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The PAU Survey


The PAU Survey studies the existence and properties of dark energy from the observations of Red-shift Space Distortions (RSD) and Weak Lensing Magnification (MAG) from galaxy cross-correlations as main cosmological probes (right panel).

The PAU Team is building an instrument, PAUCam, designed to be located at the prime focus of the 4-m diameter William Herschel Telescope (WHT) in La Palma. The simulations indicate that PAUCam at the WHT will be able to image about 2 square degrees per night in 40 narrow-band filters plus five wide-band filters to an AB magnitude depth of i~23, providing low-resolution (R~50) photometric spectra for around 30,000 galaxies, 5,000 stars and 1,000 quasars.

The PAU Survey will image an area of 100/200 square degrees with in 40 narrow band filters. Our simulations indicate that it is possible to obtain a redshift precision above 0.0035 (1+z) for 70% of the imaged galaxies. With these observations it would be possible to obtain competitive measurements of the dark-energy equation of state parameters, comparable to other much larger spectroscopic and photometric surveys now taking place or planned for the near future.

The competitive edge of our approach compared to other photometric surveys, resides in the possibility of measuring distances with the precision corresponding to the scale of the transition from linear to non-linear matter fluctuations. We want to be able to trace the linear matter fluctuations in three dimensions while the wide-band photometric surveys can only do it in two, rendering the number of independent modes that can be measured per unit surveyed area much smaller. Moreover, there are measurements such as redshift space distortions that cannot be done without precise radial distances.  PAU is also competitive when compared with the current generation of spectroscopic surveys, owing to the larger number of objects measured over a large volume of space extending to z ~ 1.

When not in use by PAU, PAUCam can be used as a community instrument, able to provide spectral energy distributions (SED) of moderate resolution for a very large sample of objects, allowing the study of a variety of scientific topics beyond cosmology. The filter system is being designed to include five wide band filters as well as additional space for mounting possible special-purpose filters provided by the users.

Please follow links above for more information.

Dark Energy Probes


The indications of an accelerated expansion of the universe have led to many proposals intended to constraint the nature of the putative dark energy that is assumed to drive such an expansion. Most of the information today comes from the combined analysis of three distinct probes: CMB temperature fluctuations, luminosity-distance of type Ia Supernovae and large-scale structure observations. With respect to the latter, it should be noted that the formation of large-scale structure is affected in many ways by the existence of dark energy, giving rise to a number of potentially sensitive probes, which, generally speaking, require observations involving the measurement of the angular position and red-shift of a very large number of galaxies in a very large volume of space.



Redshift Space Distortions

The measured redshift distance, s, to a galaxy is related by the Hubble expansion to the radial distance, r, but this relation is modified due to the peculiar velocity of the galaxy along the line-of-sight. At large scales (larger than 10 Mpc/h), the very large structures, walls and voids, give rise to coherent bulk motions with objects falling towards the dense regions. Objects between an overdense region and us will appear more distant while objects behind an overdense region will appear closer. This produces a squashing effect in the anisotropic 2-point redshift correlation function along the line-of-sight, known as Kaiser effect [N. Kaiser, MNRAS, 227, 1-21 (1987)]. The amplitude of distortions on large scales yields a measure of the linear growth of the fluctuations parameter, which is tracer of dark energy.


Lensing Magnification

The effect of gravitational lensing is to alter the area of the patch of the sky being observed and to change at the same time the measured flux of the source so that surface brightness is preserved. Both effects change the measured galaxy number density, the first by changing the area, the second by changing the number of sources N(<m) observed in a magnitude (m) limited survey. Together these effects are called magnification bias [Gunn, J., 1967, ApJ, 150, 737] and lead to a cross-correlation signal between galaxies in different redshift bins. The effect can be used to reconstruct the 3D power spectrum of dark matter fluctuations, which again depends on the existence of dark energy.