In simulated galaxies of the hydrodynamical cosmological “EAGLE”
simulation the distribution of atomic hydrogen agrees with observations
in unprecedented detail. This success means that EAGLE can aid
astrophysicists to better understand the processes shaping real
galaxies, such as the origin of their atomic hydrogen. EAGLE is not
quite perfect, however: the study also found that some simulated
galaxies contain unphysically large holes in their atomic hydrogen
discs, meaning further work for simulators to improve the models
underlying the treatment of supernova explosions and the interstellar
matter.
Fig. 1: Observed distribution of atomic hydrogen (blue) in the nearby galaxy
M74. Also shown are old stars (red) and UV radiation emitted by newborn
stars (purple) - both of these are more strongly concentrated towards
the galaxy’s centre than the atomic hydrogen. The green bar shows a
length of 15 000 light years. Image: THINGS survey
Atomic hydrogen (abbreviated as “H I”) is
an important component of galaxies, it is believed to feed the dense
‘interstellar matter’ from which stars can form. Although invisible to
optical telescopes, astronomers have been able to make ever more
accurate observations of this gas component using radio telescopes: this
has revealed, for instance, that galaxies with the same stellar mass
can differ in their H I content by more than an order of magnitude. By
combining several radio antennas into one `supertelescope’ with the aid
of interferometry it has also become possible to create high resolution
maps showing the distribution of atomic hydrogen within individual
galaxies (for an example, see Fig. 1).
But despite this wealth of observational
data, astronomers are still puzzled by the question of why some galaxies
contain so much more H I than others, and especially why `normal’ and
`H I-rich’ galaxies still appear to follow common relations, as shown
recently by the MPA-led “Bluedisk” project (see monthly highlights
May 2013 and
March 2014).
A fundamental problem is that galaxies only evolve over periods of many
millions of years, so that it is impossible to directly observe how the
H I reservoir is built up. Instead, astronomers have to try and answer
this question with the aid of models and simulations.
Fig. 2: Gas
in the large EAGLE simulation. Blue represents “cold” gas (T greater then
30,000 K), green warm, and red the hottest gas with T > 300,000 K.
The small insets zoom in towards a single galaxy, highlighting the huge
dynamic range of the simulation. © Richard Bower / James Trayford, ICC Durham
An international collaboration has
recently completed the “EAGLE” simulation of galaxies which matches
observed galaxies in several properties such as their stellar mass and
size with unprecedented accuracy (see Fig. 2). A research team led by
MPA scientist Yannick Bahe has now studied how well these simulated
galaxies agree with real ones in terms of their atomic hydrogen content:
an important test for the simulation model, which also determines
whether EAGLE can give trustworthy clues on the evolution of H I in real
galaxies.
To make this comparison, the scientists
first had to post-process the simulation and calculate how much of the
hydrogen in each simulation particle is actually atomic, i.e. not
ionised or molecular. Once this was done, the total mass of H I in over
2000 simulated galaxies could be computed and compared to observational
data from the “GASS” project. The resulting match between simulation and
data is extremely good: it represents a significant improvement
compared to previous simulations and indicates that the models used in
EAGLE provide a reasonable description of the physical processes
involved in forming galaxies.
Fig. 3: The
surface density of atomic hydrogen, plotted against distance from the
galaxy centre. Yellow and blue bands show data from the “Bluedisk”
project, comparing galaxies with normal (yellow) and exceptionally high
(blue) total atomic hydrogen content. Red and green circles show
simulated galaxies from EAGLE in the same categories. © MPA
Motivated by this initial success, the
scientists tested the EAGLE simulations in more detail by comparing not
just the total mass of H I, but also its distribution within galaxies to
observations. The above-mentioned “Bluedisk” project has shown that
this distribution is surprisingly independent of the total mass of H I
as long as the galaxies’ H I discs are scaled to a common size (so
called “self-similarity”). For an accurate comparison, the team now
‘observed’ the EAGLE galaxies in the same way as was done in Bluedisk.
As can be seen in Fig. 3, both agree surprisingly well: EAGLE reproduces
both the self-similarity between ‘normal’ and ‘H I-rich’ galaxies (red
and green symbols in
Fig. 3) and the detailed shape of the surface
density profile – at least in the outer parts of the simulated galaxies.
In the central regions, however, EAGLE
galaxies typically contain not enough atomic hydrogen. To test this
discrepancy further, the scientists inspected more than 2000 images of
the simulated galaxies, which finally gave the crucial clue: many
simulated galaxies contain ‘holes’ in their hydrogen discs that are much
larger than what is seen in observations. Once all simulated galaxies
showing these large holes were excluded, the density profiles matched
observations almost perfectly even in the centre.
Fig. 4: Synthetic
image showing atomic hydrogen in a simulated galaxy, in analogy to Fig.
1.
Clearly visible are a number of large holes in the hydrogen disc. © MPA
Why, now, do some EAGLE galaxies contain
these large holes? The scientists have not yet found a definitive
answer, but it is likely that the way in which supernova explosions are
modelled in the simulation plays a major role. This critical part of
galaxy formation is still causing headaches for simulators: to include
them in galaxy simulations in a fully self-consistent fashion, the
resolution of the simulations would need to go up by many orders of
magnitude. This will be, regrettably, impossible for a long time to come
– even the biggest supercomputers today are just not big enough (see
also
highlight August 2015).
As a result, EAGLE has to resort to using a highly simplified model for
the effects of such supernovae. Another simplified model has to be
employed for the dense interstellar matter, because a resolution level
that would allow a fully self-consistent treatment can also not yet be
achieved in simulations of a representative portion of the Universe.
Although these simplified models produce galaxies which are realistic in
many ways – such as their size – they do leave a noticeable artefact in
some of the simulated hydrogen discs: the large holes discovered by the
researchers.
It is therefore an important challenge
for astrophysicists to optimise both the simulation codes and the models
in such a way that – in conjunction with continually more powerful
supercomputers – a self-consistent treatment of the dense interstellar
medium can be achieved. Combined with improved supernova models, these
future simulations will, hopefully, produce galaxies that match the real
Universe even better than EAGLE does. However, the current study also
demonstrates that EAGLE can already give valuable insight into the
evolution of atomic hydrogen in galaxies. In a follow-up project, the
researchers will examine the formation of the simulated galaxies to find
out how and why some of them got so much more hydrogen than others.
