One of the most important discoveries of the last decades is the fact that we live in a spatially flat Universe whose expansion rate is accelerating. This acceleration is consistent with the effect of a cosmological constant, but it may also be caused by the presence of a dynamical energy component with negative pressure, now termed dark energy (DE). This accelerated expansion might also point to a fundamental modification of the laws of gravity. Presently, dark energy seems to be the dominant component of the Universe, significantly more important than either barionic matter (atoms), radiation or dark matter. The final explanation for this component will open a new window into the nature of gravity, particle physics and the fate of the Universe as a whole.

     Figure credit: NASA/HST/STScI, Dupke et al. 2011


As recognized by the U.S. Dark Energy Task Force (DETF, Albrecht et al. 2006,

"We strongly recommend that there be an aggressive program to explore dark energy as fully as possible, since it challenges our understanding of fundamental physical laws and the nature of the cosmos."

J-PAS is a large step in that direction, at a fraction of the cost of other similar efforts.



Baryonic Accoustic Oscillations

Probably the most powerful tool to study dark energy is the feature of the distribution of galaxies known as baryon acoustic oscillations (BAOs). These oscillations are very subtle ripples in the distribution of galaxies, with a length scale of approximately 150 Mpc (or about 450 million light-years). These ripples were generated at a very early epoch, when the Universe was less than one million years old (presently, its age is approximately 13.7 billion years old), and it was so hot and dense that radiation and matter constituted a single, tightly coupled fluid. The sound waves of this fluid can be seen today as the BAOs (see an artist's conception in the figure to the right).

The signature of the BAOs is a slight preference for galaxies to be found at a distance of about 150 Mpc from another galaxy. This distance can be on the angular direction (easy to measure), or in the radial direction (hard to measure, since the direction along the line-of-sight is difficult to measure accurately for such distant galaxies). Because of its vastly improved quality of photometric redshifts of galaxies, J-PAS will be able not just to measure the angular component of BAOs, but also its radial (line-of-sight) component. This will provide independent and more precise measurements of the DE equation of state parameter w.


Galaxy Evolution

The 56 filters of ~100 A FWHM effectively provide low resolution spectroscopy of all observed objects. This means that J-PAS will generate an impressive amount of data that will be extremely valuable for researchers of all areas of astronomy. This filter system will, for example, allow to measure many parameters which are relevant for the study of the evolution of galaxies: direct stellar temperatures, stellar masses, distribution of stellar ages, metallicity, dust extinction, and interstellar gas emission. The collection of spectro-photometric data of more than 300 hundred million galaxies will allow extensive studies of integrated stellar populations, and, since bright red galaxies will be observed until z~0.9, to investigate the evolution of galaxies from that redshift. This will allow detailed studies of star formation rates, galaxies mergers rates, and chemical evolution that will help understanding the relationship between the stellar components of galaxies of different types and their environments. Furthermore, stellar populations of nearby galaxies will be studied pixel by pixel, allowing us to to investigate the spatial evolution of the stellar component in thousands of galaxies.


Galaxy Clusters

Galaxy clusters are the largest collapsed structures in the universe, containing up to hundreds or thousands of individual galaxies. The redshift distribution and the evolution of clustering of massive clusters of galaxies can provide a direct measurement of the cosmic volume as a function of redshift as well as the growth rate of density perturbations. This is complementary to the measurement of the BAO scale, which is purely geometrical in nature. Comparison of theory to observations requires a calibration of the cluster masses. Clusters of galaxies can be identified optically by searching for concentrations of galaxies with the same velocities. J-PAS will provide a new window for accurate optical cluster detection and selection, based on the combination of photometric colors and good photo-z precision (equivalent to σv/c~1000 km/s) over all galaxies around each cluster, which will help to improve cluster completeness and purity. J-PAS will also provide the opportunity to self-calibrate the mass threshold of a given cluster sample in different ways, such as stacking weak lensing magnification measurements over the cluster position or using the (biased) amplitude of clustering in the same cluster sample. The photo-z accuracy for clusters will be improved in comparison to the galaxy photo-z by the square root of the number of galaxies in the cluster. This will result in a typical photo-z accuracy which is a few times smaller than that for galaxies. At the same time, one could use the velocity dispersion of the galaxies in each cluster to provide an estimate of the cluster mass. This should be accurate enough to have an estimate of the mass threshold of a given cluster sample, allowing to build a reliable mass function, the evolution of which can constrain Dark Energy parameters. 

A cluster survey carried out over the J-PAS area also constrains cosmology through the spatial clustering of the galaxy clusters. As mentioned above, this can be done with even higher photo-z accuracy than in J-PAS. The clustering of galaxy clusters reflect the underlying clustering in the DM; these correlations contain a wealth of cosmological information, much like the information contained in the LRG correlation function, including the BAO position. Even if the number density for clusters is lower than that of LRGs, this is partially compensated by the higher (biased) clustering amplitude. We will use J-PAS cluster redshift distribution and cluster power spectrum as cosmological probes to study the density and nature of the DE. J-PAS can also be used in combination with weak lensing (also from J-PAS) and other surveys to provide accurate photo-z in a sample of clusters detected by the Sunyaev–Zel’dovich (SZ) or X-ray signatures of hot gas in clusters.



Supernovae are another area of impact of the J-PAS survey. Due to the broad spectral features of supernovae, the filter system of J-PAS is ideal not only to discover them, but also to measure their light curves, to characterize their types (SN Ia/Ib/Ic/II etc.) and to extract their redshifts. J-PAS will discover and classify thousands of low- to medium-redshift supernovae, and will have a major impact in controlling some of the main systematic uncertainties associated with these standardizable candles.

