the gas surface density in one of the simulations at different spatial scales, demonstrating the complexity of the multi-phase gas structure.
The simulations suggest that the star formation reservoir (the cold and dense gas) is predominately in the atomic phase, contrary to the situation in spiral galaxies. This is because it takes much longer for molecular hydrogen to form in a low-metallicity environment. As the ISM is constantly shaken and stirred by supernova explosions, the molecular hydrogen has no time to reach its (chemical) equilibrium abundance. The supernova explosions inject energy and momentum into the gas, triggering turbulence and shocks, much faster than the gas can cool or heat through radiative processes. As such, the gas is also driven out of thermal equilibrium (Fig. 2).
The Kennicutt-Schmidt relation in dwarf galaxies, i.e. the surface density of the local star formation rate vs. the gas surface density. The black dots are the simulation results, while the coloured dots are observational results from literature. The dashed gray lines indicate timescales of 1, 10 and 100 Gyr, which agree much better with the results from dwarf galaxies, where the star formation is suppressed. The simulations agree well with observational data as long as supernovae are included, which indicates that the supernova explosions are the key factor that regulates star formation in dwarf galaxies.
Comparing the Kennicutt-Schmidt relation of these simulations with observations of dwarf galaxies one finds good agreement (Fig. 3). The longer timescales compared to spiral galaxies (which is about 2 billion years) is caused by the inability of gas to cool in the outer part of the galaxy. As explained above, this prevents the ISM to form the cold gas needed for effective star formation.
The simulations also demonstrate that, while a change in the dust abundance or the interstellar UV radiation has a dramatic impact on the molecular abundance, it does not affect the thermal gas properties. This suggests that molecular hydrogen plays little role in regulating star formation in dwarf galaxies and is not a good tracer for it – in contrast to spiral galaxies like the Milky Way.
Chia-Yu Hu & Thorsten Naab
(Stefanie Walch, Simon Glover, Paul Clark)
This work is supported by:
DFG Priority Program 1573: ISM-SPP
