Theory in the German Astrophysical VO

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.

Gerard Lemson and Wolfgang Voges, GAVO, Max-Planck-Institut für extraterrestrische Physik, Garching, Germany

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.

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.

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 10∼ 7 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 treesIn 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.lastProgenitorIdFor more details see [2] and the documentation on the website [3].

The Millennium Database

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

AcknowledgmentsWe 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.

Fig 5: A first example of a full virtual telescope configuration, the Planck simulator at http://www.g-vo.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.

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 GAVOHere 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 standardAPIs and interoperate by sendingdata structures following standarddata models. Components here are a photon source (simulationresult), foregrounds and virtualtelescope. And importantly the “pre-observation image” (POI)which models photons streamingfrom source to telescope. See[4] for more details and futuredevelopments

Results

Virtual Telescopes

Fig 4: Illustration of Virtual telescope use case, used in the IVOA theory interest group [5].