1 Scientific
rationale of the project
Clusters of galaxies
are the largest gravitationally bound structures in the Universe. Their
baryonic composition is dominated by hot gas that is in quasi-hydrostatic
equilibrium within the dark matter dominated gravitational potential well of
the cluster. The hot gas is visible through spatially extended thermal X-ray
emission, and it has been studied extensively both for assessing its physical
properties and also as a tracer of the large scale structure of the Universe.
Clusters of galaxies
are not isolated entities in the Universe: they are connected through a filamentary
cosmic web. Theoretical predictions indicate the way this web is evolving. In
the early Universe most of the gas in the web was relatively cool (~104
K) and visible through numerous absorption lines, designated as the so-called
Lyα forest. In the present Universe, however,
about half of all the baryons are predicted to be in a warm phase (105-107
K), the Warm-Hot Intergalactic Medium (WHIM), with temperatures intermediate
between the hot clusters and the cool absorbing gas causing the Lyα forest.
The X-ray spectra of
clusters are dominated by the thermal emission from the hot gas, but in some
cases there appears to be evidence for hard X-ray tails or soft X-ray excesses.
Hard X-ray tails are difficult to detect, and one of the topics for the team is
a discussion on the significance of this detection (yet contradictory) in
existing and future space experiments. Various models have been proposed to
produce these hard X-ray tails, and our team is planning to review these
processes in the context of the observational constraints in clusters. In
particular the implied acceleration phenomena for primary and secondary
particles will be examined.
While in some cases
soft excesses in clusters can be explained as the low-energy extension of the
non-thermal hard X-ray components mentioned above, there is growing evidence
that a part may also be due to thermal emission from the WHIM. The signal seen
near clusters then originates in the densest and hottest parts of the WHIM filaments,
where the accelerating force of the clusters is highest and heating is
strongest. A strong component of this emission is line radiation from highly
ionized oxygen ions, and the role of this line emission and its observational
evidence will be reviewed.
WHIM filaments not
only can be observed because of their continuum or line emission, but also through
absorption lines if a sufficiently strong continuum background source is
present. Again, there is mounting evidence for absorption present in both UV
and X-ray high-resolution spectra. Future space missions will be well adapted
to study these absorption lines in more detail.
In particular in
absorption lines the lower density parts between clusters become observable. In
these low density regions of the WHIM not only collisional ionization but also
photo-ionization is an important process.
In general, the
physics of the WHIM is challenging due to its complexity since there are many
uncertain factors including the heating and cooling processes, the chemical
enrichment, the role of supernova-driven bubbles or starburst winds,
ram-pressure stripping, the role of shocks, magnetic fields, etc. More detailed
(and sophisticated) hydrodynamical simulations with state-of-the-art spatial
(and temporal) resolution are required in order to follow the impact of some
(if not all) of these important processes. In particular chemical enrichment is
an important process to consider as it leads to many observable predictions. We
aim to review the various physical processes relevant for the WHIM, the methods
that are used to simulate this and the basic results from those models.
The reviews that we
intend to produce will be the starting point for future work in this field, both theoretical, numerical, observational and instrumental.
We plan to have our papers ready for publication before the end of 2007.
We are considering
also to request support for a possible workshop with a larger (~50) group of
people, to be held also at ISSI but in the end of 2007 or in 2008, after the
completion of the present team work. That contemplated workshop will be focused
more on a discussion of what will then be the state-of-the-art, rather than the
more tutorial review on the topic as we propose now. However, a possible
request for such a workshop must be seen independently from the present
proposal for an international team.
2 Timeliness of
this proposal
The first detections
of non-virialized components in or between clusters of galaxies such as thermal
emission or absorption from the WHIM or the presence of hard X-ray tails have
now been made. There are several initiatives for new space missions to study
the physics of the WHIM, either in emission or absorption, from the
3 List of the expected output
Our intention is to write a series of review papers on the topics
mentioned in the previous sections. These reviews will be published together in
Space Science Reviews. We ask from all participants the commitment to
contribute to these papers and to write or help writing the various pieces. In
order to make the effort of the various team members more visible and to make it
rewarding to participate, we intend to assign one first author to each review
papers and have a few (typically 2-5) other members of the team as co-authors
on these papers. In this way we will be able to produce the intended dozen of
papers (see the list of proposed topics in Appendix C), while each team member
will be first or co-author on several papers. As outlined in our schedule, we
will do every effort to have these papers in a publishable form in 2007.
4 Why a workshop at ISSI?
The reason for choosing a workshop at ISSI for our international team is
that ISSI offers excellent facilities and an inspiring atmosphere to carry out this
project. Also the role of the space missions BeppoSAX, XMM-Newton and Integral
as European key projects is an important factor for choosing ISSI. All topics
that we intend to cover are closely related, and therefore interaction between
the different sub-teams is very important. This can be done only in a series of
focused meetings that we would like to organize at ISSI, away from distracting
academic and administrative activities at the home institutes of the
participants. Moreover, the project is too ambitious and too broad to do by a
small team of only a few people; we need the complementary expertise of all 15
team members to get this project underway: at least a kick-off and closing
meeting at a well-equipped location like ISSI is mandatory.
5 Confirmed participants
The following participants have confirmed their presence and willingness
to work on this team:
Dr. Jelle S. Kaastra, Prof. Andrei M. Bykov, Prof. Johan A.M. Bleeker, Dr.
Sabine Schindler, Dr. Florence Durret, Prof. Frits B.S. Paerels, Prof. Xavier
Barcons, Dr. Philipp Richter, Prof. Yoel Rephaeli, Prof. Takaya Ohashi, Prof.
Vahé Petrosian, Dr. Joop Schaye, Prof. Stefano Borgani, Dr. Klaus Dolag, mr.
Norbert Werner.
6 Schedule of the project
We envision the initial one-week meeting to take place in Fall 2006. Each participant gives a short introductory talk
of ~0.5 hour on the topic followed by questions
and discussion. This process takes the first two days. Then small teams (3-4
people) will be formed by the participants focusing on specific topics. The
next two days these teams have splinter sessions in order to work out an
outline of the various review papers. These outlines will be reported on the
last day. During the next half year, the team members work together on the
draft papers at their home institutes, mainly by e-mail contact within the
sub-teams. The second one-week session will be held half a year later, around
March/April 2007. The drafts must be submitted before this meeting, and will be
discussed during the first part of the meeting. This discussion will focus on
completeness, clarity, and coherence and crossreferencing between the different
papers. The last part of the meeting will be spent on fine-tuning and improving
of contents and text. The team leader, with the help of co-organizers, will do
the final editorial work.
7 Facilities required
Use will be made of the standard facilities of ISSI. A meeting room for
15 people is required, equipped with a beamer and su_cient (wireless) internet access.
A few desktop computers would be su_cient as most participants are expected to
bring a laptop. Some limited printing facility is needed. For the splinter
sessions room for about 4-5 groups of 3-4 people at the same time is needed,
some of which might use the main meeting room. All these facilities are
available at ISSI.
8 Financial support requested
of ISSI
We request the standard support for international teams (per diem &
lodging) as well as travel support for the team leader.