One of the niches that the J-PAS supernova survey will explore is the excellent characterization of the supernova's environments that the 56 filters will allow. For most low-redshift objects, J-PAS will be able to distinguish the resolved stellar populations in every galaxy where a supernova is discovered.


     Figure credits: NASA/HST and High-Z Supernova Team



Weak Lensing

Weak lensing is sensitive to both the distance and the growth factor as a function of redshift. The lensing effect can be measured using either the shear or the magnification. The J-PAS camera will not be optimized to measure galaxy ellipticities. However, the accurate photometric redshifts obtained in J-PAS may be combined with ellipticity measurements obtained in other surveys for the same galaxies. This additional information will help separating shear lensing from intrinsic galaxy alignment. Gravitational lensing modulates the observed spatial distribution of galaxies. Dim galaxies that otherwise would not have been detected are brought into the sample by the lensing magnification. This increases the observed number density of galaxies. On the other hand, magnification also increases the apparent area, which leads to a drop in the observed number density of galaxies. The net lensing effect, known as magnification bias, is controlled by the slope of the number counts. The J-PAS Survey will be able to measure this effect by cross-correlating galaxy samples defined by separated redshift slices. Given the excellent seeing measured in the Pico del Buitre, J-PAS will use the best times (seeing<0.5-0.6") to perform a full high-quality image map of the Northern Hemisphere.



QSOs are another area where J-PAS will have a major impact. The narrow-band filter system is ideal to detect the broad emission lines of type-1 quasars, and we expect to identify and measure with high accuracy the redshifts of more than 3 million of these objects, up to redshifts of z~6. This quasar survey will be by far the largest in existence, improving on SDSS by a factor of >20, and allowing, for the first time, a measurement of large-scale structure with quasars alone. Even BAOs will be observable with quasars, both in the angular and in the radial directions. Moreover, the dataset will also be ideal to explore issues such as the clustering and bias of quasars, their luminosity function, duty cycles, etc. Another exciting application of the quasar survey is the search for strongly lensed systems: J-PAS will provide hundreds of multiple-image candidates.

J-PAS will provide a map of galaxies (with well defined redshifts) in the neighborhoods of the observed target lines of all quasars, which will be extremely useful to correlate absorbers with the galaxies observed in the  line of sight, providing valuable information on the gas distribution around the galaxies. The most luminous star-forming galaxies at redshifts z ~2.5 will be detectable with J-PAS narrow-band photometry by means of their Lyman continuum break, the Lyα forest absorption, and possibly Lyα emission line. This will allow the study of this galaxy population and its clustering properties over an unprecedentedly large volume.



By selecting the JPCam filters appropriately, several stellar parameters observed in J-PAS should be measurable, such as effective temperature, surface gravity, iron abundance, and α/Fe. 

The photometric data obtained by J-PAS reproduce the spectral energy distribution (SED) of the stars. The determination of these spectral parameters can be carried out through the comparison between the observed and theoretical SEDs, calculated for a large interval of spectral parameters. Some characteristics of the SEDs, such as the Balmer jump (~3647A) and the slope of the Paschen continuum (between 3800A and 8200A), that are sensitive to to the stellar parameters, will be used as primary indicators of these parameters. An analysis of a large sample of the Galactic Halo stars, will also allow us to accurately determine the density and metallicity profiles of old and metal-poor stars in the Galaxy.

Minor Planets

The distribution and chemical composition of asteroids is one of the most significant measurements of their formation and evolutionary histories, but it is also one of the most difficult quantities to be measured due the selection effects. The use of asteroid colors opened a new window for studies of the origin and evolution of asteroids. This idea was carried out recently using the 5 filters of the SDSS “Moving Object Catalog” showing very promising results in the area (Parker et al. 2008, Icarus, 198, 138; Carvano et al. 2010). The system of 56 filters of  J-PAS camera will allow to extend this study enormously, basically getting entire spectra of each asteroid, leading the exploration of this new scientific window to a new level.



Redshift Distortions

Peculiar velocities superimposed to the Hubble flow distort the mapping between position space and "redshift" space (see figure to the right, a rendering of the galaxy-galaxy correlation function produced by the 2dF Galaxy Redshift Survey Team, 2001). J-PAS's redshift accuracy will be sufficiently good to identify individual not only the galaxies, but the structures themselves (clusters, filaments, walls and voids), both in the angular and in the radial (line-of-sight) directions. Accurate measurements of the redshift-space power spectrum will be possible in the linear and mildly nonlinear regime, and a detailed comparison with theoretical predictions will be done in conjunction with the measurement of BAOs. The measurement of redshift space distortions offer an independent test of the growth history of the peculiar velocity field. This encloses cosmological information on DE (and/or modified gravity) which is complementary to that in BAOs, and which measures the background expansion history. A decomposition of the two-point correlation function in the radial and transverse directions also allows for a measurement of bias b and the amplitude of matter clustering σ8, which can be used to study the growth history of density fluctuations up to z = 1. The measurement of the amplitude of the galaxy power spectrum, P(k), as a function of redshift can also be used to determine the growth rate of structure through the cross-correlation of the galaxy data with future CMB-lensing data, or by using higher-order correlations to determine the bias parameter as a function of scale and redshift. Higher-order correlations, such as the three-point function or bispectrum, will also be used to measure the BAO feature.