LA ESPIRAL DE LA INCREDULIDAD IMPÍA

3.2.1 Background

The formation of the large-scale structure of matter can be understood as a competition of grav- itational contraction of local overdensities and the overall expansion of the Universe. The pre- dictions of numerical simulations of the dark matter evolution are consistent with observations of the distribution of galaxies and galaxy clusters, and with statistical measures of cosmic shear (weak lensing) that probes the gravitating (mostly dark) matter directly. Because the luminous (baryonic) matter is subject to non-gravitational forces, it dissipates energy and can change to different chemical and physical states. Most importantly, it is able to collapse to more compact structures such as galaxies and stars. The observed distribution of baryonic matter on scales from galaxies to galaxy clusters thus provides a wealth of information on the physics and evolution of the luminous as well as the dark Universe.

Galaxy clusters and groups are identified through optical observations as over-densities in space and/or colour, or through the X-ray emission of the hot intergalactic gas that accreted into poten- tial wells of the dark matter. Dark matter wells are identified through the shear distortion of light from background galaxies. About ten thousand galaxy clusters (defined as structures with mass larger than 1014_{Solar masses), and groups (with smaller masses) have been found. Several forth-}

coming experiments, in particular the Planck satellite, should expand this number significantly by discovering unbiased samples of clusters through a characteristic spectral distortion of the CMB

3.2. HOW DID THE STRUCTURE OF THE COSMIC WEB EVOLVE? 31 as it scatters off the hot electrons in the intracluster medium, the so-called Sunyaev-Zeldovich effect. Sunyaev-Zeldovich surveys are more sensitive to distant clusters than X-ray surveys, and are expected to produce a sufficient number of the rare distant clusters beyond redshift one that are particularly interesting to constrain cosmological parameters and to study the main epoch of galaxy and supermassive black hole formation. By 2010 such surveys are expected to produce about 30 000 clusters, with hundreds of clusters at redshifts larger than one.

A major motivation of optical, Sunyaev-Zeldovich and X-ray cluster surveys is to establish the evolution of the large-scale structure mass spectrum in order to constrain the basic cosmological parameters, in particular the dark energy equation of state (§ 2.2). It is crucial to properly relate the observables to the baryonic and total mass, which requires a good understanding of the en- ergetics of the intracluster medium as a function of time. There is evidence that at early epochs the energy density of the intracluster medium is strongly affected by merging, star formation and energetic outflows from active galactic nuclei. Conversely, the injection of energy and heavy ele- ments into the cosmic gas provided a regulating feedback that significantly affected the formation of stars and galaxies, and possibly also of massive black holes. The tidal interaction or merging of galaxies and the ram pressure of the intergalactic medium on the galactic interstellar medium adds to the complexity of baryonic structure formation.

3.2.2 Key questions and experiments

The effects of the environment on the formation of galaxies are poorly observed and not well understood. How do star formation, supermassive black holes, galaxy encounters, magnetic field generation and heavy element enrichment affect the distribution and properties of galax- ies in different environments, i.e., in clusters, groups, filaments, and voids? What are the physical properties of the diffuse gas and what is its relationship to the embedded galaxy populations? Radio galaxies, quasars and starburst galaxies trace over-densities to higher redshifts than cur- rent galaxy cluster surveys. Studying the surroundings of such objects with Sunyaev-Zeldovich, sub-millimetre, or Balmer-line imaging provides information on the environmental dependence of galaxy formation at the earliest epochs of structure formation. Studies of starburst galaxies and dusty quasars require ALMA for the sub-millimetre regime, and Herschel for the far-infrared. The growth of structures in low-density environments can be traced by absorption-line studies of low HI column density regions as demonstrated by numerical simulations of structure for- mation. The topology of the dark matter density field (sheets and filaments) and kinematics of the gaseous cosmic web can thus be derived from three-dimensional mapping of Ly-α absorbers with an ELT, using high-redshift quasars and compact, luminous galaxies as background targets. High-resolution ultraviolet (such as FUSE or HST) and X-ray spectroscopic capabilities (like the failed XIS instrument on SUZAKU) are needed to study the kinematics of the intracluster medium through spectroscopy of extended emission from ionic lines.

Large-area weak-lensing surveys with the VLT Survey Telescope (KIDS) will be complemented by near-infrared surveys with VISTA. Similar efforts in the USA include the Dark Energy Survey (DES), Pan-STARRS and LSST. Extension to all-sky surveys with depths of R(AB)> 24 and K > 19 is important to detect the galaxy concentrations of the rich clusters out to redshifts of 1.5. Deep, high-resolution multi-colour imaging with space telescopes will be required as well.

Sunyaev-Zeldovich survey instruments include APEX, the Atacama Cosmology Telescope (ACT, under construction), the South Pole Telescope (SPT, under construction), the Planck mission (launch 2008) and the interferometric projects SZ-Array and AMI. Several large-aperture single-dish (sub)- millimetre telescopes under construction or in the early planning stages will be able to provide the spatial resolution necessary for the joint modeling with X-ray and weak lensing studies, e.g., LMT and CCAT. The IRAM 30 m and the GBT could be equipped with high-resolution Sunyaev- Zeldovich imaging capabilities, and ALMA will provide key follow-up for the most interesting objects. The planned all-sky X-ray imaging survey with eROSITA will discover several hundred

Figure 3.4: The Sunyaev-Zeldovich effect can be observed by WMAP or other instruments that look at the microwave background. The microwave background will be distorted by foreground hot electrons and this distortion can be measured. Interestingly enough, the Sunyaev-Zeldovich effect is distance independent and will therefore probe all intervening hot electron clouds.

thousand clusters and groups and thereby provide a database for statistical studies of large-scale structure. The continuous availability of the X-ray space telescopes XMM-Newton and Chandra is crucial to study the hot intracluster medium and active galactic nuclei in the large number of clusters that will be discovered.

These observational programs will need to be complemented by the next generation of numeri- cal simulations of the structure and evolution of the cosmic web which include all the relevant physics. This will require substantial supercomputer resources.