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Theory in the German Astrophysical VO
Gerard Lemson and Wolfgang Voges, GAVO, Max-Planck-Institut für extraterrestrische Physik, Garching, Germany
Summary: We show results of efforts done within the German Astrophysical Virtual Observatory (GAVO). GAVO has paid special attention to the introduction of theory data (simulations) into the
Virtual Observatory (VO). The main emphasis of GAVO in this context was to investigate the use of relational database technology in the analysis of results of large scale structure simulations, as well
as in their online publication. The former may lead to direct scientific benefits to the owners of the data, the latter leads to benefits to the larger community that gets access to the data in a well defined
and standardized manner. We also show prototypes of so called Virtual Telescopes, online services to “observe” simulation results so as to produce results that can be directly, or almost directly, be
compared to observations of similar objects as were simulated.
The Millennium Database
Abstract [1]
The Millennium Run is the largest simulation of the formation of structure within the LCDM cosmogony so
far carried out. It uses 1010 particles to follow the dark matter distribution in a cubic region 500h−1Mpc on
a side, and has a spatial resolution of 5 h−1kpc. Application of simplified modelling techniques to the
stored output of this calculation allows the formation and evolution of the ∼ 107 galaxies more luminous
than the Small Magellanic Cloud to be simulated for a variety of assumptions about the detailed physics
involved. As part of the activities of GAVO we have used a relational database to store the detailed
assembly histories both of all the haloes and subhaloes resolved by the simulation, and of all the galaxies
that form within these structures for two independent models of the galaxy formation physics. We have
created web applications (see Fig. 2) that allow users to query these databases remotely using the
standard Structured Query Language (SQL). This allows easy access to all properties of the galaxies and
halos, as well as to the spatial and temporal relations between them and their environment. Information is
output in table format compatible with standard Virtual Observatory tools and protocols. With this
announcement we are making these structures fully accessible to all users. Interested scientists can learn
SQL, gain familiarity with the database design and test queries on a small, openly accessible version of
the Millennium Run (with volume 1/512 that of the full simulation). They can then request accounts to run
similar queries on the databases for the full simulations.
Time evolution, merger trees
In contrast to most observational database, time
evolution is an integral part of simulation results. In the
case of the Millennium database, this is embodied in the
storage of merger trees of both dark matter halos and
galaxies. To store such hierarchical data structures in a
relational database we invented a new method that
allows fast retrieval of the history (and future) of arbitrary
nodes in the tree. As illustrated in Fig.1, it relies on
assigning identifiers according to a depth-first ordering of
the nodes in a tree and assigning pointers (foreign keys)
pointing from each node to the last of its progenitors in
that ordering (the red arrows in the figure). As can be
easily seen , the complete merger tree for a given node
(e.g. the one with ID=1) can be retrieved by the following
query:
select p.*
from halos d, halos p
where d.id = 1
and d.id <= p.id
and p.id <= d.lastProgenitorId
For more details see [2] and the documentation on the
website [3].
Fig 1: Illustration of the merger tree structure of
objects (halos/galaxies) in the simulation. The
black lines indicate the traditional, descendant
pointers. The red lines indicate the pointer
structure used in the database model.
Fig 2: GAVO web interface for
querying
the
Millennium
databases. Results can be
obtained in a variety of formats
and can be visualised online
using the VO-India’s VOPlot
facility.
Virtual Telescopes
Results
http://www.g-vo.org/mpasims/MoMaf2
http://www.g-vo.org/hydrosims/
Fig 3: Two examples of a virtual telescope, one creating mock light-cones through a
cosmological simulation, the other creating images of hydro simulations of galaxy clusters
Virtual telescope protypes at GAVO
Here we show examples of existing web applications that illustrate the concept of the
virtual telescope. By modeling the relevant aspects of the real observational configuration,
including sources, possible absorbing/emitting/lensing foregrounds as well as the
instrumental characteristics, virtual telescopes produce mock images that can be directly
compared to real observations, possibly using tools developed for real observations.
As such, virtual telescopes form an important ingredient of the so called theory/
observational interface, through which the virtual observatory aims to bridge the gap
between observers and theorists. The examples of this page are all available on the
GAVO web pages for online execution.
Interoperability
Within the International VO Alliance (IVOA), GAVO leads the efforts to introduce theory in
the IVOA’s standards process. As an example of this serves the “virtual telescope
configuration” use case, illustrated in Fig. 4 below.
This use case describes a distributed,
modular architecture of cooperating
components, each modeling a part
of a real observation. These are
implemented according to standard
APIs and interoperate by sending
data structures following standard
data models. Components here
are a photon source (simulation
result), foregrounds and virtual
telescope. And importantly the
“pre-observation image” (POI)
which models photons streaming
from source to telescope. See
[4] for more details and future
Fig 4: Illustration of Virtual telescope use case,
developments
used in the IVOA theory interest group [5].
References and further links
[1] Lemson & the Virgo Consortium, astro-ph/0108019
[2] Lemson & Springel, 2006, ADASS XV, p212
[3] http://www.g-vo.org/MyMillennium
[4] http://www.ivoa.net/twiki/bin/view/IVOA/IvoaTheory
[5] http://www.ivoa.net/twiki/bin/view/IVOA/VirtualTelescopeConfiguration
Fig 5: A first example of a full virtual telescope configuration, the Planck simulator at http://www.gvo.org/planck. The user can enter cosmological and observer parameters as well as choose from a variety of
foregrounds to include. An online “virtual observation” results in maps in standard FITS format, for
comparison to real observations or for use in preparation of analysis software.
Acknowledgments
We thank Alex Szalay and his coworkers for their support in the development of the Millennium Database.
We thank Jeremy Blaizot, Klaus Dolag and the Planck group at MPA for their support in the development of the various virtual
telescope prototypes. The GAVO is funded by the German Federal Ministry for Education and Research.