Friday, July 01, 2016

The deficiency of star formation in dwarf galaxies

Fig. 1: 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.  © MP

Dwarf galaxies form stars very inefficiently compared to spiral galaxies like our Milky-Way. To investigate the origin of this deficiency in star formation, scientists at MPA have used high-resolution numerical simulations to resolve the evolution of the interstellar medium (ISM) in dwarf galaxies. They find that supernova explosions have a significant impact on the structure of the ISM and regulate the star formation rates of the whole galaxy. The reservoir for star formation on scales comparable to molecular clouds in our Milky Way consists mainly of cold atomic hydrogen rather than molecular hydrogen. These findings might also shed light into the birth processes of most other galaxies. Within the current paradigm of hierarchical structure formation, low mass, chemically un-evolved dwarf galaxies are the building blocks of all, more massive galaxies.

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.

Fig. 2: 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.© MPA

Fig. 3: 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. © MPA 

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.

Authors: Chia-Yu Hu & Thorsten Naab - (Personal homepage)
(Stefanie Walch, Simon Glover, Paul Clark)

Original publication:

Hu, Chia-Yu; Naab, Thorsten; Walch, Stefanie; Glover, Simon C. O.; Clark, Paul C.

Star formation and molecular hydrogen in dwarf galaxies: a non-equilibrium view

This work is supported by: