The main goal of the ALHAMBRA survey is to probe Cosmic Evolution. In order to achieve this goal it is necessary to cover cosmological meaningful volumes at all redshifts for which a large area coverage and good depth are needed. Besides, good spectral resolution is fundamental to identify the different populations of objects and large spectral coverage is important to sample enough redshift range and allow easier identifications.

There are other scientific constraints that drive the survey strategy. Although the universe is in principle homogeneous and isotropic on large scales, astronomical objects are clustered on the sky on different scales. The clustering signature contains a wealth of information about the process of structure formation. A survey aiming at studying the clustering properties needs to probe as many scales as possible. In particular, searching contiguous areas is important to cover smoothly the smallest scales where the signal is stronger and obtain an optimally-shaped window function. Measuring a population of a certain volume density is a Poissonian process with an associated variance. One would obtain different densities of the same population when measuring in different places. The variance in those measures is dictated by the the volume density of the population under study, the volume searched and the clustering of the population. In order to beat down this Cosmic Variance one needs to sample independent volumes. So there should be a balance between probing contiguous and independent areas.

We defined our survey aiming at covering a total of 8 square degrees. There are some additional points as

Low extinction
No (or few) known bright sources
High galactic latitude
Overlap with other surveys and/or other wavelengths

After taking all the previous considerations into account, the selected fields are

Field name RA(J2000) DEC(J2000) 100μm E(B-V) l b
ALHAMBRA-1 00 29 46.0 +05 25 28 0.72 0.017 113 -57
ALHAMBRA-2 01 30 16.0 +04 15 40 0.80 0.022 140 -57
ALHAMBRA-3/SDSS 09 16 20 +46 02 20 0.015 174 +44
ALHAMBRA-4/COSMOS 10 00 28.6 +02 12 21 0.90 0.018 236 +42
ALHAMBRA-5/HDF-N 12 35 00.0 +61 57 00 0.60 0.011 125 +55
ALHAMBRA-6/GROTH 14 16 38.0 +52 25 05 0.35 0.007 95 +60
ALHAMBRA-7/ELAIS-N1 16 12 10.0 +54 30 00 0.27 0.005 84 +45
ALHAMBRA-8/SDSS 23 45 50.0 +15 34 50 1.17 0.027 99 -44

Zonas del cielo observadas.

ALHAMBRA survey fields (direction and depth), compared with other surveys as DEEP2, SDSS or VIRMOS


Representación del área cubierta por diferentes mapas cósmicos frente a su profundidad. Es obvio que los mapas que toman imágenes (representados por círculos) llegan a mayor profundidad que los que toman espectros asteriscos). Los surveys como ALHAMBRA ocupan una zona intermedia.


Representación del número de galaxias observadas por un proyecto frente al volumen en el que están repartidas. Podemos ver que ALHAMBRA (en dos formatos diferentes, con completitudes espectroscópicas del 60 y 90 por ciento) alcanza un número de galaxias similar al Sloan Digital Sky Survey, pero en un volumen considerablemente menor. Por tanto, se incluyen galaxias mucho menos luminosas.

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