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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.
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).
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.