IMAGINE scientific goals
Knowledge of the strength and structure of the Galactic magnetic field
is indispensable for studies of the Milky Way's interstellar medium
and cosmic rays. It is valuable input for dynamo theories and studies
of galactic and extragalactic magnetism. In addition, the magnetic
field of the Milky Way is a foreground influencing various
extragalactic studies such as Ultra-High Energy Cosmic Ray (UHECR)
sources, Cosmic Microwave Background polarization or the Epoch or
Reionization.
A list with descriptions of the sub-projects that make up
the IMAGINE consortium can be found here.
The main goal of the IMAGINE Consortium is modeling of the Galactic
magnetic field. Major science questions that can be addressed with
sufficiently reliable knowledge of the Galactic magnetic field are the
following:
- What is the role of the Galactic magnetic field in the
interstellar medium?
Galactic magnetic fields are dynamically
important, and that their energy density is comparable to other
components in the Milky Way. Recently, the
close coupling between these components has become clear in detail:
hydrogen filaments are aligned in magnetic fields in dust as probed by
starlight polarization, by polarized
dust emission and by Faraday rotation. State-of-the-art observational data sets such as
Planck polarization maps, all-sky Faraday rotation measure
maps, and many HI and (polarized) synchrotron
studies with current and next-generation instruments necessitate
detailed knowledge on the turbulent components of the Galactic
magnetic field to ensure progress.
- How are Galactic cosmic rays accelerated and propagated in
the Galactic magnetic field?
Electrons, positrons and ions are accelerated under the influence of
small-scale magnetic fields to cosmic rays. These cosmic rays
propagate through the Galaxy mostly along field lines, but also by
diffusion and advection. They interact with the interstellar
environment through e.g. spallation or gas heating; understanding
these processes, and the creation and propagation of Galactic cosmic
rays therefore needs accurate knowledge about the local magnetic field
strength and structure.
- How are magnetic fields in galaxies amplified and
maintained?
There are a number of physical processes that can create very weak magnetic
fields in the early Universe. These fields are then amplified by a
magnetic dynamo mechanism to the microGauss strengths we observe in
current galaxies. How that magnetic dynamo works in detail and what drives it
are as yet open questions. Various dynamo models give different
predictions for (symmetries) in the large-scale structure of Galactic
magnetic fields. Hence, a good model of the Galactic magnetic field
can constrain these theories and shed light on the origin and
evolution of galactic magnetism.
- What are the sources of ultra-high energy cosmic rays?
Ultra-high energy cosmic rays (UHECRs) are extragalactic cosmic rays
with energies exceeding E ~ 1018 eV. Various cosmic sources
could possibly accelerate charged particles to these extreme energies,
such as compact transients connected to star forming activity in
galaxies, AGN, radio galaxies or galaxy clusters, but the relative
contribution of these sources is still unknown. UHECRs propagating
from their sources through the magnetized intergalactic and
interstellar medium to detectors at Earth are deflected by the
intergalactic and (in many directions dominant) Galactic magnetic
fields. The deflections in these magnetic fields currently preclude us
from following these particles back to their sources. A sufficiently
reliable model of the (large-scale) magnetic field of the Galaxy would
enable us to determine the sources of the UHECRs.
Apart from these science questions, knowledge of the magnetic field
and non-thermal components of the Milky Way can be useful more
tangentially in the study of galaxy formation. One of the most
stringent problems in galaxy formation studies is the ``missing
satellite problem'', i.e. the problem that numerical simulations
predict many more low-mass satellite galaxies than are observed. This
discrepancy can possibly be attributed to the underestimation of feed
back processes by stellar winds, supernova remnants and active
galactic nuclei in these simulations. This feedback could drive gas
out of galaxies, or preclude new gas from accreting onto the
galaxies. Studies of the non-thermal components of the Milky Way will
teach us about these feedback processes at low star formation rates.
Finally, a trustworthy model of the Galactic magnetic field will allow us
to properly subtract the (polarized) radiation fields in the Galaxy
that arise due to this magnetic field, to reveal the extragalactic sky
in more detail. We will be able to better model Galactic polarized
dust emission to benefit detection of polarized Cosmic Microwave
Background B-modes, to model polarized Galactic synchrotron emission
to aid detection of HI fluctuations from the Epoch of Reionization, or
to study magnetic fields in the cosmic web.